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Merlin's Questions for Water

Q 1.

What is the difference between an elemental molecule and a compound molecule?

And the answer is....

Q 2.

What are the three strong bonds and how do you tell them apart?
And the answer is....

Q 3.

How many atoms of hydrogen (H), oxygen (O), carbon (C), chlorine (Cl) and nitrogen (N) are found in the molecule "glycine" (molecular formula C2H5O2N)?

And the answer is....

Q 4.

Is there any difference between C2H5O2N and NC2O2H5?

And the answer is....

Q 5.

What is the difference between a nuclear and a chemical reaction?

And the answer is....

Q 6.

What were the major contributions of Bohr and Lewis to our understanding of Alchemy?

And the answer is....

Q 7.

What is special about the d and f orbitals?
In what way does this effect how they are "filled" ("completed")?
How many types of each are there and how many electrons can they hold?
Do d or f orbitals contribute to the shape of their atoms?

And the answer is....

Q 8.

What are Lewis structures?

And the answer is....

Q 9.

Ammonia is a colorless gas with a pungent order. (It stinks!)

Draw the electronic configuration, Lewis structure and symbol for ammonia (NH3).
[Recall, nitrogen has an atomic number of 7.]

And the answer is....

Q 10.

Is ammonia polar? (Does it have polar bonds?) How would that affect its ability to dissolve substances?

And the answer is....

Q 11.

Can ammonia form hydrogen bonds?

And the answer is....

Q 12.

Draw the electronic configuration, Lewis structure and symbol for silicon dioxide (SiO2).
[Note, silicon has an atomic number of 14.]

And the answer is....

Q 13.

Potassium chloride (KCl) is a simple electrovalent compound.

Explain how it is formed. You need not draw the electronic configurations, just explain it in words.
Write it out as a series of formulas showing the gain and lose of the electron(s).
(Note. Potassium has an atomic number of 19 and chlorine has an atomic number of 17.)

And the answer is....

Q 14.

Explain metallic bonds and how their unusual "electronic configuration" affects their properties.

And the answer is....

Q 15.

The molecular structure of hydrogen cyanide is easy to determine from its molecular formula (HCN).

Explain (with words, not a drawing) how these three atoms are joined.
[Hint: Think of the covalency of each atom, as determined earlier in this class. Then "solve the puzzle"!]

And the answer is....

Q 16.

Calcium (Ca) has an atomic number of 20.

Without drawing the shells, you should be able to explain how calcium can obtain the electronic configuration of a noble element. [Recall that argon, with an atomic number of 18, is a noble gas.]

Also, what is the symbol for the calcium ion? Then try to figure out the formula for calcium fluoride.

And the answer is....

Q 17.

Sulfur (S) has an atomic number of 16.

What would you expect its covalency to be, why, and what other element has the same covalency?

And the answer is....

Q 18.

Hydrogen sulfide is the foul smelling gas given off by rotten eggs (and a few other stinky things).

What would you expect the molecular formula to be for hydrogen sulfide?

And the answer is....

Q 19.

Calcium carbonate (CaCO3) is an example of an "ionic-covalent" molecule! When added to water it dissolves into its two ions: the normal cation (Ca+2) and an anion, not of a single atom, but of a covalent molecule, CO3-2.

This is something new for you - a covalent, molecular ion! (It's called a "radical". You'll learn about them later.)

Draw a Lewis structure of the carbonate anion (CO3-2) showing how the two extra electrons (donated by the calcium as it ionizes to Ca+2) aid in the bonding of the third oxygen, giving the carbonate a charge of -2 while also allowing for a third oxygen to covalently bond to the central carbon.

(Hint: start with the Lewis structure of carbon dioxide, then add in the extra oxygen and electrons to form a carbon with a double bond to each of the three oxygens.)

This isn't an easy one so take your time and give it a lot of thought.

And the answer is....

Q 20.

You may have noticed that covalent molecules, like CO2, H2O or CH4 are all drawn as specific structures, with specific bonds linking the atoms together in specific patterns. But ionic (electrovalent) compounds, like NaCl or MgCl2, are only represented by spheres. Why have I kept the shape of the ionic compounds a "secret"?

Do covalent and ionic compounds have fundamental differences that make their structures different? [Hint: Think about the "directionality" of bonds (the directions they point and patterns they can produce.)]

And the answer is....

Q 21.

What is the difference between chemical properties and physical properties?

And the answer is....

Q 22.

Arrange these interactions from the strongest to the weakest.
(If you think some have very similar strengths, place them together into a "group".)

metallic ("super-sharing")
electrovalent
covalent
van der Waals
hydrogen bonds
hydrophilic-hydrophobic interactions

And the answer is....

Q 23.

Candle wax is a mixture of very large, very complex organic compounds (carbon based compounds) collected from plants and animals. Paraffin wax is much simpler to understand and is collected from petroleum. Paraffin is a very large hydrocarbon - nothing but carbons linked into long chains "coated" with hydrogens.

What inter-molecular forces would it be able to use? How can it be a solid at room temperature?

And the answer is....

Q 24.

