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PRINCIPLES OF ALCHEMY
WATER

Part Two

The weak "bonds" (or forces) dominate the way molecules interact.

Weak interactions determine the states of matter.

VSEPR theory explains the shape of covalent molecules.

Other molecules use electrostatics to make their simple shapes.

You have learned these three bonds very well, Arthur.
Now I'd like to tell you about some other molecular "forces".

Forces?

Yes. Some Alchemists call these forces "bonds" but other Alchemists say they aren't really bonds at all, so they call them "forces". It doesn't really matter what you call them as long as you know what they are.

OK. What are they?

Weak. Very weak. The three bonds we discussed earlier (covalent, electrovalent and metallic) are pretty strong bonds. By comparison, the "bonds" I now want to tell you about are very weak.

So what good are these weak bonds?

Oh, they are very important. Weak bonds are what causes something to be solid, liquid or a gas. The "state of matter" as we call it.
[Note: with some materials the strong bonds of the molecules play a part in this too, but the weak bonds dominate the state of most matter.]
You see, the strong bonds hold groups of ATOMS together in molecules. We can call them "intra-molecular bonds", because they are the bonds internal to molecules.

Because "intra" means "internal"

Yes, that's correct.
But the weak bonds hold groups of MOLECULES together in matter! It's a very important part of Alchemy. These weak bonds are "inter-molecular", meaning "between molecules".

So, these weak bonds cause molecules to link together, not atoms.

Yes. But don't take that definition too far. Because if you are linking molecules together, you must be linking atoms together too. Right?

Oh, yeah. I see what you mean.

And you should also know that these weak bonds can act "internally" too. What I mean is that a very large (covalent) molecule, like a protein, may loop back on itself using some of these weak forces. Producing weak links to other atoms in the same molecule. That gives the molecule a specific shape and defines how it interacts with other molecules.

Sounds complex.

It's not that bad. Just remember that the strong bonds are "real". Real strong! They link the atoms into molecules. But the weak bonds have more to do with the way molecules interact with each other.

Except some big molecules, where they might be used to interact with themselves.

Yes. Understand that these exceptionally big molecules, using weak, "inter-molecular" bonds on themselves are simply behaving as if the two parts of the same molecule were actually two separate molecules.

I see. Anything else these weak bonds do?

Oh yes. They are an extremely important part of BioAlchemy, the Alchemy of Life. Most of the millions of chemical reactions going on in your body make use of these weak forces. To shape proteins. To make membranes. To dissolve substances. I could go on and on.

Yes. I'm sure you could. So what are they?!

Weak forces are caused by the strange way the electrons behave around each other. Actually, there's nothing "strange" about it. It's all electrostatics.

Again with electrostatics!

Like I said, most of Alchemy is the study of the electrons.
Consider the molecule hydrogen chloride (HCl). We could think of it as a covalent compound, sharing a pair of electrons, thus forming a covalent bond. We can even write a Lewis structure for it. Or we could think of it as an electrovalent compound, with the chlorine stealing away the electron from the hydrogen.

Yeah. Either way seems likely to me. Either way they end up completing their outer shells. So which is it?

Well, from experimentation, Alchemists have found that HCl is a covalent molecule after all. BUT...

There's always a big "BUT" to make life complicated.

And I'm not too old to kick yours! So pipe down and pay attention!

Oops. Sorry.

Where was I? Oh, yes.
As it happens the covalent bond in HCl is not a fair "sharing" of electrons. The electrons in the HCl covalent bond are more attracted to the chlorine than the hydrogen atom. Thus, the chlorine atom carries a partial negative charge and the hydrogen a partial positive charge.

Because chlorine is more electronegative than hydrogen.

Yes! Precisely. We use a Greek letter called "little delta" to symbolize these partial charges. (It looks like a wiggly "d".)Molecules with an uneven distribution of electrons are called "polar molecules" because their covalent bonds have a direction to them.

Like "north" and "south".?

Yes. But this has nothing to do with magnets or the Earth. It's just a name Alchemists borrowed. We say the bond is "polarized", it has a specific direction to it. From partially positive to partially negative.

I see. Do all molecules show this polarizing effect?

Some do and some don't. For example, diatomic hydrogen, diatomic oxygen and diatomic nitrogen are "apolar", meaning they have no pole. Can you think of why they don't have a pole?

Sure. Because they are elemental compounds - all the same atoms. Therefore, they are all equally electronegative. If both the atoms have the same electronegativity, they must end up distributing their electrons evenly between them.

Very good. You must have different atoms to have different electronegativities and you must have different electronegativities among the atoms in a molecule for it to become a polar molecule. (The shape of the molecule is important too. But we will ignore that.)

So these partial charges affect the way the molecule behaves?

Yes indeed. Especially how they behave with each other.
Guess which molecule uses its polar properties the most in Alchemy.

Water?

Right! Water is a polar molecule. You will recall that hydrogen and oxygen normally make a covalent bond when they come together. But the oxygen attracts the electrons more strongly than the hydrogen....

...because oxygen is more electronegative than hydrogen (hydrogen is such a whimp).

Yes. The oxygen pulls electron pairs in the covalent bond closer to itself than to the hydrogens.

I see. And that means the oxygen will be slightly negative because it hogs the electrons, even though they are shared.

That's right. The hydrogens, both of them, will have a small positive charge because their electron is not covering the proton very well. This leaves the positive part of the hydrogen (the proton) slightly exposed. This slightly exposed hydrogen atom, with its slightly exposed positive charge, can exert an electrostatic force on other atoms.

You mean the slightly positive hydrogens can attract a negatively charged atom? Slightly?

Yes. That is exactly what I mean. Electrostatics can work on any molecule with a charge, even a partial charge like these polar molecules. So other atoms, if they have any charge to them at all (even partial charges of their own) can interact. Got it?

Sure do. Water is a polar molecule. It has a slightly negatively charged oxygen atom and two slightly positively charged hydrogen atoms. But only slightly. Neither the oxygen nor the hydrogens are ions. Right?

That's right. They are like "almost ions". Sometimes this unfair sharing goes too far and the molecule ionizes. But let's not get into that now. Just remember that ALL water molecules have these partial charges that create polarized bonds and that's what makes water a polar molecule.

You know, water is such a simple molecule but it has so many kinds of bonds going on.

Yes, it does. But if you learn all of the bonds in water, you will not only have mastered water, but also almost every other molecule.
The weak bonds that water forms, allow water to exert an electrostatic force on other molecules, even though it isn't quite an ion. This weak electrostatic force gives water many of its most important properties. Including its ability to behave as a "universal solvent".

A what?!

"Universal solvent" means (almost) any ionic molecule will dissolve in it.
Take NaCl as an example. Recall that when you add salt to water it dissolves. The reason it does that is because the water molecules help pull the ions apart and keep them apart. All because water can create electrostatic effects of its own.

Let me get this straight.
The oxygen in water pulls hydrogen's electrons close, causing the oxygen to become slightly negative and the hydrogens slightly positive. That's the polarized bonds of water.
Sodium's cation (Na+) is attracted to water's slightly negative oxygen. Chlorine's anion (Cl-) is attracted to water's slightly positive hydrogens. That's what pulls the salt apart?

Yes, indeed! In fact, what happens is that a whole bunch of water molecules surround each of salt's ions. The Na+ is trapped by a sphere of water molecules, all pointing their oxygen atoms at it, with the waters' hydrogen's facing away. That forms a shield around the cation, keeping the anions away. We call that a "solvation shield" because it is a shield that promotes "solvation", a fancy word meaning the ability to dissolve.

I see. Water's solvation shield forms a sphere around the ion, keeping its opposite ion from reaching it. So the Cl- can't get to the Na+.

Yes. As a matter of fact, the solvation shield not only blocks the ions from meeting. It also hides their charges. So, not only are the two ions prevented from getting together, they don't even "feel" their opposite ion hidden inside the solvation sphere.
Any approaching Cl- is more likely to find and be attracted to the partial positive charge of the hydrogens, than to the hidden Na+ ion at the center.

What about the other way around? What happens to the Cl- ion?

Good question.
A similar solvation shield is formed around the Cl- ion, but with the water facing the other way, because the charges are reversed! The chlorine is solvated just like the sodium was, except everything is reversed, because the charges are reversed.

I get it. But when we remove the water, by allowing it to evaporate, the shields disappear and the ions can get together.

Yes! That is exactly right. Indeed, you can imagine it the other way around.
If you add too much salt to a glass of water, there will be too many salt ions to shield. If you keep adding salt, eventually it will stop dissolving, because all the water molecules are used up as shields. Then salt ions can stay together and the salt just drifts to the bottom of the glass. We say it is saturated (with salt) when we cannot dissolve any more into it. And that extra salt "precipitates" to the bottom. A precipitate is the stuff that comes out of the solution when you add to much salt (or other stuff) or take away too much water.

I'm impressed. Water's "solvation trick" is great! And this is all caused by polar bonds?

That's right.
"Hydrogen bonds" are a special kind of polar bond. They are the electrostatic interaction occurring in molecules that have a hydrogen bonded to an electronegative atom (oxygen, chlorine, fluorine, even nitrogen a bit).
Although we call them hydrogen bonds, and (indeed) hydrogen is the "link", it is caused by the electron-withdrawing properties of the electronegative atom covalently bonded to that hydrogen.

So hydrogen by itself can't make hydrogen bonds. It has to be bonded to an electronegative atom.

Precisely. Even molecular (diatomic) hydrogen can't make hydrogen bonds....

...because it isn't even a polar molecule!

By Jove, you're good at this!

So these hydrogen bonds are just partial ionic bonds. They are special polar bonds that allow water to use electrostatic forces to dissolve salts.

Yes. And hydrogen bonds do other things too! They are important for more than just dissolving salts. Hydrogen bonds can also attract other atoms with partial charges of their own, including other water molecules. Or even complex molecules that have partial charges. They take part in the structure and function of many chemicals important to Life. Water is just one of the many molecules that uses hydrogen bonds.

