Robert S. Mulliken

A crucial grant allowed Robert to pursue his research into isotope separation and band spectra, which led him to Harvard University. He then embarked on a transformative journey to Europe in 1925 and 1927. He met the giants of quantum mechanics—Erwin Schrödinger, Paul A. M. Dirac, Werner Heisenberg—but it was Friedrich Hund, working on quantum interpretations of diatomic molecules, who truly shaped his path.

Working with Hund in 1927, Robert developed his molecular orbital theory. This new approach assigned electrons to states that stretched across an entire molecule, fundamentally changing how scientists understood chemical bonds. The theory became so intertwined with their collaboration that it was often called the Hund-Mulliken theory.

Before their work, the Valence-Bond method, popular because it mirrored chemists’ ideas of localized bonds, was the standard. But it often struggled with molecules in excited states. Robert’s molecular-orbital method, describing electron wave functions as delocalized and symmetrical with the molecule itself, proved more versatile. It could be applied to a wider array of molecules and fragments, eventually eclipsing its predecessor.

This deep theoretical insight, which revealed how atoms bond at their most fundamental level, earned Robert Mulliken the Nobel Prize in Chemistry in 1966. He also established a new scale for measuring electronegativity in 1934, defining it as the average of an atom's ionization enthalpy and electron affinity, a concept that closely corresponded with Linus Pauling’s earlier work.

Robert continued his rigorous studies of molecular structure throughout his life, applying quantum mechanics to acid-base reactions and exploring everything from simple diatomic molecules to complex aggregates. In 1929, he married Mary Helen von Noé, with whom he had two daughters. He retired in 1985, a year before his passing.

Robert Mulliken died at 90 years old in Arlington County, Virginia, and was laid to rest in Chicago, the city where his scientific journey truly blossomed.