Describe and explain the states of matter which a solid piece of paraffin experiences as the temperature is increased (to as hot as the sun!).

And the answer is....

Q 25.

Petroleum is a mixture of hydrocarbons of various sizes. Oil refineries separate and collect these molecules into these different groups:
Refinery gas is a mixture of methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10).
Gasoline is a mix of hydrocarbons containing 5 to 8 carbons (and their associated hydrogens).
Kerosene (sometimes called paraffin oil) is a mix of hydrocarbons containing 11 to 12 carbons (and their associated hydrogens).
Diesel oil (sometimes called gas oil) is a mix of hydrocarbons containing 13 to 25 carbons (and their associated hydrogens).

How do you think they are separated in oil refineries? How can refineries use temperature to separate petroleum into these groups?

And the answer is....

Q 26.

Imagine a liquid is fairly viscous (meaning it does not flow easily - it is "thick").

What do you think would happen if a viscous liquid were quickly cooled down to form a solid, but the molecules were not able to correctly line up their bonds?

And the answer is....

Q 27.

You did a Lewis structure for ammonia (NH3) earlier and saw it had a strange shape.
Use VSEPR to determine the correct shape of NH3.
(Take your time and work carefully because VSEPR is not easy!)

And the answer is....

Q 28.

Earlier you did a detailed analysis of calcium carbonate and figured out the Lewis structure of the carbonate ion (CO3)-2. That was a difficult Lewis structure but you learned that the extra charges (from the two extra electrons) contribute to the overall structure of the ion. They allowed the carbon to make more bonds. You found that the ion is composed of a central carbon connected to three oxygen atoms by double bonds. VSEPR can be used to figure out the correct shape of the ion. Just because it is an ion, doesn't mean you can't use VSEPR!

Give it a try. What is (CO3)-2 shaped like?

Hint: The extra electrons are part of the Valence Shell. This ion is not as complex as you might think. If you are thinking it's like ammonia, you're wrong!

And the answer is....

Q 29.

Ethylene is an invisible, highly flammable gas with the molecular formula, C2H4. Draw a Lewis structure for it.

(This puzzle requires a little bit of thought to get the electrons shared properly. But you should be able to do it.)

And the answer is....

Q 30.

Use VSEPR theory to predict ethylene's shape.

Hint: Ethylene has TWO carbons that behave as "central atoms" but they are both the same shape, just pointing in opposite ways. If you figure out the shape of one half of the ethylene molecule, you can easily figure out the other.

And the answer is....

Q 31.

Chlorine dioxide (ClO2) is an explosive (!) gas used to bleach flour and wood pulp (for paper). Chlorine has an atomic number of 17 and oxygen has an atomic number of 8. Draw a Lewis structure for chlorine dioxide.

(Warning: This is NOT easy. Take your time and keep thinking about the valence shell electrons.)

And the answer is....

Q 32.

What will be the shape of the molecule, chlorine dioxide?

And the answer is....

Arthur's (and Merlin's) Answers

A 1.

Elemental molecules are made of all the same element, like H2 and O2.

Compound molecules are made of more than one element, like CH4 or H20.

Back to the question

A 2.

Covalent bonds are formed by the sharing of pairs of electrons between two atoms.

Electrovalent (or ionic) bonds are formed by the transfer of one (or more) electrons between two atoms.

Metallic bonds are created due to the "supersharing" of electrons among all the atoms (cations, actually).

Back to the question

A 3.

Glycine (C2H5O2N) has 2 carbons, 5 hydrogen, 2 oxygens and 1 nitrogen.
It has no chlorine (otherwise it would be written in the formula).
The sole nitrogen doesn't need to have a 1 as a subscript, because it is assumed that there is a 1 hidden there.

Back to the question

A 4.

Not really. C2H5O2N and NC2O2H5 are the same molecule with the same atoms ordered differently.
Some (advanced) Alchemist would argue that there is a proper ("Correct") way to order the elements, so they obey certain rules of "chemical nomenclature" (the way chemicals are named). Unfortunately, this requires a more detailed understanding about electronegativity and the Periodic Table. Maybe later!

Back to the question

A 5.

Nuclear reactions involve changes in the nucleus (plural "nuclei") of atoms. They release radiation as nucleons are changed or released. The result of a nuclear reaction is a transmutation of one element into another.

Chemical reactions involve changes in the electron (shells) of atoms, with the breaking of electron "chains" and reforming them in a different pattern. These have nothing to do with the nucleus (directly), nothing to do with radiation, and do not change the atoms (only their arrangement).

Back to the question

A 6.

Bohr discovered the fact that electrons orbit atoms in very specific shells.

Lewis discovered that atoms seek to complete (fill) their outer electron shells.

Back to the question

A 7.

Atoms don't "like" to use their d and f orbitals unless they absolutely have to be used.
That means atoms would rather put electrons into the s and p orbitals of other, higher shells than continue to place them in the same shell. This is what makes assigning electrons to shells so difficult with large atoms.

There are five different types of d orbitals (capable of holding a total of 10 electrons) and seven different types of f orbitals (capable of holding a total of 14 electrons).
These orbitals are used by larger atoms as a place to store electrons (in order to balance the charges from the protons).

