What is the difference between an elemental molecule and a compound molecule? |
What are the three strong bonds and how do you tell them apart?
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How many atoms of hydrogen (H), oxygen (O), carbon (C), chlorine (Cl) and nitrogen (N) are found in the molecule "glycine" (molecular formula C2H5O2N)? |
Is there any difference between C2H5O2N and NC2O2H5? |
What is the difference between a nuclear and a chemical reaction? |
What were the major contributions of Bohr and Lewis to our understanding of Alchemy? |
What is special about the d and f orbitals?
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What are Lewis structures? |
Ammonia is a colorless gas with a pungent order. (It stinks!) Draw the electronic configuration, Lewis structure and symbol for ammonia (NH3).
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Is ammonia polar? (Does it have polar bonds?) How would that affect its ability to dissolve substances? |
Can ammonia form hydrogen bonds? |
Draw the electronic configuration, Lewis structure and symbol for silicon dioxide (SiO2).
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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.
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Explain metallic bonds and how their unusual "electronic configuration" affects their properties. |
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.
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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. |
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? |
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? |
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. |
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.)] |
What is the difference between chemical properties and physical properties? |
Arrange these interactions from the strongest to the weakest.
metallic ("super-sharing")
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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? |
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!). |
Petroleum is a mixture of hydrocarbons of various sizes.
Oil refineries separate and collect these molecules into these
different groups:
How do you think they are separated in oil refineries? How can refineries use temperature to separate petroleum into these groups? |
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? |
You did a Lewis structure for ammonia (NH3) earlier and saw it had a strange shape.
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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! |
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.) |
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. |
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.) |
What will be the shape of the molecule, chlorine dioxide? |
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. |
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). |
Glycine (C2H5O2N) has 2 carbons, 5 hydrogen, 2 oxygens and 1 nitrogen.
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Not really. C2H5O2N and NC2O2H5 are the same molecule with the same atoms ordered differently.
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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). |
Bohr discovered the fact that electrons orbit atoms in very specific shells. Lewis discovered that atoms seek to complete (fill) their outer electron shells. |
Atoms don't "like" to use their d and f orbitals unless they absolutely have to be used.
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).
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. |
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. |
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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!
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. |
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 ...) |
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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".
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-
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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. |
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
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
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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.
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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! |
This diagram is really of an "intermediate" molecule. It lasts for only a very brief time. The carbonate ion forms very quickly.
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. |
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.
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. |
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. |
Electrovalent and covalent are the strongest bonds, with
metallic bonds (usually) a little bit weaker.
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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.
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. |
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.
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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.
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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.
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. |
So, begin by drawing a simpler Lewis structure like this one, below.
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.)
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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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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.
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! |
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!)
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!) |
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.
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.
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. |