Then Pauling made a daring leap. In the spring of 1928 he wrote a brief note to the
Proceedings of the National Academy of Sciences in which he outlined what he called
Heitler and London’s "simple theory" of the chemical bond and noted that it was, "in
simple cases entirely equivalent to G. N. Lewis’s successful theory of the shared
electron pair." Nothing much new there. But at the end of the note, in a single paragraph,
he announced a significant advance. His calculations, he said, showed that quantum
mechanics could explain the tetrahedral binding of carbon.
This woke readers up. Carbon was a much-studied element, the linchpin of all organic
chemistry. Strings of carbon atoms formed the backbone of proteins, fats, and starches–the
major constituents of living systems. Carbon chemistry was the chemistry of life.
Carbon was also the subject of a debate between physicists and chemists. It was known
that each carbon atom carried a total of six
electrons, the first two of which had nothing to do with forming bonds; they paired
in a stable two-electron inner shell. The remaining four electrons should be, in theory,
at the next energy level, in the next higher shell of the atom.
But physicists’ studies showed that carbon’s four binding electrons actually existed
in two slightly different energy levels, or subshells. The two lower-shell electrons
should pair with each other, the physicists said, leaving only the remaining two in
the next subshell to bond to other atoms. Carbon, the physicists argued, should have
a valence of two.
Chemists understood, however, that carbon more commonly offered four strong bonds
to other atoms, creating the shape of a three-sided pyramid, or tetrahedron. Carbon
had a valence of four. This was the structure of methane, for instance: four hydrogen
atoms at the corners of a tetrahedron with a carbon atom at the center.
The physicists’ evidence was undeniable, as was the chemists’. Both groups somehow
had to be right. Reconciling the physicists’ carbon and the chemists’ was a major
challenge, and Pauling was determined to meet it.