Molecular Bonding--Or Mere Attraction?

Molecules and atoms form groups using what we call "bonds." This blog will discuss intramolecular bonds, or bonds within a molecule (intra- means "within"). I'll discuss intermolecular bonds, or bonds between molecules in the near future.

Bonding Atoms: Intramolecular Bonds
There are three types of intramolecular bonds: (1) covalent bonds, (2) ionic bonds, and (3) metallic bonds. A covalent bond is caused by a sharing of electrons between molecules. An ionic bond is caused by the attraction of positively charged and negatively charged particles. Metallic bonding is the free sharing of electrons between masses atoms of a metal.

A covalent bond is created when two atoms both need more electrons to be in a stable state. Most atoms want 8 electrons within their reach for their last filled shell. Oxygen for example, in neutral state has only 6 electrons in its outer shell (2 in the inner shell, and six in the outer shell). The neutral state sounds pleasant, but really it just means electronically neutral--it means there are the same number of electrons as protons. The neutral state is not stable--the oxygen wants 8 electrons in its outer shell, so it needs to find 2 more electrons somehow. One way to get those electrons is by sharing them with another oxygen atom. So two oxygen atoms, each wanting two more electrons for their outer shells can get real close to each other and share enough electrons to give both atoms access to 8 electrons in their outer shells. In the case of oxygen atoms, each atom shares two pairs of electrons. Each atom, then, has two lone pairs of electrons in its outer shell, and two shared pairs of electrons in its outer shell: 8 electrons total in the outer shell. The resulting unit is an oxygen molecule (O₂). The oxygen molecule is still neutral because there are 16 electrons total (two in each atom's inner shell, two lone pairs on each atom, and two shared pairs between the two atoms) and 16 protons total. The resulting molecule, then, is both neutral and stable!

Note that, in this case, the oxygens shared two pairs of electrons--that's a double bond. If they only shared one pair, it would be a single bond. Up to three pairs of electrons may be shared between atoms (triple bond). A triple bond is stronger than a double bond, which is stronger than a single bond. The more electron sharing there is, the stronger the bond will be.

Ionic bonding is different. It does not involve sharing as much as greed! Consider, for example, chlorine. It has seven electrons in its outer shell in the neutral state, but would prefer to have 8. Sodium has only one electron in its outer shell when neutral. It would take a lot of effort for sodium to get seven more electrons, so it doesn't even try. Instead the sodium just tries to pawn off its extra electron on someone else, ridding itself of its current outer shell, preferring to get along with the next shell down. With the one electron gone, sodium's next shell down would then be the outer shell, and it has eight electrons in it--ahhh, the stability of a full outer shell.

To form an ionic bond, then, a chlorine atom just steals an electron from a sodium atom. The chlorine atom holds a negative charge (after stealing an electron, it has more electrons than protons) and the sodium atom holds a positive charge (after losing an electron, it has fewer electrons than protons)--positive charges attract negative charges, and that's an ionic bond! That's all it is, just attraction.

When molecules use ionic bonds we call them salts. Table salt, for example, consists of sodium and chlorine bonded ionicly. Road salt is often potassium chloride. In another post, perhaps I'll discuss why table salt doesn't work as well for melting ice on the roads.

Other Intramolecular Bonds
"I thought he said there are just two types of intermolecular bonds...." You're right, but really the two types are just the two ends on a continuum of electron sharing. Think of the perfect covalent bond as being at one end of the continuum (equal electron sharing), and the ionic bond as being at the other end (no electron sharing). Bonds may also be somewhere in between--unequal electron sharing.

Water for example--in a water molecule, two hydrogens are covalently bonded to an oxygen. The oxygen is more electronegative than the hydrogens. So it shares electrons with the hydrogens, but it doesn't share them equally--the oxygen hogs them. The difference between a covalent and an ionic bond is really just the degree of electron sharing. The degree of electron sharing depends on how tightly the electrons in the molecule hang on to their electrons. So if two atoms share electrons a little bit, but one of the atoms hogs the electrons the vast majority of the time, that is closer to the ionic end of the continuum. So we would call that an ionic bond even though there is a little bit of covalent (sharing) character to it. On the other hand, in water, even though the electron sharing is unequal, we call the bond covalent because there is still enough sharing going on that it is still toward the covalent end of the spectrum.

When one atom in a covalent bond hogs the atoms considerably, but not enough to call the bond ionic, we would call the bond a polar covalent bond. "Polar" just means that one atom in the bond is hogging the electrons. The greedy atom, then, would be more negative than the other atom in the bond.