I can see why you've chosen water as our second topic. It's a simple molecule but it does so many things in so many ways. Water is a covalent molecule, yet it has a few ionic (electrovalent) properties and some partial ionic (polarized bond) properties.

Yes indeed. You see, water is a versatile molecule. And it nicely illustrates that bonding is often a continuous range (a continuum) of bonds.

Any other weak bonds?

Yes. Let me tell you about a bond described by a Dutch physicist in the 19th century. His name was Johannes D. van der Waals.

Funny name.

Well, we can't all be called Arthur or Merlin.
Anyway the van der Waals force or van der Waals bond can be made when two molecules come very close together, causing their electron shells to push against each other. The electrons of one molecule try to repel the electrons of the other. This repulsion pushes the electrons slightly away from the nucleus of each atom, causing a very, very slight positive charge from the nucleus to be exposed.

Very, very slight, is it?

Yes, very. And what do you think the electrons of the "invading" atom do when they see a very, very slight positive charge on the other atom?

They get very, very slightly attracted to it. Because of very, very, very slight electrostatic forces.

Yes, that is correct.

By this "very, very, slight" stuff, are you trying to tell me these van der Waals forces are weak?

Yes. Van der Waals forces are the weakest of bonds. As a matter of fact, some Alchemists don't even believe them to be a bond at all! But regardless, van der Waals forces definitely affect molecules and their bonding. Weakly. And it only effects molecules when they are very, very close to each other. The two interacting molecules must fit together, like a key in a lock.

OK. Van der Waals forces are the weakest and require the molecules to fit well together. I suppose that is so lots of atoms can take part.

Right. Many of these weak forces work best when there are lots of them working together.
The van der Waals force can hold small molecules together only if those small molecules are not wiggling very much.

I get it. It's easy if you just think about how these electrostatic forces work. Now that I know that, I've mastered all the bonds!

Yes, you have. Any questions? Anything about water?

Yeah. Why doesn't water mix with oil?

Good question! It's because water, being a polar molecule, has a slight charge. But oil is not a polar molecule. (You may not have known that until now.) So oil has no charge at all. Remember, not all molecules are polar. Oil is apolar.

So?

So all the water molecules are attracted to each other. As they clump together, these water molecules exclude the oil molecules. Oil is not attracted to water, because oil doesn't have a charge. Not even a partial charge. Oil is not polar. It's apolar. Oil has no polarized bonds (because it is made of atoms with low electronegativity, so it has no chance to become polarized).

But water is a polar molecule, so the water molecules are attracted to each other's partial charges on the oxygen (delta minus) and hydrogens (delta plus).

Right. That leaves the oil excluded from the water. Fortunately, oil is less dense than water so it floats to the top. The difference in density helps the separation. But it is the underlying differences in polarization between oil and water that causes them to never mix.

I see. The "density thing" just helps.

Yes. I'm glad you mentioned this "oil and water separation thing". It is a very important part of Alchemy.
For example, many salts (like NaCl) will dissolve in water, but not in oil. That's because water is a polar molecule and is able to dissolve many ionic compounds very well. But oil cannot form a solvation shield around salt or its ions. So salt will dissolve in water but not in oil. We say salt is "hydrophilic" (pronounced "hi-drow-fill-ick") meaning......

... it loves water! "Hydro" means "water" and "philic" means "love". I took Greek you know.

I know. I know. So tell me "Mister smart-Greek-speaker", what would you call oil, or any other molecule that does not "love water"?

I'd call it "hydrophobic"(pronounced "hi-drow-fobe-ick"). That means it is "afraid of water".

Correct. You obviously learned your Greek well. And now you've found a use for it. But even if you know nothing about Greek, you could easily remember that all chemicals can be classified as either hydrophilic (they like water) or hydrophobic (they fear water).

Is this hydrophobic or hydrophilic difference caused by a bond?

No, not really. I suppose it is a force. We could say that a hydrophilic force attracts the water molecules (and salts) to each other, but a hydrophobic force attracts the oil molecules together.

Hmmmm.
Now wait a minute. This hydrophobic force, this "water fearing force", isn't real! It's just a lack of hydrophilic character, a lack of polarized bonds, which produces this hydrophobic "force".

You are absolutely correct.
And now I think you can see why the weak forces are often hard to describe as proper bonds.

Yeah, they are not proper bonds at all.

That's right.
The strong bonds link up the atoms to make molecules. They are responsible for the chemical properties of a substance.
The weak bonds are hardly bonds at all. However, the weak bonds are very important because they affect so many of the physical properties of chemicals.

What do you mean by "chemical properties" and "physical properties"?

Chemical properties describe the ability of one substance to change into another completely new substance.

So Alchemy is really about the strong bonds, not the weak forces. Right?

Not quite. Both strong bonds and weak forces are important in Alchemy.
Strong bonds are responsible for linking ATOMS together into molecules, so it is easy to think that all of Alchemy has to do with strong bonds. But that is only half the story.
The weak forces link the MOLECULES together and it is the linking together of these molecules that give substances most of their physical properties.

What do you mean by "physical properties"?

Physical properties describe a substance "as it is". Like its hardness, color, even smell!

Oh, I see. Then these physical properties are very important things to know. And they are caused by the weak forces?

Yes, for the most part.
Atoms, of course, are too small to see and therefore do not have any properties we can easily measure. The same is true of most molecules. And those molecules are simply atoms linked together by strong bonds. Their chemical properties change when they undergo chemical reactions, which change the molecule.
But the physical properties of a substance may change due to changes in the interaction BETWEEN the molecules. That interaction is, for the most part, due to the interactions caused by the weak forces.

Why do you keep saying "for the most part"?

Well, the distinction between strong bonds and weak forces does not always parallel the distinction between chemical and physical properties. Strong bonds can affect physical properties too. Can you think of a physical property caused by strong bonds?

Yeah, the "metallic" properties of metals. Things like iron and gold are malleable and that would be a physical property. Right?

Absolutely right.
Malleability is due to the way metal bonds work, and metal bonds are strong bonds. At least that is how we have defined them.
But bear in mind Arthur, there is no distinct cut off between strong and weak. There is some overlap. The physical properties caused by metal bonds are a good example.

But it all has to do with how the atoms or molecules interact. Right?

Yes.
It is easy to get confused because the universe is not as nice and neat as we would like it to be. Let me summarize the bonds and their strengths.
The weakest of the weak forces is the van der Waals forces.

So molecules held to other molecules by van der Waals forces are the easiest to separate.

Right. Materials made of molecules that are attached to each other only by van der Waals forces are so delicate they fall apart almost as fast as they are made. But they are very important forces at the molecular level. Van der Waals forces control many of the complex and delicate Alchemy that occurs in Life itself!

What about the other two weak forces, hydrogen bonds and hydrophilic interactions?

Both of them are about ten times stronger than van der Waals forces and, because of that, you can actually see their effect on some of the more delicate materials.

Like what?

Like raindrops.
All drops of water are held together by hydrogen bonds and hydrophilic forces. These forces are weak and easy to break but they shape every rain drop you see. They are also very important in the Alchemy of Life.

What are the next strongest bonds?

Next come covalent and ionic bonds. They are ten times stronger than hydrogen bonds or hydrophobic forces.

So covalent and ionic bonds are a hundred times stronger than van der Waals forces.

That's right. There is no doubt that strong bonds are the foundation upon which molecules are built. They hold atoms to each other very tightly. A salt crystal (NaCl) is held together by ionic bonds. Each crystal of salt is really a group of cations and ions held together by lots of ionic (electrovalent) bonds. So a crystal of salt is harder than a drop of water.

Is there a way to explain this by example?

Maybe. You could easily split a drop of water with a hair. Right?

Yeah, I suppose a drop of water that fell across a hair would be split in half by the hair.

That's right. But a grain of salt would bounce off the hair. It can't be so easily split by the hair because its molecules (of NaCl) could not be broken by the molecules that make the hair.

But the rain drop would be split because only weak forces hold water drops together. I get it!

Good. Now get this. What holds hair together?

I don't know. You tell me.

No, you tell me. Are the molecules in the hair held together by weak forces (hydrogen bonds, hydrophobic/hydrophilic or van der Waals forces)?

Ah... , gee. No! They couldn't be held by any of the weak forces.
If hair molecules were held together by weak forces then a drop of water would bounce off it or it might even break the hair. This doesn't happen so hair molecules must be held together by something stronger than the weak forces. Strong forces.

That's right. Hair is held together by covalent bonds - strong bonds. Hair is made of protein, and proteins are large molecules of hydrogen, carbon, oxygen and nitrogen atoms covalently linked to each other. Hair also has lots of sulfur atoms covalently linking it. And these covalently attached sulfur atoms will cross-link to other sulfur atoms in other proteins using covalent bonds.

So each hair is a large net of protein molecules, cross-linked by sulfur covalent bonds.

Yes. Hair is strong because of those strong covalent bonds. The same bonds that hold each atom to the other in the hair molecule, also hold the hair molecules to each other.

Makes it hard to keep track of where the molecule ends. Doesn't it?

Aye, but it doesn't matter.
So now you have a pretty good understanding of the forces and how they interact to produce matter of various types.

Hey, what about metal bonds?

Oh yes, metal bonds. Well, metal bonds are a sort of strong bond, but not as strong as covalent or ionic. Metal bonds are kind of in between.

Probably because they have that strange super-sharing of electrons.

That's right.
But let's get back to the weak forces. Van der Waals forces, hydrophobic and hydrophilic interactions, and hydrogen bonds are all weak. They don't link together the atoms of a molecule. These weak forces are involved only in making the molecules interact with each other.

And it's this interaction of molecules which causes all the physical properties?

MOST of the physical properties, not all.
For example, the specific positioning of the covalent bonds between carbon atoms in a diamond cause it to be very hard. The diamond's covalent bonds have a favorable geometry that makes them hard to twist or buckle. So diamonds are hard because of the covalent bonds.

