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 cubbyholes.
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.