So how do you decide how much electron sharing there will be? Just look at the electronegativity of the atoms in the bond. If one atom is far more electronegative than the other atom in the bond, then there will be more stealing and less sharing of the electrons. Here's a rule of thumb, things on the left side of the periodic table form ionic bonds with things on the upper right side of the periodic table. Things on the upper right side of the periodic table form covalent bonds with other things also on the upper right side of the periodic table. Things on the left side of the periodic table don't tend to form bonds with other things on the left side of the periodic table because they all are trying to get rid of their electrons, so nothing else on the left side of the table can take a spare electron from another to form a bond.

Remember that elements closer to fluorine are more electronegative than elements closer to francium. Fluorine, being on the upper right of the periodic table is very greedy for electrons, but sodium does not care for electrons much at all. Accordingly, the two would form an ionic bond. Oxygen is also greedy for electrons, so if it bonded with fluorine, the two would form a covalent bond.

One molecule may have both covalent and ionic bonds. For example, sodium acetate (NaCH₃COO) looks like this:

The bonds between the carbons, the hydrogens and carbon, and between the oxygens and carbon are all covalent bonds. The bond between the oxygen on the sodium is an ionic bond. The entire molecule is a salt, because it contains one ionic bond, even though there are also covalent bonds in the molecule.

For more detailed information on intramolecular bonding, follow these links:
Wikipedia article on ionic bonds
Wikipedia article on covalent bonds
Wikipedia article on metallic bonding (I didn't really discuss metallic bonding in this blog, but it is easy to understand, and this Wikipedia entry is clear)
A more in-depth look at bonding (this page discusses unusual bonds, such as ones that don't follow the "octet rule")
Details needing for making calculations regarding covalent bonds (this site discusses bond lengths and energies for covalent bonds)

The Unique Properties of Water

This is a brief easy to understand explanation of why water (as in H₂O) has the unique qualities of being less dense in its solid form than its liquid form, and remaining liquid at temperatures than can sustain human life.

Molecular Shape
Let's start at a molecular level for this. Many of water's special and unique attributes are derived the components of the water molecule. Water is composed of two hydrogen atoms covalently bonded to an oxygen atom. In addition, to the hydrogen atoms, the oxygen also has two lone pairs of electrons attached to it. So we could say the oxygen in a water molecule has four electron complexes attached to it (two hydrogen atoms and two lone pairs of electrons). Each of the electron complexes is "tied" to the oxygen, but repulses the other electron entities. Imagine the oxygen as a ball in the middle of the four electron complexes, with each complex at the end of a string tied to the ball--for each complex to be as far from each other complex as possible, it will form this shape, which we call a tetrahedron:


But, in this case, two of those complexes are lone pairs of electrons. We don't count lone pairs when looking at the shape of a molecule. The shape of a water molecule, then, is composed of the center oxygen with two hydrogens attached at an angle--it's a bent line. We call this molecule's shape "bent." [link to more on molecule shapes]


Molecular Attraction
Remember those lone pairs of electrons on the oxygen atom in a molecule of water? Think of that portion of the water molecule as the "top," and think of the portion with the hydrogens as the "bottom." The top portion of the molecule has an oxygen molecule in it, which is relatively electronegative, or attractive to electrons. [link to electronegativity] Oxygen is more electronegative than hydrogen, so it doesn't share the electrons quite equally with the hydrogen atoms--it hogs the electrons, you might say, sharing with the hydrogens only enough to mostly keep them attached. Due to this electron hogging and the presence of lone pairs of electrons, the top portion of the water molecule--where the oxygen is--is relatively more negative than the bottom portion of the molecule.

As we know, a positively charged object tends to attract a negatively charged object. Well, the ends of a water molecule have a positive and a negative change. So each end of a water molecule attracts the opposite end of nearby water molecules--the top of one is attracted to the bottom of the next, much like positive and negative ends of magnets. In the case of a water molecule there are two positive points sticking out at an angle (the hydrogen atoms), and one negative portion (the oxygen atom).

Hydrogen Bonding
The positive-negative attraction between the top and bottom of a pair of water molecules is called a "hydrogen bond" because it comes from the attraction of a hydrogen in one molecule to the oxygen in another. In liquid water, in which the molecules are moving quickly, hydrogens are constantly being formed and broken. But when water molecules are moving sufficiently slow [link to the relation between temperature and kinetic energy] the attraction between portions of water molecules becomes strong enough that more permanent hydrogen bonds can form between the molecules. This bond isn't as strong as a covalent bond (the bond within one water molecule, caused by the sharing of electrons between the oxygen and the hydrogen), but it is strong enough to cause a lattice to form when many different molecules are attracted together in this way. This is what happens when water turns into ice--the individual water molecules work together, using the attraction between a hydrogen in one molecule to the oxygen in another to form a lattice structure. Each hydrogen in one water molecule will attach to a different oxygen atom in a different water molecule, and each oxygen atom in the lattice would have several hydrogens attracted to it. Molecules are moving slow enough to begin formation of the lattice structure when the temperature of the water is below four degrees Celsius.