That's a physical property worth understanding.

Yes, it is, but it would take a lot of talk about crystals and their structure to appreciate the full explanation. That's best left for an Advanced Alchemy course.

But I want to know how molecules interact to cause all these important physical properties!

I appreciate your enthusiasm to learn, but you must walk before you can run. So today, I'd like to teach you how weak forces contribute to the most important of physical properties - the states of matter.

What are the states of matter?

Actually, you already know three of them; solid, liquid and gas.

Oh, solid, liquid and gas are the three states of matter. I see. And there are only three states of matter because that's all something can be; either a solid, a liquid or a gas. Right?

Ah, sort of right.

What?! What else is there for matter to be but solid, liquid or gas?

Calm down. Calm down. Don't get your knickers in a twist.
Actually, you've made the same mistake most Alchemists have made for centuries. We assumed that there were only three states of matter because that was all we were familiar with. But there is another form of matter called plasma.

Plasma? What's that?

In the 20th century, Alchemists learned to use very high temperatures to strip the outer electrons away from gases. What's left is a plasma.

Plasma is just ionized gas?

Aye, it is. It would be fair to say that plasma is an unusual state of matter. It is rare, but it can be found wherever there is a huge amount of energy released - enough to strip away electrons. The outer layer of the sun is a plasma. All the atoms in the sun's outer layer (mostly hydrogen) have been stripped of electrons, so they are a plasma.

I'm not likely to do much Alchemy work on the surface of the sun. Is plasma found on earth? Naturally? Not just in some 20th century lab?

Well, yes. Lightning releases a huge amount of energy and strips away electrons. So lightning creates plasma from the air. But, like most plasma on earth, they don't last long. The cations quickly gather up electrons and become normal atoms again. Sometimes they will gather electrons in such a way as to share them, making new covalent bonds and new molecules.

But plasma is rare. Like lightning.

Yes, plasma is rare in our time. However in the future...

OK. This state of matter called plasma doesn't seem to have much to do with chemistry.

Well, it is a very unusual state of matter and Alchemists, even in the 20th century, spend most of their time working with the other three states of matter.

Let's stick with those three then.

OK, but be aware that plasma is a fourth state of matter. Plasmas have unique properties and behaviors of their own and their own particular way of glowing! And they can be moved around by magnetic and electric fields. It's quite magical, really!

Yeah, but we don't want to turn this into a physics or technology class so you won't go into it any further.

Correct!
Now then, the state of matter (even plasma) depends upon the temperature of the matter. A substance may change its physical state as temperatures change, but it still keeps its chemical identity.

How do you mean?

Well, take water. Its different forms are fine examples of the states of matter.

Water really is your favorite molecule, isn't it?

Sure is. Water changes its state as the temperature changes. You have ice at cold temperatures, water at "comfortable" temperatures and steam at high temperatures. All three of those states are easy to demonstrate with water.

And you can change states by changing temperatures.

Aye, that you can. So changes in temperature can cause molecules of water to interact with each other in ways that cause them to form a solid, a liquid or a gas. What do you think causes that?

I don't know. All you change is the temperature.

And what is temperature?

You know. Hot, cold. That's temperature.

NO, that's not.

Yes, it is! A hot cup of tea feels hot and a cold glass of milk feels cold. That's temperature!

No. That's just how we describe it (when we aren't using numbers).
We, as animals, sense various things and one of them is the ENERGY of our general surroundings. If the air is full of energy we describe it as "hot" weather and if the air has little energy we say it is "cold" weather.

So if I'm enjoying a nice, warm, sunny day, and a cold breeze comes along, I sense the change in the air's energy?

That's right. In your example, you would feel the air loose energy as it cooled. That's a change in temperature.

But you are not talking about the energy of the wind, are you?

No, I'm not. Wind is just the movement of the molecules themselves as the cold air sweeps over you. I'm talking about the energy in the air molecules themselves. Cold air has molecules with less energy than warm air.

What does this energy do?

Good question. Energy causes the molecules to wiggle. The more energy, the more they wiggle, and it is the amount of wiggling that determines the state of matter.

How's that?

Well, solid materials have very little wiggle in them. They wiggle in place and barely wiggle at all!
Liquids have enough wiggle in them to keep the molecules interacting, but they can still slip past each other and change places with neighboring molecules.
Gases have so much energy in them they not only wiggle, they go flying off and away from any other molecules they bump into.

So ice, water and steam are all just molecules of H2O with different amounts of wiggle.

Yes. They are in various states of wiggle because of the energy in them.
Now tell me, Arthur, how do you make steam?

Well, I just put a pot of water on the fire and the steam rises off the water.

Yes. What happens to the water's energy?

Oh, I see what you're getting at.
The fire supplies energy to the liquid water and causes it to wiggle so much that the molecules go flying off.

Yes, that's exactly right. The energy in steam molecules is enough to overcome any tendency for two neighboring molecules to stick to each other by the weak forces.

I see. So steam has too much energy in it for water molecules to stick to each other by van der Waals forces, or even hydrogen bonds!

That's right. In a gas the energy is too great. But as the steam cools it looses its energy and those weak forces come into play. At some special temperature the amount of energy has come down low enough to allow the molecules to interact with each other through weak forces.

So, in the liquid state the weak forces of a molecule allow it to interact with other molecules. But liquid molecules have enough energy to keep them moving and wiggling

Yes! And that is why they behave as a fluid. A fluid is something that flows. Liquid molecules will flow over each other and eventually fill the bottom of a container, even taking the container's shape.
Gas won't just sit on the bottom of a container. Steam, or any gas, will fill ALL of the container. A gas must be contained completely because it will escape through a hole.

I guess a liquid like water can only escape through a hole if the hole is in the bottom of the cup.

Yes. (On Earth.)
You can pour a liquid from one container to another and it will take on the shape of the new container. The liquid molecules stick or "cohere" (as we Alchemists like to say) to each other through their weak forces. Liquids have a definite volume but can take any shape.

Meaning you can pour a pint of milk into any pint bottle and it will fit, no matter what the shape of the bottle.

Exactly.
The molecules in a liquid state constantly make and break their weak bonds, so they constantly change neighboring molecules. They wiggle, make a weak bond, break a weak bond, and wiggle some more. Make a weak bond, break it, and wiggle a bit to another molecule. Make a weak bond....
Over and over again.

I see. But molecules in a gas state....

Ah, Arthur. We say a "gaseous" state not a "gas state". Texas is a gas state.

Perhaps Texas should watch what she eats!
OK. Molecules in the "gaseous" state move apart from each other and fill all the spaces they can find. They don't "cohere" to each other because their weak bonds can't hold those energetic, wiggling molecules together for even a brief moment of bond making.

Yes, and the amount of wiggling depends upon the temperature. Remember?

Oh, yeah.
The hotter it is the more energy there is in the molecules.
The more energy there is the more each molecule wiggles.
The more the molecules wiggle the harder it is for weak bonds to form between them.
So if you keep heating something up it will eventually turn into a gas!

That's right! Eventually you will have put so much energy into the material, its molecules will no longer cohere and the liquid will disappear into a gas.

Any liquids? Will any liquid behave that way?

Well, any liquid made of molecules that will survive those temperatures without their strong bonds breaking.
So summarize how temperature affects the states of gas. Use H2O as an example.

Steam is hot. Steam molecules have more wiggle energy than bonding energy so they do not cohere at all. Steam is a gas because the water molecules have too much energy to hold on to each other through their weak bonds. And steam behaves like a gas, filling all of a container and escaping through any hole it finds (such as the spout of a teapot).

Yes, that's right. Now what about liquids? What about liquid water?

The colder it is the less energy there is and the less the water molecules wiggle.
Liquid water molecules have about the same amount of wiggle energy as bonding energy, so they cohere to each other, but only briefly. That's why liquid water behaves as a liquid, taking the shape of the container.

Right as rain.
Now, tell me. What happens if you cool the liquid water further.

It freezes and turns to ice.

Why? Tell me what you think happens with the water molecules as they turn from a liquid state to a solid state of ice.

Simple.
The water molecules cool down so much that they stop wiggling. And then they can make permanent bonds even though they are only weak ones.

Almost right. You are correct to say that they cool down enough to form permanent, weak bonds. But the molecules do not stop wiggling. Molecules in all materials wiggle. Even solids. The only way to stop a molecule (or atom) from wiggling is to freeze it to the lowest temperatures in the universe. It doesn't get that cold here in this forest!

But if they are still wiggling, how can they form bonds?

We'll they don't wiggle violently. Molecules in solid substances wiggle just a tiny bit. A slight vibration, actually. That vibration is not enough to tear weak bonds, just strain them a bit. As a liquid cools to a solid state, the molecules start to take up definite positions with respect to each other and form permanent weak bonds.

So ice molecules still wiggle, but not very much. Not enough to break weak bonds.

Yes. In the case of water, the molecules start to line up their oxygens and hydrogens, forming hydrogen bonds between each molecule. Because of the shape of water molecules (a subject we will study later), ice water molecules form a regular, flat six sided crystal.

Notice how these water molecules are arranged. The oxygen atom (the fatter circle in each molecule) is hydrogen bonded to the hydrogen of another water molecule. And so on and so on. That causes the water molecules to set up into pretty patterns with six sides.

I've never seen any six sided crystals in a chunk of ice.

That's because neighboring ice molecules push against each other and break up the crystals before they grow big enough to see. To make big six sided crystals of ice you have to freeze the water very gently and give it plenty of room to grow so it wouldn't bump into neighbors.

Where does that occur?

In snow clouds water cools to its freezing point. That's the temperature at which molecules of water are able to form hydrogen bonds between each other. As they link up their hydrogen bonds, tiny six sided crystals are formed and they fall to earth.

I get it. Snowflakes are six sided because the water molecules form their hydrogen bonds in a pattern of six at a time. A hexagon!

