Mulliken , Robert Sanderson

(1896–1986) American physicist and chemist

Mulliken was born in Newburyport, Massachusetts, the son of an organic chemist. He was educated at the Massachusetts Institute of Technology and at the University of Chicago, where he obtained his PhD in 1921. After working briefly at Washington Square College, New York, he was appointed to the staff of the University of Chicago, where he served as professor of physics from 1931 until he retired in 1961. From 1961 he was Distinguished Service Professor of Physics and Chemistry at Chicago and Distinguished Research Professor of Chemical Physics at Florida State University.

It was Mulliken, in 1922, who first suggested a method of isotope separation by evaporative centrifuging. Most of his research career was concerned with the interpretation of molecular spectra and with the application of quantum theory to the electronic states of molecules.

Mulliken, with Friedrich Hund, developed the molecular-orbital theory of chemical bonding, which is based on the idea that electrons in a molecule move in the field produced by all the nuclei. The atomic orbitals of isolated atoms become molecular orbitals, extending over two or more atoms in the molecule. He showed how the relative energies of these orbitals could be obtained from the spectra of the molecule.

Mulliken's approach to finding molecular orbitals was to combine atomic orbitals (LCAO, or linear combination of atomic orbitals). He showed that energies of bonds could be obtained by the amount of overlap of atomic orbitals.

Another of Mulliken's contributions is the application of electronegativity – the ability of a particular atom in a molecule to draw electrons to itself. He showed that this property was given by the formula ½(I+E), where I is the ionization potential of the atom and E is its electron affinity.

He also made major contributions to the theory and interpretation of molecular spectra. In 1966 he was awarded the Nobel Prize for chemistry for “his fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular-orbital method.”