Because large atoms fill the s and p orbitals of their next larger shell BEFORE assigning electrons to their d and f orbitals, only the s and p orbitals are found as the outer electron cloud. The d and f orbitals are not involved in the shape of atom. They (the d and f orbitals) are just a "basement" for storing electrons below the important (outer) shell.

Back to the question

A 8.

Lewis structures are diagrams or cartoons of the outer shell of an atom, used to figure out the assignment of electrons in the formation of bonds. Each atom is represented by its abbreviation and surrounded by x's or o's representing the outer shell electrons. The xo's are arranged to produce pairs of shared electrons, each pair representing one covalent bond.

Back to the question

A 9.

Your drawings should look something like the one on the right (although the exact geometry may vary).

Nitrogen's 7 electrons are distributed with 2 in the K-shell and 5 in the L-shell, leaving room for three more. All three hydrogens have a single electron in their K-shell and require one more for a complete shell. Thus three covalent bonds are formed, all of them N-H bonds.

We'll come back to this later to do a proper VSEPR analysis.

Back to the question

A 10.

Yes, ammonia is a polar molecule because it has polarized bonds. Even though nitrogen is not as strongly electronegative as oxygen it is, nonetheless, more electronegative than hydrogen. Therefore, the nitrogen will draw electrons towards it, sharing the pairs unequally with each hydrogen. This means that ammonia, like water, is a polar molecule and will be able to dissolve other polar or electrovalent molecules.

Ammonia is not AS polar as water. In fact, ammonia is poorly polar because the N-H bonds of ammonia are not as polar as the O-H bonds of water. But it's still polar!
Don't worry if you didn't get that one. Ammonia's polarity is so poor that it is easy to overlook it.

Some students will argue that ammonia is a gas, not a liquid, and thus unable to dissolve anything! However, ammonia can be liquefied (by reducing its temperature) and once it is turned into a liquid, it can dissolve polar molecules and electrovalent molecules as expected.

Back to the question

A 11.

Sure can! Hydrogen bonds can form whenever hydrogen is covalently bound to an electronegative atom. The hydrogen bond is really an extension of the polarized covalent bond, causing the hydrogen to have a partial positive charge and attract any (partial or whole) negative charge (by electrostatic attraction). It doesn't even have to be a liquid (but it might help if it were).

Ammonia's N-H bonds don't make hydrogen bonds as strong as that made by water's O-H bonds. But ammonia can still make hydrogen bonds (weaker than water's).

By the way, ammonia has to be colder than water in order to become a liquid. That is, you need colder temperatures to turn ammonia from a gas to a liquid than you need for water. You should know why. (Think. If ammonia's hydrogen bonds are weaker than water's hydrogen bonds ...)

Back to the question

A 12.

Your drawings should look something like the one on the right.

Silicon has 2 electrons in its K-shell, 8 in its L-shell and the remaining 4 in the M-shell, requiring 4 more electrons to complete the shell.
Both oxygens have 2 electrons in the K-shell and only 6 electrons in their L-shells, requiring 2 more (each) to complete their shells.
All three atoms achieve the electronic structure of a noble element by sharing. The silicon shares two of its M-shell electrons with each oxygen, which in turn share two of their L-shell electrons with the silicon.

You may have noticed that silicon dioxide is very similar to carbon dioxide. The only thing different is that silicon uses its M-shell in the bonding while carbon uses its L-shell. However, carbon dioxide is a gas and silicon dioxide is a solid! (Sand and glass are made of silicon dioxide.) Silicon's extra shell makes a big difference!

Back to the question

A 13.

Potassium (K) will have complete K, L and M shells (holding a total of 18 electrons) and the last electron goes into the next shell. We haven't discussed the next shell, but you may have guessed is called the "N-shell".
Remember, the d and f orbitals in the M shell aren't used (yet) so that last electron goes into the N-shell. (Specifically the N-shell's s orbital).
BUT, that last electron is lost in order to make a complete M-shell!

Chlorine has 7 electrons in its outer (M-shell) so it requires only one more to obtain a "noble electron configuration".

This is achieved by stealing the single outer electron from the potassium, causing both atoms to have complete outer shells (and both of them M-shells, it just so happens). The oppositely charged ions are then attracted to each other by electrostatics (opposite charges attract).

K ------> K+ + e-
Cl + e- ------> Cl-
K+ + Cl- ------> KCl

Back to the question

A 14.

Metal bonds are formed because the metal's outer shell electrons are shared among millions of atoms (not just the one atom next to it). The electrons tend to simply wander around the piece of metal, with no particular atom as its "home". These wandering electrons can be "encouraged" to move (quickly) in a direction by applying voltage (more electrons) to one end while draining them away from the other end. This is an electric current.
All metals can carry an electric current.

Also, the "cloudy" nature of these wandering electrons means that the cations have a slight amount of positive repulsion between themselves. Therefore, metals can be easily shaped (they are malleable and ductile) by simply pushing the cations over each other.

This "super sharing" of electrons and the "mega-cations" they produce give metals a shiny appearance (luster).