Precisely.
(In fact, most raindrops start out as snowflakes, high up in the clouds where it is very cold. As they fall down they pass into warmer layers of air. By the time they reach the ground they have melted. Only when the air temperature stays cold all the way to the ground do we get snow, instead of rain.)

So the three states of gas, liquid and solid depend upon whether the molecules at that particular temperature have enough energy to make them wiggle a lot, a little or hardly at all.

Yes. Well said. At least in terms of the molecules' wiggle. But how would you explain the three states in terms of their bonds.

Gas forms no weak bonds because it wiggles too much.
Liquids form temporary weak bonds but eventually the molecules' wiggle causes them to break. So the bonds in liquids aren't permanent, just temporary.
And solids have permanent weak bonds between the molecules because these molecules don't wiggle so violently as to break them up.

Right.
Now is a good time to point out that each kind of molecule has its own temperature at which it forms either a solid, liquid or gas. (Actually, pressure is important too, but we will leave that complication to an Advanced Alchemy course.)
The molecule water has its own physical properties that are derived (that means "comes from") the molecule itself. The atoms and their arrangement in the molecule determine how the molecule's weak forces will interact with other molecules.

So what? I don't see what you are getting at.

OK.
In the winter it gets cold enough to freeze water (H2O) but not air, which is made of molecules of O2 and N2. They are different molecules, and each kind of molecule has its own properties that determine how well it forms weak bonds. Water is liquid at comfortable temperatures (like in this room). But air is still a gas. O2 and N2 will become a liquid if you cool them enough, and a solid if you cool them further.

But you can't freeze air or even make it turn into a liquid!

Oh yes, you can! It isn't easy but you can make liquid air and even solid air! You just have to go to very, very cold temperatures to do it. But those low temperatures don't happen here in the forests of England, or anywhere on Earth in the 5th century. However, in the 20th century they have machines capable of cooling the air and forcing it to "rain" liquid oxygen (O2) and liquid nitrogen(N2).

It must be very, very cold to do that.

Yes, it must be.
Tell me what you think would happen to molecules of air (either O2 or N2) as they got very, very cold.

Well, they would wiggle less and less until they wiggled so little that they could form weak bonds between themselves.
But, wait a minute. O2 is just two oxygen atoms and N2 is just two nitrogen atoms. They can't even make hydrogen bonds.

That's right. So what weak forces must they rely upon in order to form bonds between molecules of O2 or N2.

Ah, van der Waals forces. All they can do is bond by van der Waals forces.

Yes, that's right. And how do van der Waals forces compare to hydrogen bonds in terms of strength?

Oh, they are much weaker. Van der Waals forces are about one tenth (1/10) as strong as hydrogen bonds.
Oh, now I see. These air molecules must wiggle very little if they depend on the very weak van der Waals force to hold them to each other. That's why you need very, very cold temperatures to cause O2 or N2 to become a liquid. Or a solid!

That's absolutely right! And you have shown a good understanding of the nature of these "inter-molecular" bonds (bonds between molecules).
Imagine you have a mixture of very hot air containing steam (O2, N2 and H2O), all as gases. As you lower the temperature of this air, the water will be the first to turn to a liquid and then to a solid. All because the water molecules can use the strongest of weak bonds - their hydrogen bonds.

Yeah, but the air molecules can't form hydrogen bonds. They have to wait until the temperature drops much further before they can use their weak van der Walls forces to cohere to each other. I got it!

You got it.
In the winter it gets cold enough to cause the water on the pond to freeze...

... but it never gets cold enough to cause the air molecules of O2 and N2 to slow down their wiggling enough to form even temporary bonds using the weakest of the weak bonds, van der Waals forces.

Excellent.
Even on the coldest winter night, the air molecules have too much wiggle to interact with each other. They stay as a gas, at least on this planet.

This is the only planet I intend to live on.

Yes, I suppose you are limited at this time. But in the 21st century...

Yeah, yeah. Anyway, getting back to Earth, is water the only molecule which has all three states at easy to reach temperatures?

Well, there are a few exotic compounds that can be solid, liquid or gas at "comfortable" temperatures. But there's no need to go into them. Water is a fine molecule to use as an example of the three states of matter.

If you heat it enough, will water become a plasma? Will it ionize?

I thought you didn't want to talk about "unearthly" temperatures.

Well, just suppose you heated steam up even further. As hot as the sun. I bet it would lose an electron and become a plasma.

You're thinking is very good and I understand why you would think that but, actually, if you heat steam up too much, the water molecules fall apart. They break up into hydrogen and oxygen because the heat is so great it breaks the covalent bonds that hold them together. That's true of most plasma temperatures. Most plasmas are ionized atoms, not ionized molecules. Plasma temperatures are too much for most covalent bonds!

I see. But those atoms of hydrogen and oxygen would eventually become a plasma as you raised the temperature.

Yes. But that would not be the water molecule in the state of a plasma. That would be the atoms (hydrogen and oxygen) as plasma.

I see. So it is possible to break up molecules before they reach all the possible states.

Yes. Especially plasma temperatures. They are very hot!
But most Alchemy occurs at relatively cool temperatures. Plasma temperatures are really the realm of a 20th century physicist.

OK. I can live with that.
Is this understanding of water's different states and the temperatures to make them, of any real use?

Oh, yes! Alchemists use their understanding of the states of water and the states of other molecules dissolved in water, to do a great deal of Alchemy. Water is central to most Alchemy. Its unusual properties make it a very useful molecule for Alchemists. So we Alchemists spend a great deal of time learning about water and its properties.
Any questions about water and other molecules?

Yeah, how come water left out for some time will disappear?

You mean it evaporates.

Yeah. It doesn't have to be boiling hot out, so it can't be turning to steam. So where does water go? How does evaporation work?

Oh yes, evaporation. It wouldn't be a complete lesson about water if we ignored evaporation and precipitation.

So, how does water evaporate at room temperatures? It isn't hot enough to boil away as steam.

That's right. Evaporation is not boiling. Some folks get them confused. But steam is a gaseous state.

So what is evaporation?

Evaporation is a "vapor state".

Huh?

Lets start with a plate of water left outside. The temperature of the water is far too low to boil away. But the water can disappear as a vapor. The water escapes like a gas but it does so without boiling. The air just carries the water molecules away. The molecule behaves as if it were in a gaseous state, but it's below the temperatures needed to make it boil. This is called a vapor. All solids and liquids give off vapors. Molecules or atoms that have evaporated from the "condensed" form are called vapors. When you smell something, you are sensing its vapors.

That doesn't make sense. If it isn't hot enough to boil, then the water molecules will just stick together by their hydrogen bonds. They will be a liquid.

That is true of MOST of the water molecules in general. But some of the water molecules, at the surface of the water, will wiggle enough to leap away complete from their neighbors and fly off into the air. They don't have a chance to bond to another water molecule.

So these few wiggly water molecules at the surface just "lose touch" with their neighbors? And get carried away by the air before they can rebond back to the other water molecules?

Yes, that's right. It's a statistical thing really, and has to do with the way the energy in a liquid (or anything) is distributed.
You see, when we think of temperatures and wiggling molecules we often think about their average temperature and therefore the average amount of wiggling. But in fact some water molecules, among the billions at the surface of a cup of water, will be so full of energy they wiggle free from their neighbors. Never to return.

So the water molecules in a cup of water may wiggle at different rates. Some will wiggle more than others...

... and some will wiggle less than others. So, as an average, we simplify our picture of them and think of them as wiggling at the same rate at a particular temperature. But in fact, some are wiggling more and some less than the average.

I think I see. This all has to do with the large numbers of molecules and what you call statistics.

Yes. Statistics is a form of mathematics that Advanced Alchemists find very useful. But we need not learn those details now.
Oh, another thing. Water vapor has to do with the air too. Imagine there is a lot of water vapor already in the air. What do you think might happen to those water molecules evaporating from the surface of a cup of water?

Hmm. Well, if there is enough water as vapor, floating in the air, maybe two water molecules will meet each other and form hydrogen bonds to each other.

That's right. Air can carry only so much water vapor. We say it becomes saturated with water. At that point the water molecules in the air start to meet each other and hydrogen bond together. If the temperature isn't too high (so the molecules don't wiggle too much), hydrogen bonds will form between them. Tiny water droplets will be created and start to fall out of the air. That's called precipitation.

Rain! Rain is precipitated water vapor.

Exactly. (But rain can also be made from snowflakes.) Let's review the water cycle. It is very important.
We say that water evaporates into the air as water vapor. This is not boiling because it is at (average) temperatures far below that needed to get all the water molecules to break away.

But some of the more energetic molecules leap up from the water surface and are carried away in the air. That's evaporation.

Yes. Eventually the air is saturated with water molecules. The air just can't hold anymore without the water molecules meeting up. We say the air is saturated with water vapor. Another way of saying the same thing is to say the air has 100% humidity.

Humidity?

Yes. Humidity is a measure of how much water vapor is in the air. When the humidity reaches 100% the air is saturated with water.

I see. And this saturated air cannot hold any more water vapor molecules without them meeting and bonding to each other as a liquid. So the water molecules form hydrogen bonds to each other and precipitate into drops. It rains!

Yes, it does. And dust helps.

Dust?!

Yes. The water molecules stick to dust so the dust helps form raindrops. Dust in the air acts as a meeting place for water molecules.
If the water droplets precipitate onto bits of dust in the air, we get clouds and fog and eventually rain. However, if there is no dust to help form drops the water precipitates on anything it can find, like blades of grass.

The morning dew!

Exactly. Morning dew is caused by precipitation of the water molecules from the saturated air that is (relatively) clean of dust.

And all this can be explained by the weak forces acting on the molecules, their temperatures and their wiggle caused by that temperature. (Or simply, energy.)

Right. When you heat up a pot of water you increase the number of water molecules able to evaporate, because you give them more energy. So more vapor is given off by a hot pot of water.

