The first type of bond we discussed in the previous section, where the electron leaves the first atom and joins the second, is called an ionic bond. We see this kind of bond in molecules like sodium chloride (salt), where an atom that easily loses its outer electron (such as sodium) combines with an atom that has an empty slot very close to the nucleus (like chlorine) that is very good at attracting electrons.
The second type we discussed, where two nuclei share an electron, is called a covalent bond. These are very strong bonds, and hold molecules together very well. Bonds between carbon atoms, or between carbon and hydrogen are usually covalent bonds.
The third type of bond we discussed is called a metallic bond, because it is characteristic of the bonds we see in metals.
In all three of these bond types, we are dealing with bonds where one or more electrons is involved. There are other kinds of bonds, usually much weaker than the first three, which form when, on average, less than a whole electron is involved.
We looked earlier at the hydrogen bond, which forms between molecules, rather than between individual atoms. Molecules are required because this kind of bond only happens when an electron spends more of its time around one atom in the molecule than around another atom. This makes one side of the molecule a little more negative, and the other side a little more positive. The positive ends of one molecules are then attracted to the negative ends of another, to make the hydrogen bond.
A similar set of effects, collectively called van der Waals forces (named after the Dutch scientist Johannes Diderik van der Waals) can happen even to single atoms. The electrons around two atoms can become correlated, so that when an electron in the first atom is on one side, so is an electron on the other atom, and when the electrons spin around the atom, they do so in synchrony, so that there is always a positive side of one atom facing the negative side of the other, creating an attraction.