Back to the question

A 15.

Hydrogen has a covalency of one, nitrogen three and carbon four. There is only one way all three of these atoms can share their outer electrons to complete their shells. When trying to figure out molecular structures it is always smart to start with the element(s) of highest covalency (carbon in this example) and work down to the element(s) with the least covalency (hydrogen in this example).

Carbon can form three bonds with nitrogen. This would mean that the nitrogen is using all its covalency (its three sharing electrons) and carbon is using three too. But that means the carbon still has an electron looking for a partner to share in a covalent bond. Hydrogen fits the bill! Hydrogen's one electron is shared with carbon's last remaining "shareable" electron to form a single bond.

Therefore, hydrogen cyanide is formed by a triple bond between the carbon and the nitrogen and a single bond between the carbon and the hydrogen.

Back to the question

A 16.

Calcium would need to lose TWO electrons in order to have the electronic configuration of argon. Therefore, calcium will donate two electrons to any atom able to accept electrons. Calcium ionizes like this
Ca ----> Ca+ + e- and then further ionizes like this
Ca+ ---->Ca+2 + e- for a total of
Ca ---> Ca+2 + 2e-

Fluorine (F) requires only one electron to complete its outer shell (so it can "look" like neon). Therefore one fluorine atom accepts one electron like this
F + e-----> F-
But notice that the calcium must shed two electrons, not just one. So another fluorine ion is created. Thus one calcium will ionize to Ca+2 and the two electrons it gives up are accepted by a pair of fluorines creating 2F-. Electrostatics bring the three atoms together into a single, neutral electrovalent compound CaF2.
Calcium fluoride (CaF2) occurs naturally as the beautiful purple crystal "fluorite" (some folks call it "fluorspar"). When added to toothpaste it fights bacteria!

Back to the question

A 17.

Sulfur needs two more electrons to complete its outer shell (so it would have the electronic configuration of argon, atomic number 18). Therefore, sulfur would be expected to have a covalency of two - looking for two electrons to share, to form two shared pairs (two covalent bonds).

This is the same as oxygen (O). Although oxygen is a whole shell smaller than sulfur (and, thus seeks the electronic configuration of neon), oxygen nonetheless has the same covalency as sulfur.

Here's something new.
Sometimes sulfur has a covalency of 6, not 2! That means under some conditions the sulfur has a "Lewis structure" different from expected. Indeed, this kind of sulfur wants an outer shell of 12 electrons, not eight. This is a property called "hypervalency" and it comes about because some of the bigger atoms (like sulfur) make use of their inner d and f orbitals on some occasions.
Don't worry about the details. Just be aware that big atoms can be very funny about their inner "basement" orbitals.

Back to the question

A 18.

Hydrogen sulfide is made of hydrogen and sulfur, just as the name suggests. The trick is to determine how many hydrogens are attached. Starting with the atom of higher covalency, sulfur, we realize that it has two bonds to make, but hydrogen has only one. Therefore, two hydrogens must bond to each sulfur in order to satisfy the desire for complete outer shells. Sulfur shares one electron to a hydrogen and the other to another hydrogen. Of course, the hydrogens get a complete shell (looking like helium) due to the sharing.

If you thought that sulfur was hypervalent with hydrogen, you guessed wrong. The hypervalent properties of big atoms doesn't happen with hydrogens. (But you wouldn't have know that until now.) So, H6S doesn't exist.

The molecular formula for hydrogen sulfide is therefore, H2S. Just like the formula for water. Oxygen and sulfur have the same covalency because they both have similar outer shells (even though sulfur's is larger).

It is interesting to realize that these two molecules, H2O and H2S are similar in formula but so different in their properties. Refreshing water or stinky gas!

Back to the question

A 19.

Start with carbon dioxide. We drew its Lewis structure during the dialogue, but here it is again, along with some new details for you to see. The covalent (shared) electrons are drawn closer to the oxygens than to the carbon. This is because oxygen is more electronegative than carbon. Therefore, a partial positive charge is created on the carbon. The "little deltas" are there to remind you about these polar bonds. The partially positive carbon attracts a third oxygen and any free electrons (which just happen to come from the calcium).

It is good to keep in mind that the bonds in molecules are very "dynamic". That is, they are not simply "chains holding ships together" (as Arthur likes to think of it).

The third oxygen (with its six outer electrons) needs two more electrons to fill its shell. Carbon normally has only 8 electrons in its outer shell, but in this case it drags in an extra two more, which it then shares with the third oxygen, forming a pair of covalent bonds in the process. There are now 12 electrons involved with the carbon (!); 8 from the carbon dioxide (the way it started), 2 from the new oxygen and another 2 electrons that come from "outside". Because all the shared electrons (including the two extra electrons) are drawn to the oxygens more than the carbon, the carbon doesn't really have 12 electrons in its outer shell (although the Lewis structure might make you think they do). Instead it is best to think of the carbon as "juggling" the two extra electrons by passing them to the three oxygens as quickly as possible.
Just as a juggler is constantly throwing a third ball in the air as he tries to keep three balls all at the same time; the carbon appears to be holding onto 12 electrons, when in fact it is passing the extra electrons to the oxygens and taking them back again. (That's what a covalent bond is all about. Right?). The three oxygens are "happy" to take on the extra electrons because they are electronegative elements. Meanwhile, the two electrons from the calcium are also attracted to the partially positive carbon.