Hey, that means a hot cup of tea will evaporate faster than a cold cup of tea.

That's right.

What about the steam I can see rising off a hot cup of tea? Is it really steam?

What do you think?

It can't really be steam! Otherwise the tea would be boiling. I like my tea hot but not boiling!

Right! That's excellent thinking.
Many people see the vapors rising from a hot cup of tea and call it steam. But that's not steam, it's vapor.

Hmm. I see.
A steaming hot cup of tea isn't really steaming, it's "vaporing"!

Well, yes. But it can still be very hot.

How about the "steam" that comes out of my mouth on a cold day. I bet that's not steam at all.

You're right. That's the vapor of your breath.

Why do I only see it on a cold day?

Good question.
Your breath is very warm - body temperature. But the air around you is much colder. That temperature difference is very important. You see, your breath inside you is not at 100% humidity. (It's close to 100% but not quite.) When you breathe out, you mix your moist hot air with the colder air...

And it precipitates! It makes a tiny cloud!

Exactly! The difference in temperature causes the "cloud" to form.
"Humidity" is a funny thing. It has to do with both the amount of water vapor AND the temperature.
The water molecules in your warm lungs have lots of energy, but when they cool outside your lungs they can make their hydrogen bonds. Inside your lungs they are slightly less than 100% humidity, but outside your lungs they are more!

I get it. Inside my lungs it is so warm that the water molecules wiggle too much to form hydrogen bonds. They stay as vapor. But when they get outside my body they cool down enough for the hydrogen bonds to form between them and they make tiny clouds.

Yes, that's right. The same is true for the vapor off a hot cup of tea. What you see as a "vapor" is really the cloud made as the vapor condenses and precipitates. The tiny cloud is carried up and away by the left over heat.

Hey.
You know, if you look carefully at a pot of water as it heats up, it tends to make "vapor clouds" before it boils. Doesn't it?

Yes, it does. The heat makes lots of vapor escape from the pot but it quickly makes clouds.
As you continue to heat it, eventually ALL the water molecules have enough energy, enough wiggle, to pull away from all their neighbors and fly away as steam. It boils.

Water is very strange.

Yes, it is. To fully understand things like evaporation, precipitation and vapors, you need to learn more math and take the air pressure into account. It gets too complex for a course like this one. It takes up a great deal of the science of meteorology.

Meteorology? What's that?

Oh, it's a branch of Alchemy that studies, and tries to predict the behavior of, the Ancient Element AIR. Maybe some other day I will teach that to you.

OK. I think I've learned enough about water today. I can see why you think so much about it. Water has some very complex behaviors.

Yes. And we have only touched on the most obvious ones. Water is a wonderful molecule to learn about the states of matter and their inter-molecular interactions through their weak bonds.

Yeah, and not only does water have all three states of matter at convenient temperatures, but water also has every imaginable kind of bond and force. Except metallic bonds.

That's right. Water is a great molecule.

I think we've covered a great deal about molecules but we haven't talked about the shape of molecules yet.

Don't orbitals and their shapes affect the way the atoms bond and the shape of the molecules they make with their bonds?

They sure do. In the last Ancient Element, AIR, we talked only about atom shapes. Now let's move on to molecules. In molecules, the outer (valence) electrons are controlled by more than one atom. For most of this lesson we will be talking about covalent molecules because they make the most complex shapes. In covalent molecules the atomic orbitals overlap to form molecular orbitals.

Do molecular orbitals control the shape of covalent molecules?

Aye, they do! All of the electrons in the outer valence shell are involved in making the bond but it is the subshells (orbitals) that determine the direction of the bonding and (thus) the shape of molecules.

What about Lewis structures? Are they worthless? Do they have anything to do with what we are talking about?

Lewis structures are a rough estimate of the bonding. We can build upon the idea of Lewis structures in order to figure out how their bonds are positioned. The eight electrons used to draw Lewis structures are all from the same (outermost) shell - 2 from the s orbital and 6 from the p orbitals. Many of the covalent bonds formed between atoms are produced by molecular orbitals that are a hybrid of the s and p orbitals.

What do you mean? What's a hybrid?

A hybrid is a mix or combination of two things. Hybrids have some properties of the two things from which they are made. But, because they are a MIX of two different properties, hybrids have their own properties that make them an "in between" thing.

I think I understand. But then maybe I don't.

OK. Let's start with molecules that do NOT have hybrid molecular orbitals. Once you understand them, you will be ready to understand hybrid orbitals. OK?

OK. What molecule doesn't have a hybrid molecular orbital holding it together?

H2. The two atoms in a molecule of hydrogen are identical and very easy to understand.

I thought the two atoms in the molecule of hydrogen just share the two electrons between them.

You're right. That's all there is to know about the H2 molecule. It is so simple that it is hardly worth mentioning.

So why mention it?

To remind you. Tell me what kind of a shape would you expect of H2?

Ah, it's just a straight line, isn't it? Two points form a line.

Right! That's all it could be. Here's what the molecule would look like. I've included the electron cloud from both atoms. See how they overlap? Together the two atoms make a line. H2 is so simple that a line is the only shape it could possibly be.

Hey, wait a minute! ANY two atoms held together can only be a straight line! I can't imagine it any other way.

Right! All diatomic molecules are linear, meaning they form a line. There is no other way to connect them.

How dull!

Well, just think about it. Most of the air you breathe is O2 and N2. Both have a linear shape.

Easy. Let's move on to something hard (and more interesting).

OK. The shape and direction of ALL molecular orbitals, including hybrid orbitals and even the "dull" linear ones, can be explained by VSEPR theory.

What theory?

VSEPR. It stands for Valence Shell Electron Pair Repulsion. It is based upon the fact that pairs of electrons, the pairs in each orbital, arrange themselves to be as far away as possible from other pairs (other orbitals).

Oh, this has to do with electrostatic repulsion. Doesn't it?

Aye, it does. That's what the EPR part of VSEPR is all about. Electron Pair Repulsion (EPR) is the electrostatic force that causes electron pairs to repel other pairs. But notice I am talking about PAIRS of electrons being repelled.

Yeah, so?

So some folks get confused at this point. (It's easy to be confused here.)

Why? The pair of electrons in an orbital must repel each other. They have the same negative charge. That's what you are talking about. Right?

Wrong. But I can see why you would make that mistake. You see, the pair of electrons in an orbital are there because of Pauli's exclusion principle.

Yeah, I know that. Pauli says only two electrons per orbital.

Right. They are not the pairs we are talking about as repelling each other. Due to quantum mechanics, the electrons which share an orbital don't repel each other. But they do repel other electrons in other orbitals.

OK. I think I see what you mean.

Good. VSEPR is concerned with how Pairs of Electrons in an orbital Repel other Pairs of Electrons in other orbitals. That's what we mean by Electron Pair Repulsion.

What about Hund? You know, the electrons get distributed first as individuals to all orbitals of the same energy. That means if you have three electrons to distribute to the 3 p orbitals, each orbital gets only one.

Yes, that's Hund's rule and it applies very well with atoms. But it doesn't work well with covalent molecules.

Why not?

It's a long story. The short story is that all covalent bonds use PAIRS of electrons. Even the electrons which are not involved in bonds (directly) prefer to be in pairs.

So Hund's rule should only be applied to individual atoms, but not to atoms in molecules.

Right. In covalent molecules electrons LOVE to be in pairs. To fill MOLECULAR orbitals, you try to fill them in pairs.

OK, Hund's rule doesn't apply to covalent molecules or VSEPR Theory.

Correct. Electron Pair Repulsion accepts the fact that orbitals have pairs of electrons whenever possible. Each pair fills an orbital. Pauli says so. But now the orbitals can repel each other!

I understand. That's why you speak about Electron PAIR Repulsion. It's not the repulsion of the electron pair IN an orbital but the repulsion of electron pairs BETWEEN orbitals. Is that right?

Absolutely right! The two electrons within an orbital cannot get away from each other. They are locked in by quantum mechanics. But those locked up pairs can repel other pairs of electrons. It is the repulsion of the pairs of electrons, the orbitals, which shape an atom and the covalent bonds it makes.

I see. But what about the first part of VSEPR theory. The VS stands for.... what?

Valence Shell. And what is a valence shell?

It's the outer shell. The one that has the electrons which do all the bonding. It's the valence shell which gives us the Lewis structures.

Very good. It is the Electron Pair Repulsion of the orbitals in the Valence Shell that determines the shape of the bonds it makes!

I get it. VSEPR theory predicts the shape of molecular orbitals by taking into account how the orbitals repel each other. Because only the valence shell (the outer most) orbitals are involved in bonding, we only bother with them. We ignore the ones in deeper, inner shells. It is the repulsion of the valence (outer shell) orbitals (the electron pairs) which causes the shapes.

Right! The shape and direction of all molecular orbitals, including the hybrid molecular orbitals, can be explained by Valence Shell Electron Pair Repulsion theory (VSEPR theory). Fundamental to understanding VSEPR is the fact that pairs of electrons (the pairs in each orbital) will arrange themselves to be as far away as possible from other pairs (other orbitals) due to electrostatic repulsion.

And exactly how is that shape made?

That depends on the atoms and their outer (valence) orbitals. You must use VSEPR theory to figure out the structure for each molecule. Let's try the molecule methane (CH4). To use VSEPR we start with a Lewis structure.

OK. Carbon is at the center of the Lewis structure and has the 4 hydrogen's surrounding it.

Right. You're familiar enough now with Lewis structures to know how to arrange those shared electrons. There's nothing new to that.

Yeah, but the way I have it drawn, it looks flat. Is that right? Is methane flat?

No, it is not. Real molecules have all three dimensions to work with. So do you. Now we get to use VSEPR theory to predict the correct shape. The whole point of VSEPR is to arrange the electron pairs to be as far away as possible from each other.

