Hooks and eyes: That was the description of bonds between atoms that Pauling learned when he was an undergraduate at the Oregon Agricultural College. Each atom had a certain number of hooks that allowed it to attach to other atoms, and a certain number of eyes that allowed other atoms to attach to it. A chemical bond (a bond between atoms that held them together as molecules) resulted when a hook and eye connected.

This described a bond, but it explained very little. At least it took into account the century-old ideas of the great English chemist John Dalton, who in the early nineteenth century theorized that atoms — which came in distinct sizes, called elements — combined with other atoms in simple whole-number proportions to form larger molecules. As is known today, two hydrogen atoms and one oxygen atom join to form water, four hydrogens and one carbon form methane, and so forth. The element’s combining capacity or "valence" — its number of hooks and eyes — was set somehow by nature. But no one knew why elements combined in just these proportions, or what forces held them together.

Linus Pauling intended to solve these mysteries by applying the new physics he had learned in Europe.