This diagram is really of an "intermediate" molecule. It lasts for only a very brief time. The carbonate ion forms very quickly.
It is this "juggling" of the extra electrons that allows carbonate to have a charge of -2 while still satisfying carbon's (and the oxygens') desire to have a complete outer shell. The positively charged calcium ion (Ca+2) is attracted to the carbonate anion. If there is enough water around, solvation shields form around both the calcium and carbonate ions and the two ions stay "in solution". So calcium carbonate is not formed. If the water evaporates the shields disappear and a mineral called "calcite" precipitates out of the solution. Also, many creatures collect these two ions from the water to make their shells (snails, clams and many other creatures of the oceans, lakes and rivers). This accumulation of calcium carbonates by living creatures forms huge reserves of limestone (chalk) and marble.
The way this ion forms is so "radical" it needs to be repeated. It would appear that the carbon now has an outer shell of 12 electrons! In fact, the three oxygens tend to hold the extra electrons among their outer shells. It is impossible to tell which oxygen has the two extra electrons (donated by the calcium). That's because the three oxygens constantly swap the electrons around via the carbon. In fact, most of the time the extra two electrons are hanging with the oxygens more than with the carbon (even though they do share with the carbon, a bit). Remember that oxygen is very electronegative. So the oxygens tend to draw the electrons away from the carbon anyway. Therefore, the carbonate ion is an anion held together by the sharing of electrons.

This was a particularly difficult question. So don't feel bad if you had trouble with it. Just read it again and look carefully at the drawings to understand how this complex ion is formed.

Back to the question

A 20.

Covalent bonds form in specific directions because the sharing of electrons requires the atom to be in specific positions in order to share their electrons. On the other hand, ionic bonds are simply the (electrostatic) attraction of one atom to another.
Covalent molecules are often pictured as "stick models" with the covalent bonds connecting the atoms together like beams and struts in a "scaffold" of atoms. Each atom is bonded to another in a specific direction and the bonds are "fixed" in POSITION. (However, they are dynamic in the sense that the electrons are constantly being swapped back and forth.) Each carbon atom in carbon dioxide is bonded to two specific oxygens. These individual atoms do not swap their electrons with other atoms, to form other bonds. Covalent molecules have an "identity" in their structure.

Ionic molecules, however, are not a simple matter of one atom bonded to another. The attraction between oppositely charged ions means they are drawn to each other without directions to the bonds. That is not to mean that ionic compounds are not arranged in a pattern. They often are. Many of the ionic molecules we discussed are packed in specific geometries (producing beautiful crystals). These geometries are due to the way the spheres pack together, not the way the bonds are "directed". Ionic bonds have no direction.

Perhaps an example would help. NaCl crystals are tightly packed spheres of Na+ and Cl-. Imagine a box of oranges and apples all arranged so one orange is always separated from the other by an apple. That is what a crystal of NaCl is like. It is a cube made of the spheres of the two ions arranged in alternating fashion (or stuck in groves between them). Now imagine that each apple has a stick connecting it to an orange. Each orange is thus attached to two apples (like a molecule of carbon dioxide). These two types of fruit groups are different. In a box full of apples and oranges, arranged in alternating manner, it is impossible to tell which two are actually joined together. Is this orange attached to the apple on the right or the left? You can't say. However, those fruit linked together by sticks can be easily identified as belonging to one or the other.

This may be difficult to picture. I hope you will agree that ionic compounds lack directions to their bonds and also lack any true "identity" as to where each molecule begins and ends. Covalent molecules, on the other hand, have specific directions among their atoms and you can imagine just one molecule at a time.

Back to the question

A 21.

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

Physical properties describe a substance as it is; like its hardness, color, density, and many more.

Back to the question

A 22.

Electrovalent and covalent are the strongest bonds, with metallic bonds (usually) a little bit weaker.
Hydrogen bonds and the hydrophilic-hydrophobic interactions are about 1/10th (10%) as strong as covalent and electrovalent bonds.
Van der Waals forces are the weakest of forces (only about 1% as strong as a covalent or electrovalent bond) and are the first "bonds" to "melt" when the temperature of a substance increases.

Back to the question

A 23.

Paraffin, being made of nothing but hydrogen and carbons, has only van der Waals forces to hold it together. It can't even make hydrogen bonds! If the paraffin was in water, the hydrophobic interactions might come into play. But here we are only dealing with dry paraffin wax.
However paraffin is solid at room temperature, while other molecules that only have van der Waals to hold them (O2 and N2, CH4, for example) are gases! You could be forgiven for being confused. How does paraffin stay solid at room temperatures? (You may ask.)

Paraffin is a very LARGE hydrocarbon and is thus able to make MANY van der Waals forces. This collective use of van der Waals forces holds large molecules to each other and is common among the molecules of organic and biochemistry.

