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)