But they are! There are four pairs of electrons (four bonds) in methane. The best way to separate them is to put them at the four corners. Then they are at "right angles" to each other. That means they are 90 degrees apart. Because there are only 360 degrees in a circle, you can't separate them any further!

That's a good argument but you are thinking in two dimensions. If you think in three dimensions you will see that they can be positioned further apart. Four bonds that repel each other, equally and in all three dimensions, will form a tetrahedral shape.

A what?

A tetrahedral (pronounced "tet-rah-heed-rule) is a very important three dimensional shape. "Tetra" means four. Tetrahedral is the shape formed when four objects are arranged equally around a center. Any object with a tetrahedral shape is called a tetrahedron.

Like the four hydrogens around the carbon.

Right. Because we are dealing in all three dimensions, the angle between these bonds is actually 109 degrees, not 90.

Oh, I think I'm beginning to see what you mean. It just isn't easy.

I agree. The problem is that there is no easy way to draw a three dimensional object on a flat surface. Notice how I've drawn these bonds. I picture the hydrogen directly above the carbon as being in the plane of the paper (blackboard or screen). Notice that the line connecting the carbon to that hydrogen above it is just a simple line. It is the same width from top to bottom.

Yeah, but the other three bonds are shaped like triangles.

That's right. Alchemists use wedges (triangles) to represent the idea that the bonds are sticking into or out of the paper. In this picture, I've drawn the hydrogens on the right and left sides to be slightly behind the carbon. Look carefully at those bonds on either side of the carbon. You see how the bond is thicker at the carbon than at either of those hydrogens?

Yeah, I see what you mean. It gives the impression that the hydrogens are a little bit behind the carbon.

That's right. Now look at the hydrogen above the carbon. See how it is a normal looking line. All the same width.

Yeah.

That means the carbon and that hydrogen are in the same plane. They are in the plane of the paper. But the other three hydrogens are not. That's why their bond width changes as you move along the bond. That is meant to give it perspective.

Perspective?

Yes, perspective. I don't want to turn this into an art class....

Art class!?

Aye. Perspective was developed by Italian artists in the 15th century (the Renaissance) to draw objects on a flat surface so they appeared to have their natural dimensions and relations. Alchemists draw bonds in perspective. Imagine the bonds are SUPPOSE to be the same width. Then you begin to imagine that the reason some are shaped like wedges is because the perspective changes their width.

I think I'm getting the hang of it. Take that hydrogen below (and slightly to the left) of the carbon. Its wedge is backward from the two on the sides. It is as if that bond were sticking out of the page. Is that right?

Absolutely! That's perspective. Now you can imagine (by looking at this picture) that the hydrogen I've drawn below the carbon is sticking out of the page....

and the two on the sides are sticking into the page, behind the carbon. But the hydrogen above the carbon is in the same plane as the carbon atom. It isn't sticking in or out of the page so its bond is the same width - not a wedge.

Right.

You said this shape is very important in nature?

Yes, it is. A tetrahedron is the simplest three dimensional object you can make with flat sides.

A sphere is simpler.

A sphere doesn't have flat sides.

Oops! Right. I suppose it's important to really understand the shape of a tetrahedron. It looks a bit like a pyramid.

That's exactly what it is! A tetrahedron is a pyramid made of 4 triangles.

Like the Great Pyramids of Cheops!

No. The Egyptian pyramids are square pyramids. That's a common mistake. The Pyramids of Cheops have a square base and triangles arising from each of the four sides. The Pyramids of Cheops have five sides - the square side being on the ground.

Oh, I'll have to look at them again sometime.
Is there an easy way to picture or make a tetrahedron? It seems so important I don't want to forget it.

Yes. Easy. Show me three fingers.

Huh? You mean just hold up three fingers?

Right. Most people naturally show three fingers by holding their thumb against their tiny, outer finger and letting the three middle fingers stick up.

OK. So what? That's not a tetrahedron.

Not yet. To make a tetrahedron "tent", spread your three fingers down onto the table top so the tips of your fingers touch an imaginary triangle. Position your finger tips to make a triangle with three equal sides. (An equilateral triangle.) Try to spread them a few inches apart.

I have to really spread them out. Especially the middle one.

Aye, but don't hurt yourself. When you have your three fingers arranged on the table with the tips making a triangle, you have a made a triangular pyramid. A tetrahedron.

I see! Great. Each fingertip is where methane's hydrogens go with the forth hydrogen at the top of the tent. I have a tetrahedron with me wherever I go! That's pretty neat.

I think so too. Tetrahedrons (objects with tetrahedral shapes) have amazed great minds for centuries. Even the Ancient Alchemists were excited by tetrahedrons. A common symbol in their old books was a drawing of the four Ancient Elements arranged in a tetrahedron. They thought it was magical.

Was it?

No, of course not. But it is pretty. Now let's get back to methane.

Methane is shaped like a tetrahedron. The carbon is in the middle and it arranges its orbitals to be as far apart as possible. So they form a tetrahedral pattern. The hydrogens are stuck on the ends of carbon's four orbitals, so they are arranged around the carbon in a tetrahedron. Easy.

Yes. Notice that those four bonds are made of the eight electrons shared by the carbon and the hydrogens. That means all of carbon's L-shell electrons are involved in the bonding.

Yeah. That's the Lewis structure. Magic number eight.

Not magic. Mathematics! Anyway, I want you to recall that it is the entire L-shell involved in making those orbitals from the carbon. That means both the s and p orbitals are involved.

Hey, I'm suddenly confused! S orbitals are spheres and p orbitals are double lobes! How do you get a tetrahedral pattern out of that?

Hybrid orbitals.

Hybrids. I almost forgot. Are you saying that the electrons in the s and p orbitals mix together to make the tetrahedral pattern?

That's exactly what I'm saying. You see, orbitals can hybridize. They can mix.

What about ionic molecules and metals? Do they make hybrids and molecular orbitals?

No, don't get yourself confused. (It's so easy to get mixed up here.) Recall, electrovalent compounds don't share electrons. And metal bonds "supershare" their electrons. Neither of them make bonds in specific directions.

I see. So, metals and electrovalent compounds are made of simple atoms. They have simple shapes.

Right. The orbitals in single lone atoms can only make simple orbitals. Electrovalent and metal bonds don't make these complex molecular orbitals. Their outer shells are shaped by the simple atomic orbitals we discussed before I mentioned VSEPR.

OK. So VSEPR is only used to figure out the shape of covalent molecules.

Right. Now, let's get back on track with molecular orbitals and VSEPR.

OK. Molecular orbitals and VSEPR theory have to do with covalent molecules only.

Right! And molecular orbitals can hybridize. As a matter of fact, most molecular orbitals are hybrid orbitals.

I see. Carbon by itself has normal s and p orbitals. But when carbon forms molecules it can hybridize those orbitals to make the best shapes for VSEPR.

Absolutely right. The eight electrons in methane (four from the carbon and one from each hydrogen) are directed towards the corners of a tetrahedron by VSEPR predictions. Because each of these four hybrid orbitals is made of a bit of one s orbital and a bit of the three p orbitals, we call them sp3-hybrid orbitals.

So, sp3-hybrids are a mix of all the orbitals in carbon's valence shell (the L-shell). And VSEPR makes sure they are arranged in a tetrahedron.

Right. I think you are getting the hang of it. The sp3-hybrid orbitals and the tetrahedrons they make are a very important part of molecular structure.

What if you don't have four atoms around the central atom? What if you have only two? Does it form a line?

Sometimes the molecule is linear and sometimes it is not. It depends on VSEPR. Let's take carbon dioxide as an example. Show me how you get started.

OK. First I draw the Lewis structure and assign pairs of electrons to form the bonds. Gee, it looks a lot more complex than methane. There are double bonds. And now I have to deal with oxygen atoms. They have L-shells as the valence shell. That makes them more complex than hydrogens. Help!!

Don't panic. You'll see how easy it gets. You are right that the oxygens have more complexity than the hydrogens. But in this example they really add nothing to your problem. They are hanging onto the side of the central carbon. It is the carbon that decides the shape of CO2.

Why?

Well, think about it. It is the bonds made by the carbon on which the oxygens hang on. The oxygens will be positioned wherever the carbon demands them to go. It is the molecular orbitals of the central atom, the carbon, which really matter.

So those extra electrons hanging off the oxygens (the white x's) don't matter?

Right. They don't matter because they aren't involved in making the bonds with carbon, are they?

Oh, I see. It is only the shared electrons (in blue) that matter because they are the only ones involved in the bonds.

Right. At least in this example.
Always define which atom is critical to making the bond and the electrons it uses to make those bonds. The only electrons that matter are the ones owned or shared by the "linking" atom. (Carbon in this case.)

OK, but what about the double bonds? Do they matter?

Well, they might. Just remember that double bonds are two molecular orbitals shared between two atoms. That really doesn't change anything. They will still form a single repulsion axis.

A what?

We've been talking about repulsion axes. ("Axes" is the plural of "axis".) But we haven't given them that name. Every electron or pair of electrons in an orbital can make a repulsion axis. It is an imaginary line of electrostatic repulsion. Methane has four repulsion axes, four separate bonds, four orbitals. Because they repel each other equally, methane's carbon has its four repulsion axes arranged as a tetrahedron.

I see. Repulsion axis is just the line of repulsion.

Right. Now back to our problem. Double bonds in CO2, or any where, make a SINGLE repulsion axis. By that I mean they work together to produce a single (but pretty powerful) axis of repulsion.
Think about them. Four shared electrons make double bonds between the carbon and an oxygen. So they must lie side by side in order to make a connection to each other. Two bonds but one axis. Now let's think about how the repulsion axes work.

Hmm, all of carbons' 4 valence electrons are used up in those double bonds and the 4 electrons it gets from the oxygens are all used up too. It looks to me that there are only two repulsion axes here. One on the right and one on the left. Is that right?

Absolutely right. Carbon in carbon dioxide has two repulsion axes. Both axes are made of two bonds. All together those two double bonds account for all the eight electrons shared in the bonds.