If a molecule is large enough to establish 10 van der Waals interactions with another molecule (of the same kind), it will be held together by a force similar to that of a single hydrogen bond. (That's because each van der Waals force is only 1/10 as strong as a hydrogen bond.). Each paraffin molecule has over 30 sites for van der Waals forces to operate. That would be equivalent to three hydrogen bonds (or maybe as strong as a weak covalent bond). So, at room temperature, paraffin is a solid substance.

Back to the question

A 24.

Molecules of paraffin in a solid state wiggle a little (vibrate) but not enough to stop them from forming stable van der Waals "bonds" between each other. Any material that is made of molecules linked together by stable bonds is a solid regardless of what kind of bond (or force) is used to link them.

As the temperature rises, the paraffin molecules will wiggle too much to maintain stable bonds. At this temperature the paraffin melts. The melting point of any substance is the temperature at which inter-molecular bonds are no longer stable. All pure substances have a specific melting temperature (although the atmospheric pressure can effect it). Now the molecules can only form unstable, very short-term bonds and the entire substance is a liquid. The liquid paraffin wax will now fill the bottom of a container and take its shape.

Raising the temperature further causes the paraffin molecules to wiggle and whirl so violently that no bonds can form and no molecules can interact. At this temperature the paraffin boils. The boiling point of a substance is the temperature at which inter-molecular bonds can not form. All pure substances have a specific boiling temperature (although the atmospheric pressure can effect it). Now the molecules just bounce off each other and the entire substance is a gas. This gas of paraffin will fill a container and escape out any hole it finds.

Raising the temperature still further causes paraffin to actually break its intra-molecular covalent bonds; the bonds holding the atoms of hydrogen and carbon together. At this point the paraffin disappears into a cloud of atoms! The paraffin is gone, and that's the end of the answer about paraffin.
(Of course, if you keep raising the temperature, these atoms will lose electrons and the substance becomes a plasma.)

Back to the question

A 25.

Petroleum is a mixture of hydrocarbons of various sizes. Oil refineries separate and collect these molecules into these different groups by a process called "distillation". You may not have heard that name before, but you should have been able to guess the process. Distillation takes advantage of the differences in boiling point (temperature at which each chemical boils) between all the substances in the (liquid) mixture. Molecules with few van der Waals forces will have lower boiling points than molecules with many van der Waals forces.

Refinery gas is a mixture of methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). It is already a gas at room temperature so it need only be collected before it gets away! At room temperatures the van der Waals forces for these small molecules can not form even temporary bonds.
Gasoline is a mix of hydrocarbons containing 5 to 8 carbons (and their associated hydrogens). At room temperature gasoline is a liquid, its van der Waals forces can form temporary bonds to other molecules. If you raise the temperature (through a range of 40C to 180C), these molecules wiggle too much to interact even temporarily. They become a gas, and can be collected just like the refinery gases above. Larger molecules able to form more van der Waals bonds are left behind in the liquid phase.
Kerosene (sometimes called paraffin oil) is a mix of hydrocarbons containing 11 to 12 carbons (and their associated hydrogens). After the gasoline molecules are collected, the temperature is increased further (through a range of 180C to 250C). This causes the molecules in the size range of 11 or 12 carbons long, to lose touch with their neighbors. They boil away and are collected as a gas. Larger molecules (over 13 carbons) are still able to form temporary bonds with their neighbors, so they remain a liquid (diesel oil and others).
Diesel oil (sometimes called gas oil) is a mix of hydrocarbons containing 13 to 25 carbons (and their associated hydrogens). Raising the temperature still further (through a range of 250C to 350C) releases these diesel oil molecules as a gas and they are collected.
What is left behind (the residue) is a mixture of very large hydrocarbons (including paraffin wax).

Back to the question

A 26.

A viscous liquid cooled too quickly for the molecules to make the most favorable bonds, will still solidify (become a solid). But its molecular arrangement will be irregular, even random! This is called a glass. Some people use the word "amorphic" or "amorphous" to describe glasses because it means "without shape".

Viscous liquids are still liquids, because their molecules can still slip past each other. Most of the molecules in a viscous liquid are involved in large clumps (aggregations) of molecules, that can move only slowly. Water molecules cannot form these clumps so (pure) water is never viscous. On the other hand, silicon dioxide (SiO2) has important viscous properties allowing it to form two very different kinds of solids.

Sand is made of tiny crystals of quartz, which is silicon dioxide arranged in a regular pattern. Crystals are solids with their molecules (atoms or ions too) formed in a regular pattern. This regular pattern is determined by the formation of the best inter-molecular bonds. Most solids, including sand and the rocks from which the sand was made, are really clumps of small crystallites or grains. A good magnifying glass will show the grains in sand and most rocks. For these bonds to form correctly, the material must have cooled very slowly, giving plenty of time for the silicon dioxide molecules to form their regular patterns.

To make window glass, sand is heated until it melts. This produces a very viscous liquid of silicon dioxide that is then cooled very quickly. Much too quickly for it to form the best bonds in a regular pattern, so it solidifies as a glass (from which we get its name). This irregular molecular arrangement is still a solid.
Note: Not all glasses are clear (most are not).

