Atoms combine into molecules because by doing so they achieve a state of lower energy. Making molecules from atoms is a "downhill" process, and tearing molecules apart again into atoms always requires energy to go back up the energy hill. We usually can think of molecules as being held together by bonds between pairs of atoms within them. A key question in chemistry is: Which atoms will combine with one another, in what way, and why?

At the beginning of this century chemical bonding was still a mystery. One of the triumphs of quantum mechanics, a shatteringly unorthodox theory developed between 1900 and 1926, was the successful explanation not only of atomic structure, but of bonding between atoms in molecules.

We can take some of the pictorial conclusions from quantum mechanics and use them to predict the behavior of atoms in molecules, without becoming involved in the mathematics. This is done in Chapters 7-9.