And the unshared electrons from the oxygens (the white x's) don't affect it.

Right. As it turns out, those other electrons (the white x's) are not involved in the bonds and they end up arranged neatly away from the bonds. Now tell me, how would carbon arrange those two repulsion axes in order to have them as far apart as possible?

In a tetrahedron?

No. A tetrahedron has four repulsion axes. That was good for the carbon in methane because methane has four repulsion axes. But carbon dioxide has....

two repulsion axes. Gee, to make them as far apart as possible, I guess they would be on opposite sides of the carbon. 180 degrees apart. Yeah, that way they would be as far apart as possible.

That's right.

Then the structure of carbon dioxide is linear. It just looks like a straight line. It looks just like I've drawn it and there is no third dimensional worries.

Right. I told you this is easy.

Well, it seems I went to a lot of trouble to discover it!

Yes, you did. But in the process you learned something. You learned how to use VSEPR to determine the shape of the molecule. Like before, these bonds are really a hybrid of the s and p orbitals in the valence shell. We could even give them a name like we did for the hybrid that makes the tetrahedron, but we don't need to know all those details. Let's just stick with the idea of VSEPR theory.

So all molecules linked up from a central atom are linear. Right?

Wrong. That's a common mistake. You must use VSEPR theory for each new molecule you come across. There are no short cuts or simple rules for groups of three or more atoms.

But a linear chain is the only possible outcome for three atoms linked by a central one. Isn't it?

Nope. But I can see why you might think that.
Let's do a VSEPR analysis of the water molecule and you'll see what I mean.

It will look like CO2, won't it?

Give it a try.

OK. The Lewis structure looks like this.
I'll give the oxygen the o's and hydrogens the x's (for the electrons).

Good. I see you have lots of extra electrons around the central atom but they aren't involved in bonding.

Are they still in orbitals?

What do you think?

I suppose they have to be somewhere. I guess they must be in orbitals. But they aren't bonding orbitals.

Right. All electrons must be in orbitals. (Where else could they possibly be?!)

Are they in pairs?

Yes, they are in pairs. When talking about molecules we can ignore Hund's rule and put all electrons into pairs.
Of course, some atoms will have an odd number of valence electrons (think about nitrogen) and we will end up with one odd electron (because you can't make complete pairs with odd numbers).
But that hasn't happened in this molecule. They are all in pairs, but not all of the pairs are involved in bonding.
We call any pair of electrons in an non-bonding orbital a lone pair. Lone pairs do not have atoms attached to their ends.

So the central oxygen has two bonding pairs and two lone pairs. Is that right?

Yes, it is. And each of those four orbital pairs will make a repulsion axis. So tell me Arthur, what shape is formed by four repulsion axes?

Well, all four of them will want to be as far apart as possible. They will form a tetrahedral pattern. Just like methane. That doesn't seem right.

But it is right! (Or very close.) Even though the oxygen only has two atoms attached to it, it has four repulsion axes. The lone pairs still make orbitals and VSEPR shapes them all into a tetrahedron.

But the lone pairs don't have atoms attached to their end.

Right. But they ARE Electron Pairs in the Valence Shell which Repulse each other. So you have to count them in the VSEPR. However, you are right to point out that there are only two atoms. Those two atoms, the hydrogens, are held by the two bonding orbitals.

It looks to me that the orbitals form a tetrahedron, but the atoms by themselves make up only a part of it.

Right. It is as if parts of the tetrahedron were invisible.

So the molecule of water isn't linear at all. It looks like an angle. That's why you've been drawing it bent.

Absolutely right! The water molecule is angular but the carbon dioxide molecule is linear.

That's weird. But I understand it.
The carbon in carbon dioxide used all its orbitals in bonds so there were atoms at the ends of the repulsion axes to see them. And it had double bonds that gave it only two repulsion axes, so VSEPR proved it was linear. But the oxygen in water has no double bonds and all four repulsion axes to play with. VSEPR says four repulsion axes will arrange in a tetrahedron. But because there are no atoms at the ends of two of them, the water molecule looks like it is just a bent line.

Right. There are several important lessons to learn here. For example, you must take into account ALL the electrons in the outer (bonding) shell when using VSEPR theory. That includes the lone pairs and as well as the bonding pairs.

What about an orbital with only one electron in it? Does that happen?

Yes, that does happen. An electron all by itself in an orbital is called an odd electron. You will see some molecules with an odd (unpaired) electron every now and then. (I'll give you one in the question section later.)

That poor, lonely electron. I mean odd electron (I don't want to get it confused with a lone pair, because that's different.). Does an odd electron make a repulsion axis?

What do you think?

Yeah, I think it does. If it is in an orbital all by itself it still has electrostatics going on. I just think that it wouldn't be as pushy as a pair of electrons.

You're right and that is a very good point. Imagine you had an atom with four orbitals. Three of those orbitals were full but the last orbital had only an odd electron. Would that change the shape of the molecule? Would it still be a tetrahedron?

Yes! It has to be a tetrahedron because even the odd electron has a repulsion axis. Even though it isn't a very powerful one. An atom with four repulsion axes will arrange them in a tetrahedral pattern. It has those hybrid orbitals, sp3.

Right, but it wouldn't be a nice, neat tetrahedron. It would be a little lopsided because the odd electron is not as "pushy" as a pair. Instead of forming a perfect tetrahedron like the carbon in methane, an atom with an odd electron in an orbital would be shaped like a slightly flattened tetrahedron. It would still be a tetrahedron, but a shortened one.

I see. The odd electron has a weaker repulsion axis than any pair so it gets pushed around more. It can't hold its own against the pair.

Right. All orbitals make a repulsion axis but not all repulsion axes are equal. The axis made by an odd electron (that is an unpaired electron) is the weakest repulsion axis of all.

What about the strength of the two kinds of paired orbitals (those with two electrons)?

Good question. As it turns out, lone pairs have more repulsive force than bond pairs.

But water has two lone pairs and two bond pairs in its tetrahedron.

Right. And that affects the final shape of the water molecule. Remember I told you that a perfect tetrahedron has the four axes positioned at 109 degrees.

Yeah. That means all four axes are forced 109 degrees apart. That is as far apart as you can get them in three dimensions.

Right. You proved that water's H-O-H formed an angle of 109 degrees (the hydrogens at 109 degrees from each other when measured from the oxygen). That's close but not completely right. The two lone pair orbitals use VSEPR to push a little harder on the bond pair.

Why? How?

Think about it. Lone pairs aren't shared by other atoms. They aren't tugged between two nuclei, so they can hang out better and repel other electrons better.

So the bond pairs, the orbitals holding the hydrogens, are pushed closer together than 109 degrees by the more powerful lone pairs?

That's right. It seems strange that the lone pairs, the ones you can't really "see" because they don't have atoms attached to them, are stronger at electrostatic repulsion. But they are. And it is because they are not wasting their time around other atoms that they are so much stronger (better) at repelling!
Careful measurements (using tools I don't want to go into) show that the angle formed between the two hydrogens in water is actually 104.5 degrees.

Could you have predicted that angle from VSEPR theory?

Well, yes and no. You now understand that the angle would be less than 109 degrees because of the more pushy lone pairs. But to predict exactly how much they push requires a lot of math. And even then it might be hard to get the prediction that accurate.

So VSEPR theory isn't good enough. What a waste of time!

Not at all. With VSEPR theory you can predict or explain molecular shapes. You may not get the exact angle right (even with tons of math to help you), but at least you can get close.

OK, I'll accept that. VSEPR theory is just a matter of finding the repulsion axes (from the Lewis structure) and remembering that the final angles may be a little bit off because of the different electrostatic powers of the three kinds of orbitals.
Lone pair orbitals are the most pushy (even though you can't "see" them) .
Then comes the bonding pair (which you can "see" because they have atoms at their ends).
But the weakest orbital of all is one with an odd electron (which you can't "see" because there's no atom at its end).

Right. Isn't that simple? It's just a matter of thinking it through. Electrostatics explains it all. VSEPR theory explains it all!

Hmm, what about those double bonds we talked about earlier? You know, the ones in carbon dioxide. Are they more repulsive than a single bond? Do double bonds make a more repulsive axis than single bonds?

What do you think?

I think they are. A single bond has only two electrons in its repulsion axis, but a double bond has four. So a double bond should be much more repulsive than a single bond. It has more electrons.

Absolutely correct. You see, you haven't even looked at a molecule with a double bond pushing on a single bond. But when you come across one you will know that the double bond forms a more powerful repulsion axis. That's electrostatics.

I bet a triple bond is even more repulsive than a double bond because the triple bond has six electrons in its repulsion axis.

Fantastic! You see. You are making very good predictions using VSEPR theory by keeping in mind the electrostatics that go on. That is the way to determine molecular shape.
Before we call it a day, review for me how to use VSEPR theory to predict a molecule's shape.

OK. First draw a Lewis structure and define the repulsion axes. Repulsion axes include all the axes made by the valence shell electrons. It includes the bond pairs (stuck to other atoms), lone pairs (not stuck to other atoms) and even the odd electron (which is all alone in the orbital).

Right. Also keep in mind that any bond between atoms (single, double or triple) makes only one axis.

I know that. Once you have figured out the repulsion axes you can decide what is the best shape to keep the electrons as far apart as possible.

That's right. An atom with only two repulsion axes will be linear like carbon dioxide. An atom with four repulsion axes will arrange them in a tetrahedron.
Oh, here's a thought. What kind of shape would you get if the atom has three repulsion axes?

Three?! Hmm, we haven't done an atom with three axes. I guess they would all stick out as far as possible from each other. Wouldn't that make a simple, flat triangle?

Yes, it would. An atom with three repulsion axes forms a triangle with its orbitals all in a plane. The three orbitals will stick out of the atom in a triangle. Any atoms on those bonds will be separated by 120 degrees.