Some substances cool at just the right speed such that tiny crystals form in some parts, but not in others. These crystals become trapped inside a glassy structure as the rest of the substance solidifies forming irregular bonding patterns. This substance would be described as devitrified, small crystals held in a glassy structure.

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A 27.

Earlier we drew the Lewis structure of ammonia as a triangle. A Lewis structure is not necessarily a true shape for the molecule. (It rarely is the correct shape.) Instead, most people like to draw their Lewis structures to look "symmetrical" because it looks "neat". That's OK, but for VSEPR analysis it is better to make it clear what is going on by keeping all the similar bonds in a clear pattern. The Lewis structure here looks nice but it is a bit confusing because it looks like the lone pair electrons are on opposite sides.

So, begin by drawing a simpler Lewis structure like this one, below.

Like all Lewis structures we only count the valence electrons. This might confuse you. It is true that only six electrons are involved in making the three covalent bonds but there are also two more electrons in nitrogen's valence SHELL. They are not bonded to any atoms but they affect the final shape of the orbitals. ALL electrons in the valence SHELL must be taken into account for VSEPR. The Lewis structure shows that six electrons are involved in bonding to the hydrogens and another pair of electrons are simply hanging around in their own orbital. That gives the nitrogen (in ammonia) a total of four repulsion axes; three from the bonded pairs and one from the lone pairs. Any atom with four repulsion axes will arrange those orbitals into a tetrahedron (like methane).

If the molecule had identical orbitals (like methane's four bond pairs) we would expect them to repel each other equally. But lone pairs are more repulsive than bond pairs. That means the lone pair (white) will push harder on the other three orbitals. Instead of them being 109 degrees apart (as in methane), they will be pushed a little closer together. It is hard to say how much they are pushed, but you would be right to expect the three hydrogens to be less than 109 degrees from each other (measured from the central nitrogen).

The final structure of ammonia is a that of a tetrahedron with the top cut off. The missing top is really the invisible lone pair. Alchemists call this molecule's structure a "trigonal pyramid". Notice that the orbitals are arranged as a tetrahedron, but the atoms are not. (The atoms are in the shape of a trigonal pyramid.)

Also, because of the (invisible) lone pair, this trigonal pyramid is a wee bit squished together. That is, the three hydrogens are closer together making the molecule look like a tall (uneven) pyramid.
This is the same effect you saw for the water molecule, but here it is in three dimensions. The water molecule was angular (flat) but the ammonia molecule is a pyramid (3 dimensional).

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A 28.

Although the Lewis structure for this ion looks complex, the final bonds it makes are really very simple. The carbon is connected to the three oxygens by double bonds. Instead of 8 electrons in the valence shell, there are 12. That's because of the extra electrons. If you tried to figure out how to fit all those electrons into s and p orbitals, you would have been confused because it is not easy to understand. Besides, it's not important for VSEPR theory. The bonds will be hybrid molecular orbitals anyway so you don't need to know much about them in order to use VSEPR theory on them.

The Lewis structure proves there are three double bonds involved. That is, the central atom (the carbon) has three repulsion axes. Remember, single, double and even triple bonds all have only one repulsion axis each. So the question is, "What is the best way to arrange those three axes to have them as far apart as possible?" (After all, that's what VSEPR is all about!)

A little bit of thought would have shown that the axes must be arranged in a simple flat, triangle. Alchemists call this a "trigonal planar" arrangement meaning it forms a flat triangle. The carbon is at the center of the triangle and the oxygens are at the corners. Simple!

Notice that ammonia (from before) and the carbonate ion have similar formulas (a central atom surrounded by three others) but their shape is very different. VSEPR theory takes into account hidden forces (lone pairs) involved in the ammonia molecule. The carbonate ion, although very complex as a Lewis structure, simply has three repulsion axes with no hidden orbitals (because there's an atom at the end of each axes).

By the way, because the carbonate ion has all the SAME kinds of bonds, they are EQUAL in the amount of repulsion. You may recall that double bonds are more repulsive than single bonds. All three axes in the carbonate ion are equally pushy because they are all the same. That means they can't push each other around because they have the same strength. Therefore, the triangle is a perfect (equilateral) triangle. The three oxygen atoms in carbonate are 120 degrees apart from each other (measured from the central carbon), a perfect "trigonal planar" molecule.

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A 29.

Both carbons have four valence electrons and each hydrogen can contribute only one. That makes eight electrons from the two carbons and another 4 electrons from the hydrogens for a total of 12 electrons. That's enough to make 6 bonds (with two electrons shared per bond).

This puzzle is not easy until you realize that the carbons must be sharing more than one pair of electrons between them. In this diagram I have used o's for the electrons donated by the hydrogens and x's or +'s for the electrons from the two carbons. Start by making the four x's and +'s around the two carbons. As you the add in the hydrogens (with their electrons) you will see that there are extra electrons available for the carbons and they are needed to fill the valence shells of both carbons. That makes the double bond between the carbons.

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A 30.