Here's an illustration of liquid water on a molecular level:


Note the random placement of the molecules. Hydrogen bonds are forming and being broken constantly in this picture as hydrogen molecules get close to oxygen molecules in the frenzy.

Here are a couple illustrations of the lattice structure:


Note that the lattice structure is not space efficient. When the movement of the individual water molecules becomes more erratic--as the water becomes warmer--the hydrogen bond is not strong enough to hold the lattice structure in place. Some molecules, for example, could flow right in the middle of the hexagonal structures in the lattice. When there are molecules in those holes in the lattice, the unit is more dense--it has more stuff in the same amount of space. If you pack more stuff in the same amount of space, then you've made that material more dense. In this case, that is the reason why ice floats--it is less dense than liquid water. When the temperature of water is low enough that the water molecules are moving slowly enough for the lattice structure to form, the low-density ice lattice is formed. But when the temperature is raised, making the individual molecules more erratic, the hydrogen bonding fails to hold the molecules in place causing the low density lattice to break down, melting the ice into higher density water.

What does hydrogen bonding mean to my life?
Without hydrogen bonding, water would not be a liquid at temperatures in which humans can survive. Even though the hydrogen bonds in liquid water are not strong enough to form ice above zero degrees Celsius, they still play an important role in the properties of liquid water. The attraction of the water molecules to each other (or cohesiveness) causes the molecules to be reluctant to separate into a gas. Without hydrogen bonding water would probably boil at a temperature similar to that of other substances of similar molecular weight. Carbon dioxide, which is significantly heavier than water--which means it would boil at a higher temperature than water--boils at about -72 degrees Celsius. Life on this Earth as we know it requires the use of liquid water. Without hydrogen bonding, the Earth would either be barren or life would be sustained by a means that doesn't require liquid water.

Without hydrogen bonding ice would also be totally different. I'll use a bucket of water to explain: Because colder water is more dense than warmer water, colder water sinks to the bottom of the bucket. If I left a bucket of water out on a winter day in Alaska, the molecules of water at the top would cool down, become more dense, and sink to the bottom, but wouldn't freeze because, at the bottom of the bucket the water is not exposed to the cold air. Other molecules would then be at the surface; these would also cool down and sink to the bottom without freezing. This process would occur over and over again, circulating the water until the whole bucket was four degrees Celsius. At this point, the water at the top, exposed to the cold air, would be further cooled below four degrees, causing it to begin to form the lattice structure. The surface water, then, because it is forming the lattice, would be less dense than the water below it, and as it cooled would stay at the top instead of sinking to the bottom, like it would do above four degrees. Those surface molecules would then form a thin layer of ice. The top layer of ice acts as an insulator from the cold for the water below it, but slowly the energy will drain from the water causing a thicker layer of ice to form (remember this energy being "drained" is just the molecules slowing down). At some point, if the bucket is big enough, a thick enough layer of ice will form that it insulates the water below, keeping it from freezing, even though the outside temperature may be very cold. This is how many fish survive in frozen lakes--if the lake is deep enough it won't freeze through, even in Alaska. There are other factors as well, keeping the deeper water liquid--for example pressure. The pressure on the water below from the water and ice above keeps it from going into a less dense state like ice.

Without hydrogen bonding, however, the low density lattice structure would never form. Accordingly, when cycle of the surface water cooling and then sinking would continue below four degrees Celsius. Eventually, as the molecules became sufficiently slow, the water would feel thicker to the touch, and as it got colder, it would get thicker still--much like oil. Eventually, the water would become thick enough that it would all be hard, but since there would be no lattice structure, the ice would not form from the top down, it would form relatively uniformly throughout--because of the constant circulation of the densest water going to the bottom.

See these helpful links for more in-depth information regarding the topics discussed in this blog:
All about the intracacies of the water molecule
Showing how strong the hydrogen bond can really be (a tribute to the power of the hydrogen bond!)
Wikipedia article on water
Why is water blue? (discussion of hydrogen bonding's role in determining the color of water)
Everything--seriously everything--about water (this is a great site covering a wide variety of topics concerning water in depth)