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.
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