|A Different Approach
Robert Mulliken was one of the few researchers who, like Pauling, knew both physics
and chemistry. Mulliken, the son of an MIT chemist, roomed next to Slater while on
a fellowship at Harvard and, also like Pauling, made a pilgrimage to Europe in the
late 1920s to learn quantum mechanics.
At Göttingen, Mulliken had come under the influence of one of Born's assistants, Friedrich
Hund, who was thinking through an approach to the chemical bond different from Pauling's.
Hund was interested in molecular spectroscopy, the study of the characteristic light
absorbed and emitted by molecules, and he found that viewed this way, molecules behaved
in important ways like individual atoms. Hund and Mulliken came up with a concept
of the chemical bond that seemed radically different from Pauling's. Instead of electrons
concentrating between two nuclei to bind atoms together, Hund and Mulliken theorized
that binding electrons were spread around the molecule's surface, forming what Mulliken
would call molecular orbitals. They conceived of the hydrogen molecule, H2, for instance, not as two hydrogen atoms approaching each other and forming a bond
by pairing their electrons, as Heitler and London had proposed, but as a two-electron
helium atom splitting into two nuclei with its surrounding electron cloud reshaping
into a new molecular orbital.
"In general no attempt is made to treat the molecule as consisting of atoms or ions,"
Mulliken wrote in 1932. "Attempts to regard a molecule as consisting of specific atoms
or ionic units held together by discrete numbers of bonding electrons or electron
pairs are considered as more or less meaningless." This was radical thinking; the
molecular-orbital concept seemed diametrically opposed to everything chemists had
thought about the nature of the chemical bond for decades. It did, however, fit the
spectroscopic data, and Mulliken stuck with his ideas after returning to the United
States to teach at the University of Chicago.