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
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.