Robert Sanderson Mulliken (1896-1986), an American chemist, was awarded the 1966 Nobel Prize in chemistry for his molecular-orbital theory, which explains how atoms combine to form a molecule. Most scientists now accept Mulliken's theory that the electrons in the outermost shell of a molecule orbit the entire molecule rather than the individual atoms in the molecule. Scientists engaged in biological and industrial research have used Mulliken's theory to study the structure of proteins, plastics, and other complex compounds.

Mulliken was the son of Samuel Parsons Mulliken, a distinguished professor of organic chemistry at the Massachusetts Institute of Technology (MIT). While still in high school, Robert became familiar with the names of organic compounds, and learned much about organic chemistry in general, by proofreading galleys of his father's textbooks, which became classics in the field.

In 1913, the year Mulliken graduated from Newburyport High School, Danish physicist Niels Bohr published his model of the atom, in which electrons orbited the nucleus in shells. The title of Mulliken's salutatory address, delivered at his graduation ceremony, was “The Electron: What It Is and What It Does.” Mulliken would devote much of his future research to the electron's role in molecular spectra and structure. Ultimately he extended Bohr's atomic model to molecules, which he pictured as formed by electrons moving in orbits that encircle all the nuclei of the constituent atoms.

Mulliken attended MIT and graduated with a B.Sc. in chemistry in 1917. The United States had recently entered World War I (1914–1918), and Mulliken took a wartime job with the U.S. Bureau of Mines as a junior chemical engineer, doing research on poison gases at the American University in Washington, D.C., under the direction of James Bryant Conant, who was later to be president of Harvard. Following the war, he worked briefly as an assistant in rubber research at the New Jersey Zinc Company.

In 1919, Mulliken began graduate work in chemistry at the University of Chicago. After he earned his Ph.D. in 1921 with a dissertation on the partial separation of mercury isotopes, he remained at Chicago until 1923 as a National Research Council (NRC) fellow. During this period, he extended his research on mercury isotopes. He built an “isotope factory,” the first of its kind. This apparatus was based on the different behavior of isotopes under the processes of evaporation and diffusion through a membrane.

In 1923, Mulliken left the University of Chicago for Harvard University's Jefferson Physical Laboratory as a National Research Council fellow. There he helped physicists Edwin Crawford Kemble and Raymond Thayer Birge prepare their 1926 report for the NRC on the spectra of diatomic molecules. In the course of his experimentation, Mulliken identified a new molecular fragment, boronvmonoxide, and introduced the concept of zero point energy, an energy in molecules at the temperature of zero degrees Kelvin, which was soon incorporated into the new quantum mechanics. His studies of the spectral lines of diatomic molecules at Harvard contributed to their classification into families and to an understanding that similar electronic structures were related to corresponding energy level systems.

In 1926, Mulliken, who was trained as a chemist, joined the department of physics at New York University (NYU). As head of a productive research team at NYU, Mulliken became a leader in the diagnosis of band spectra. This name was given to the spectra of molecules because the absorption into the continuous spectrum was broad, like a band, instead of narrow, as the spectral lines of atoms are. During this time, he became friends with German theoretical physicist Friedrich Hund, who was studying molecular structure in terms of quantum mechanics. Hund's work provided theoretical support for Mulliken's hypothesis that molecular formation could result in drastic changes in electronic quantum numbers. In 1928, Mulliken introduced his theory, which proposed a new model for molecular structure. Previously, scientists had believed that the atoms bonding together to form a molecule retained their independent characteristics. Mulliken proposed that the electrons that once belonged to one or another atom became part of an overall molecular structure and lost their atomic identity. After explaining the assignment of quantum numbers for electrons in molecules, Mulliken went on to correlate molecular and atomic electron states. This work would eventually lead to Mulliken's idea of “molecular orbitals.”

Also in 1928. Mulliken joined the faculty of the University of Chicago, first in the physics department and later in chemistry. Except for Guggenheim fellowships in Germany and Europe (1930, 1932–1933), a Fulbright fellowship at Oxford (1952–1953), and an appointment as the scientific attaché to the U.S. embassy in London (1955), he spent most of his career at Chicago.

Mulliken not only played an instrumental role in defining the basic concepts and methods used to study molecular structure, but also helped develop its notation and shape its language. He developed correlation diagrams to relate the state of the molecule with the separated atoms and the united atom descriptions. The importance of these diagrams for classifying diatomic molecules was later compared to that of Dmitri Ivanovich Mendeleev's periodical table for classifying atoms. Mulliken was instrumental in securing two international agreements on molecular notation, in 1930 for diatomic molecules and in 1955 for polyatomic molecules.

During World War II (1939–1945), Mulliken was the director of editorial work and information for the University of Chicago's Plutonium Project, a division of the U.S. government's Manhattan Project, the secret research program to produce an atomic bomb. After the war, he resumed his experimental and theoretical analyses of molecular structure and spectra. He developed the charge-transfer interpretation of spectra of donor-acceptor molecular complexes, calculated spectral intensities, explained the selection rules characterizing transitions in molecular spectra, and worked out the theory of hyperconjugation in organic molecules as well as the concept of population analysis. In 1952, he used quantum mechanical theory to analyze the interaction between Lewis acid and base molecules. In the 1960's, he studied the structure and spectra not only of molecular hydrogen, helium, nitrogen, and other small molecules, but also of complex molecular aggregates.

Mulliken was elected to membership in the National Academy of Sciences in 1936 and was also a member of the American Academy of Arts and Sciences, the American Chemical Society, and the American Philosophical Society; a foreign member of the Royal Society of Great Britain; an honorary member of the Société de Chimie Physique; and a corresponding member of the Royal Society of Science of Liége. He was also a fellow of the American Physical Society and the American Association for the Advancement of Science, and an honorary fellow of the Chemical Society of Great Britain and the Indian National Academy of Science.

In 1965, Mulliken began spending part of each year as Distinguished Research Professor of Chemical Physics at the Institute of Molecular Biophysics at Florida State University in Tallahassee. In addition to the Nobel Prize, awarded in 1966 for his “fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular orbital method,” Mulliken received five awards from the American Chemical Society during the 1960's: the G. N. Lewis Medal, the Theodore William Richards Medal, the Peter Debye Award, the John Gamble Kirkwood Award, and the J. Willard Gibbs Medal.