Pauling's reputation grew with each paper. In the spring of 1932 he took up Slater's
invitation to become a visiting professor for a term at MIT. His eastern visit introduced
a number of figures in the Harvard-MIT chemical establishment to his new ideas, and
he continued working on them nonstop between lectures and dinner parties. He had been
thinking of ways to estimate the relative contributions of ionic and covalent bonds
to any molecule, developing with a fellow Caltech faculty member Don Yost a system for estimating the theoretical strength of pure covalent bonds. With his
new numbers in hand, Pauling could now compare his theoretical numbers to the real
behavior of different pairs of elements as they formed compounds. The real-world bonds
were always stronger than predicted — an added strength, Pauling assumed, that arose
from the stabilizing effect of resonance with an ionic form of the bonds. The greater
the deviation, the more ionic character the bond had and the more the two elements
differed in their ability to attract electrons. Using this system, it was now possible
to answer old questions such as whether hydrochloric acid, HCl, was an ionic or covalent
compound. He discovered that it was both, in the ratio 20:80.
A greater ionic character meant that one of the atoms involved in the bond had a much
stronger ability to attract electrons than the other. He began mapping the electron-grabbing
ability of atoms on a scale. Fluorine, for example — the most electron-hungry of
all elements — was at the far end of the scale. Lithium was toward the other. The
bond in the compound they formed, lithium fluoride, was almost 100 percent ionic.
Iodine was somewhere toward the middle of Pauling's scale, and the lithium iodide
bond therefore had more covalent character. By comparing a number of such pairs, he
was able to map a relative property he called electronegativity and assign values
to various elements. These values in turn could be used to predict the bond type and
strength in many molecules, including those, for which no experimental data were available.
Between the lectures at MIT, he wrote up his ideas in a paper that would become "The
Nature of the Chemical Bond IV. The energy of single bonds and the relative electronegativity
of atoms," finishing it just a few days before boarding the train back to Pasadena.