Pauling was now using the term "resonance" in place of "electron exchange" when writing
about the chemical bond, and he was expanding the concept into new areas. Heisenberg
had used the electron-exchange idea to account for the interchangeability of electrons;
Heitler and London had used it to explain the covalent chemical bond; Pauling and
Slater employed it to account for the energy needed to form hybrid bonds like those
in the tetrahedral carbon atom. Now Pauling proposed that when certain criteria were
met, resonance could exist between the ionic and covalent forms of a molecule. Hydrogen
chloride, for example, could be viewed either as a hydrogen linked to a chlorine atom
through a purely covalent bond or as a positively charged hydrogen ion and a negatively
charged chloride ion linked with a purely ionic bond. The actual molecule, Pauling
proposed, is a sort of hybrid, a structure that resonates between the two alternative
extremes. And whenever that happened, "whenever there is resonance between the two
forms," Pauling said, "the structure is stabilized."
For Pauling, the entire chemical landscape now began to shift. Resonance, he realized
excitedly, could be applied as well to the relationship between single and double
bonds — they did not have to be one or the other but could resonate between the two
forms, leading to a stabilized partial double bond with its own peculiar properties.
Resonance explained all kinds of structures that didn't fit into the old classical
Virtually all of chemistry could be reevaluated in the light of this new idea, and
Pauling set about doing it through the early 1930's. By applying his resonance ideas
to various types of chemical bonds, then cross-checking and amending his theoretical
results to fit what was known empirically about bond lengths and strengths, Pauling
was able to produce a string of papers that set chemistry on a new course.