First thing you should do is realize that there are TWO carbon atoms at work here and they will both have a shape. Fortunately, they are identical. So the shape of one will also be the shape of the other. Start with one side of the molecule (one carbon). Think through the VSEPR analysis on just one carbon. Naturally, you must take into account the electrons it shares with the other carbon. Don't think about the other carbon (yet), just the electrons it shares.

You see each carbon has two single bonds (one to each hydrogen) and a double bond to the other carbon. All the valence shell electrons are accounted for. There are no lone pairs and no odd electrons. (Right?) All the valence shell electrons are in bonds. (Right?)

As you look at the Lewis structure you should be thinking, "How many repulsion axes does it (the one carbon) have?" It has three. Two axes are made by the single bonds to hydrogen and the third axis is made by the double bond to the other carbon. Remember the double bond is a single repulsion axis. Therefore the carbon has three repulsion axes and that means it will form a flat triangle.

You will recall that a double bond has more repulsion force than a single bond. That's because there are four electrons involved in the repulsion axis of a double bond but only two in the axis of a single bond. If all three axes were equal in strength they would arrange themselves in a perfect (equilateral) triangle, like the carbonate ion. But the axes in ethylene are not equal. The axis with the double bond will push the single bonds back further than the 120 degrees of a perfect (equilateral) triangle. I don't know how far back. You would be right to think that the two hydrogens on the carbon are closer than 120 degrees (as measured from the carbon).

Oh, then there is the other carbon and its VSEPR. Well, it has the same shape as the carbon you just did. Simply turn it around. Easy.

So the entire ethylene molecule is a pair of triangles joined by their double bonds. Also, notice that the entire molecule is flat. All the atoms are in the plane of the page. That might not seems obvious to you but think about the double bond as a pair of sticks and you will see that they force themselves into a flat plane. The hydrogens must also be in that same flat plane because that's the only way to keep the carbon orbitals' triangular shape.

The hydrogens will be swept further back than 120 degrees but they wouldn't both get swept ALL the way back because the single bonds will start to repel each other too!

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A 31.

If you had some trouble with this Lewis structure, don't feel bad. It's a hard one!

Chlorine has 7 valence electrons. They are in the atom's M-shell (because 2 electrons are in the K-shell and 8 in the L-shell, so that leaves 7 for the M-shell). The oxygens have 6 valence electrons (in the atom's L-shell). That gives a total of 19 valence electrons. The trick is to figure out how they are used! You know that every bond and lone pair are made of pairs of electrons. That would require an even number of valence electrons, but you have 19 to work with, so there must be (at least) one odd electron. The remaining 18 valence electrons could be used to make 9 bonds but if you tried to connect them with all the atoms you would have been frustrated. You can't connect these three atoms using all 9 bonds!

That's because there are lots of lone pairs in this molecule. To solve this puzzle think first in terms of the covalency. You know oxygen has a covalency of two. If you think about it, those two bonds, from each oxygen must attach to the chlorine. (If the two oxygens were attached to each other by the double bonds the chlorine couldn't get in!)

Once you realize that the bonds are arranged as O=Cl=O, you can start to put in the electrons. I've used o's for the electrons from the oxygens and x's for the electrons from the chlorine. The electrons involved in bonds are blue.

Notice the chlorine has 11 electrons around it. That is a lot of electrons around just one atom, but the chlorine manages to juggle them. At least for a while. (That's why it is so explosive!)

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A 32.

Like all VSEPR theory predictions, start with the Lewis structure. You will notice that there are a lot of electrons that are not involved in bonding. For VSEPR predictions we need only consider the electrons around the central atom, the chlorine.

Chlorine has two double bonds, one to each oxygen. Chlorine also has one lone pair of electrons and one odd electron all by itself. (Remember that odd number?) Now that you have identified the important electrons and their distribution, you should ask yourself, "How many repulsion axes does the chlorine have?"

There are four repulsion axes here; two axes are formed by double bonds, one axis is made by the lone pair and the odd electron forms the forth axis.

What kind of shape do you get from an atom with four repulsion axes? A tetrahedron, of course. But this will not be a nice looking tetrahedron around this chlorine atom. The angles between these hybrid orbitals will not be 109 degrees (like in methane). The double bonds are very powerful and they will try very hard to repel each other. If it weren't for the lone pair and odd electron, the double bond axes might push the oxygens completely to opposite sides. The lone pair pushes back from one side of the tetrahedron and the odd electron pushes too (very weakly). All this produces a lopsided tetrahedron.

Because this molecule has invisible orbitals (axes with no atoms at the end of them) the arrangement of the orbitals (a lopsided tetrahedron) will appear as an "angular" molecule. It is important to understand that the shape of orbitals determine where the axes go, but there may not always be atoms at the ends of all axes. So, it is possible to get molecular shapes that are different from the orbitals shapes (axes). That's because of the "invisible" orbitals.

It is impossible to predict exactly the angle between the two oxygens (measured from the chlorine), but it will be a lot larger than 109 degrees.

That was a very hard molecule. Its Lewis structure was complex. VSEPR predictions proved the orbitals were positioned to give a lopsided tetrahedron. Because there were some invisible orbitals, the final molecule is a flat angular molecule.

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