I see.
You know, an atom with two repulsion axes makes linear bonded molecules. That's just one dimension.
An atom with three repulsion axes has its orbitals arranged in a flat triangle. That's two dimensions.
And an atom with four repulsion axes arranges them in a tetrahedron. That's three dimensions.
Do any atoms have five or six repulsion axes? Do they need more dimensions? Is that possible?

Ah, yes, no and no. What I mean is, yes, there are atoms with five or six repulsion axes. But no, they are not in higher dimensions. There are no more than three dimensions in space. At least in this universe!

So what do they do? How do they distribute their orbitals?

By using VSEPR theory. I don't want to go into those structures. They are complex and can involve the d and f orbitals with all their strange shapes. But if you want to think about them you can use VSEPR theory to help you figure them out.
The one with five axes makes a shape called a "trigonal bipyramid". It's like a tetrahedron but has the extra axis sticking down, below the one on top.

Sounds complex. Would that be what you would get if you placed a tetrahedron on a mirror?

Ah, yes, that would give you an image of a trigonal bipyramid. Another way to think about a trigonal bipyramid is to imagine three axes in a triangle and the other two axes stick out of the triangular plane, above and below it.

Yeah, that's what you get if you placed a tetrahedron on a mirror. I like my way of describing a trigonal bipyramid.

Then use it!
The atom with six axes makes a pattern like the corners of a cube. It's called a simple "octahedral".

Simple to who?

Let's leave those for another time.

Fine by me. But I got a question. All this talk about VSEPR theory seems to be about covalent bonds. Right?

Right.

VSEPR theory is used to predict the shape of covalent molecules.

What about other molecules? What about other bonds?

Well, VSEPR theory is designed for covalent molecules only. But the rules of electrostatics apply to everything and it's the simple electrostatic forces that shape other molecules.

What's the shape of NaCl?

Ionic molecules are very simple. As a matter of fact, the hardest part about ionic molecule structures is figuring out where the molecule begins or ends.

How do you mean?

Well, a crystal of NaCl is made of millions of sodium cations and millions of chlorine anions. They are drawn toward each other by electrostatic attraction.

Yeah, I know that. It's the opposite of repulsion.

Right. It has nothing to do with VSEPR because it has nothing to do with molecular orbitals. When the two ions meet they continue to attract others of opposite charge. That causes them to form alternating patterns in long stretches.

Like this?
NaClNaClNaClNaClNaClNaClNaClNaClNaClNaCl

Yes, that is sort of right, but the electrostatic attractions are not linear.
You have to take into account the 3 dimensions.

OK, so table salt is really,
.NaClNaClNaClNaClNaClNaClNaClNaCl..
.ClNaClNaClNaClNaClNaClNaClNaCl..
.NaClNaClNaClNaClNaClNaClNaClNaCl..

Yes, that's better. But that's only a 2 dimensional pattern. That layer will have layers above and below it. The layers and chains arrange themselves into alternating patterns in order to get the cations as close to the anions as is possible. And at the same time, that separates the ions of the same charge from each other by placing an opposite charged ion between them. Get it?

I think I do. These ionic compounds are just spheres of opposite charge arranged to give the best electrostatics.

Well, close. Remember, all of these ions have shells and subshells. They are atoms and atoms have the atomic orbitals we discussed earlier. The ions of some elements will have outer subshells with s orbitals, so you can think of them as a sphere. But most ions have other orbitals in their valence (outer) shell.

What about sodium's ion (Na+)?

Think about the sodium ion's shells and subshells. Remember, it has 11 electrons as a neutral atom.

Right. But as an ion it has only 10 electrons. Two of them will be in the s orbital of the K-shell. The remaining eight will all be in the L-shell. Hey, that means ALL its outer orbitals will be full. There will be two electrons in the s orbital and all three p orbitals will be full too.

Right. We can write that like this
Sodium's K-shell (s2)
Sodium's L-shell (s2, x-p2, y-p2, z-p2)

I see. If the sodium were neutral (not this ion) it would have an electron in the s orbital of its M-shell.

Right. The neutral sodium atom would be a sphere. But the sodium ion has a valence shell that is NOT a sphere. Its outer shell would have those p orbitals sticking out of it in all three dimensions

It would look like a "jack".

A what?

You know. A "jack"! The game you play with a bouncing ball as you sweep up jacks with the one hand. It has six points on it. Three axes.

Oh, yes. I've seen them. They form a simple octahedral pattern.
(Note to those who care: An octahedron has eight sides but six points! An object's shape is defined by its sides not its points. A cube has six sides but eight points. Only a tetrahedron has the same number of points as sides. This is just a wee bit of 3-D geometry for those who wonder about my comment.)
So a "jack" has an axis in each dimension.

That's what I said!

Oh, OK. Anyway, you see that sodium's ion is not a sphere. What about the chlorine ion (Cl-)? It has 18 electrons. (Because the neutral atom has only 17 electrons)

Easy
Chlorine's K-shell (s2)
Chlorine's L-shell (s2, x-p2, y-p2, z-p2)
Chlorine's M-shell (s2, x-p2, y-p2, z-p2)
So it is shaped like a jack too. Just a bigger one.

Right! Now you see that these atoms are not simple spheres. But when they are attracted to each other and form their electrovalent (ionic) bonds, these atomic shapes don't really matter. The ions just bond to each other as if they were spheres

What!? Why have you wasted my time describing these two ions as jack-shaped when I can imagine them as spheres?

They ARE "jack-shaped" (have three axes) but their atomic shape doesn't affect the way they bond. The chlorine anion will be attracted to these sodium ions from any direction. Equally. Therefore, they pack as if they were just simple spheres.

They pack like balls but they are shaped like jacks.

Right. I'm sorry if that irritates you. I just don't want you to go away thinking that these two ions are sphere shaped. They aren't. But they can be imagined as spheres if that helps you imagine their arrangement, because their complex shape doesn't affect they way they bond to each other. They can bond from any direction. Like a sphere.

OK, it's just a way to imagine them. We can imagine ions as balls even though they are really jacks.

Right! And we don't need complex geometry to explain them.
NaCl and all other ionic molecules make groups of neatly arranged ions. They form pretty crystals based upon simple electrostatics. Most of the time they arrange into simple patterns. Mostly these "spheres" pack together as cubes (like NaCl) or variations of a cube.

Like oranges and apples in a box. Each apple has an orange as a neighbor and each orange has an apple as a neighbor.

Right. That's a good way to think of them.

What about metal bonds?

They are usually simple crystals too. Both ions and metals are best thought of as spheres with charges. (Even though they may actually be more complex.) Ionic molecules are arranged in alternating patterns. But metal atoms can be all the same, usually neatly arranged cations with a sea of electrons around them.

Metals are easier to imagine than ionic compounds.

Yes, they are. At least to the level of Alchemy I'm trying to teach here. In point of fact, very advanced Alchemists use the shape of each metal's atomic orbitals to figure out EXACTLY how they are arranged. The specific position of the atomic orbitals and how they interact is an important subject. It gives certain metals amazing properties. Especially when mixed with other metals as alloys or ceramics.

Hmm, I have a thought.
I might be wrong here, but aren't metal and ionic bonds only shaping solids?

You're right!
Metal and ionic bonds are only important in determining the arrangements in their solids. Liquid salt or metal flows and has no real structure. The gases are even more "unarranged".
But covalent molecules have real structure at all the states of matter. Do you know why?

I think so.
The covalent bonds are strong intramolecular bonds that link up in specific directions because of their shared electrons, molecular orbitals and VSEPR theory.
Ionic compounds and metallic compounds aren't linked to specific atoms, so in a gas or liquid state they could easily swap around and take on any shape they wanted.

Right.
You see, both the ionic and metallic compounds are held together without direction. Only when they are solid do they settle down into specific patterns.

Yeah, and those patterns are a lot easier to understand because the orbitals don't affect the direction or patterns. Not directly.

Correct.
But don't go away thinking that solids of ionic or metallic compounds don't have specific shapes. These solids make beautiful crystal patterns and are arranged in specific ways. However, I think it's best that we leave those shapes until Advanced Alchemy.

OK, let's keep this simple. What about weak bonds?

Weak bonds affect how molecules are arranged, not the atoms in them. But some of them influence the direction in which the two molecules bond. And, if it is cold enough, you can make solids using the weak forces.

What about hydrogen bonds? Do they affect a molecule's shape?

They can. They affect the shape of ice. Hydrogen bonds are very "linear". Look at the way a hydrogen bond is made. You will see that it MUST make a straight line between the electronegative atom (which draws electrons towards it), the hydrogen and the other atom that is connected by the hydrogen bond. That affects how the molecule bonds to another molecule. Hydrogen bonds must line up.

I see what you mean. Hydrogen bonds are lines. OK, what about the bonds made by van der Waals forces?

Van der Waals forces are caused by very specific, detailed shapes of electron clouds from many atoms all working at once. It is the pattern of the molecules themselves that form the outer part of the molecules involved in van der Waals forces. So I guess it is the underlying covalent bonds that cause the van der Waals forces to be the made.

So VSEPR theory predicts the shape of the covalent molecule. And the shape of that molecule causes the specific van der Waals interactions?

Yes, I suppose you can think of it that way.

There's a lot of Alchemy in this section. I feel like an expert on molecules!

You should. You've learned a great deal So, any final questions?

Only about a million of them!
How do you remember all this trivia!? Is there any order to all this stuff?

Good questions, Arthur. You've been patient and worked hard. I can see why you may find this all a bit overwhelming. It's a great deal of information and things may seem disconnected and without any clear rules. That will all change when we move on to our next Ancient element, EARTH. Next time I'll teach you how to make sense of all the different types of elements and their behaviors.
In today's lesson, I just wanted to teach you about all the kinds of bonds and how they affect molecules.
I think we have covered a great deal of material in this lesson. You should read through your dialogue (again), write some notes and answer the questions I'll give you.


This work was created by Dr Jamie Love and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License Creative Commons Licence.