Born on 7 June 1896 in Newburyport, Massachusetts, Robert Mulliken was a physical chemist who earned a Nobel Prize for his groundbreaking work on molecular structure. Mulliken studied chemistry at MIT, earning his BS in 1917. His studies were interrupted by World War I, during which he worked on poison gases for the US government. He then enrolled in graduate school at the University of Chicago, where he earned his PhD in 1921. Mulliken taught briefly at New York University before returning to the University of Chicago, where he would join the faculty in 1928 and remain until 1985, long after his official retirement in 1961. It was in the 1920s that Mulliken turned his research focus to molecular spectra and chemical bonding. By 1928 he had solidified his molecular orbital (MO) theory, in which he proposed that the valence electrons of a molecule are spread in orbitals over the entire molecule rather than being confined to particular atoms. Because of its flexibility and applicability to more types of molecules, MO theory would eventually win out over the competing valence bond method to explain chemical bonding in quantum mechanical terms. In recognition of this work, Mulliken was elected to the National Academy of Sciences in 1928 at age 32, the youngest member ever at that time, and he would go on to receive the Nobel Prize in Chemistry in 1966. During World War II, Mulliken researched plutonium in the University of Chicago’s Metallurgical Laboratory, which was part of the Manhattan Project, and in 1952 he created the University of Chicago’s Laboratory of Molecular Structure and Spectra, which became a world-famous research center for theoretical chemistry and electronic spectroscopy. He was a prolific writer, publishing several monumental series of articles on molecular science over a nearly 60-year period, and he was also an early proponent of computer technology for uncovering molecular properties. Over his career, Mulliken received a number of honorary degrees and awards and was a member of numerous academic and professional societies. He died in 1986 at age 90. (Photo credit: AIP Emilio Segrè Visual Archives, Physics Today Collection)
