One long-standing puzzle in chemistry concerned the relationship between two seemingly different types of bonds between atoms, ionic and covalent. In Lewis's scheme, the bond was covalent when two atoms shared a pair of electrons equally. When one electron-hungry atom pulled the entire electron pair to itself, resulting in a net negative charge on one atom and a positive charge on the other, the result was an ionic bond based on electrostatic attraction between the two atoms. The question was whether ionic or covalent bonds were separate species with a sharp dividing point or merely, as Lewis thought, points along a continuum.

In Pauling's third "Nature of the Chemical Bond" paper, published in early 1932, he showed that quantum mechanics again supported Lewis. Pauling’s equations in "The Nature of the Chemical Bond III. The transition from one extreme bond type to another" showed that intermediate "partial ionic" bonds, links with both ionic and covalent characteristics, were compatible with both quantum mechanics and observed properties. Bonds were not either/or; they could show the characteristics of both types of bond. In other cases he found that the jump between bond types could be discontinuous; it depended on how strongly the elements involved attracted the electrons. He backed up his arguments with a number of real-world examples and a set of conditions necessary for such intermediate bonds to form.