Friedrich Hund

Friedrich Hermann Hund was born on the fourth of February 1896 in Karlsruhe, a Baden city already known for the technical institute where Heinrich Hertz had detected the first radio waves a decade before. His parents pushed him toward a teacher’s training in mathematics, physics, and geography, three subjects he would still be writing about seventy years later. He took his early studies at Marburg, then walked north to Göttingen in time for the moment that small university town stopped being a quiet algebra outpost and became, almost overnight, the second capital of the new quantum theory.

The first capital was Copenhagen, where Niels Bohr held court. The second was the seminar room of Max Born. Born had been recruited to Göttingen in 1921 and was assembling the most extraordinary group of young theorists in Europe: Werner Heisenberg, Pascual Jordan, Wolfgang Pauli passing through, and a steady current of visiting Americans, Italians, and Russians. Hund joined them in 1925, the year Heisenberg invented matrix mechanics on Helgoland and Born, with Jordan, wrote it down in proper form. Hund became Born’s assistant. His first project, set by Born himself, was the quantum interpretation of the band spectra of diatomic molecules, the dense ladder of lines that small molecules like nitrogen or hydrogen radiate when excited. It was an unglamorous assignment. Atomic spectra had captured the public imagination. Molecular spectra were a thicket. But the thicket was where the new mechanics had to prove it could handle more than a single electron orbiting a single proton.

Hund worked with prestigious physicists, as Erwin Schrödinger, Paul Dirac, Werner Heisenberg, Max Born, and Walther Bothe. At that time, he was Born's assistant, working with quantum interpretation of band spectra of diatomic molecules. After his studies of mathematics, physics, and geography in Marburg and Göttingen, he worked as a private lecturer of theoretical physics in the University of Göttingen (1925), professor in the University of Rostock (1927), Leipzig University (1929), University…

In 1926 Hund travelled to Copenhagen and spent the winter with Bohr. Bohr’s institute on Blegdamsvej was the place where the philosophical implications of the new mechanics were being argued out in real time, often over evening pipes after dinner. Hund returned to Göttingen with the sense that the new theory was not merely a calculational scheme but a rebuilding of the concept of matter from the bottom up. He habilitated in 1927 with a treatise on line spectra and the periodic system of the elements that contained, almost in passing, the three statements every chemistry undergraduate now learns as Hund’s rules.

The first rule, the rule of maximum multiplicity, is the one that mattered. It said something the empirical chemists had long suspected but could not derive: when electrons fill a degenerate set of orbitals (the three 2p orbitals of carbon, say, or the five 3d orbitals of iron) they fan out into separate spatial states with parallel spins before they pair up. Carbon’s ground state is a spin triplet, not a singlet. The rule explained the magnetism of iron and the colour of transition-metal complexes and the bond angles of the methyl radical. It also explained, more deeply, why the world we touch is built from many-electron atoms whose ground states are not just sums of one-electron wavefunctions. The exchange interaction had a sign and Hund had pinned it down.

The same year, 1927, he made a discovery the textbooks were slower to credit him for. While analysing the inversion spectrum of ammonia he realised that a nitrogen atom sitting above the plane of three hydrogens could pass through the plane to the equivalent position below without ever having enough energy to do so classically. The barrier was real but the particle could be found on the other side. He was the first to write down what is now called quantum tunnelling, eight months before George Gamow used the same idea to explain alpha decay. The mechanism had a strange consequence that he immediately recognised: chiral molecules such as sugars or amino acids should not be stable as left-handed or right-handed forms. They should tunnel between the two and live as superpositions. They do not. Hund worked out why. For molecules above a certain size the tunnelling time stretches to longer than the age of the universe and the question is academic. His paradox of chirality is still discussed in the foundations literature today.

Hund left Göttingen in 1927 to take a professorship at Rostock, then Leipzig in 1929. At Leipzig he overlapped with Heisenberg, who had taken the chair there, and with the American visitor who would become his closest scientific partner: Robert S. Mulliken. The two men, working independently but in constant correspondence, built what is now called the molecular orbital theory. The competing valence-bond approach, championed by Pauling, drew bonds as shared electron pairs localised between atoms. Hund and Mulliken drew the bonds as delocalised one-electron orbitals spread over the whole molecule, filled in order of energy by the same exclusion principle that governed atoms. The MO picture was harder to teach but it explained things the valence-bond picture struggled with: the paramagnetism of molecular oxygen, the spectrum of butadiene, the conductivity of graphite. Mulliken received the 1966 Nobel Prize in Chemistry for the theory. In his Nobel lecture he said publicly what he had said in private letters for decades, that Hund’s work had shaped his own and that he would gladly have shared the prize.

Hund married mathematician Ingeborg Seynsche (1905–1994) in Barmen on 17 March 1931. The family had six children: chess player and mathematician Gerhard Hund (1932–2024), Dietrich (1933–1939), Irmgard (b. 1934), Martin (1937–2018), Andreas (b. 1940) and Erwin (1941–2022). The chess woman grandmaster Barbara Hund (b. 1959) and chess player Isabel Hund (b. 1962) are his…

Hund stayed in Germany through the Hitler years. Unlike Born, who was Jewish and forced out, Hund kept his chair through the war by keeping his head down and his politics private. He moved to Jena in 1946, then Frankfurt, and finally returned to Göttingen in 1957 to take the chair Born had once held. In 1931 he had married the mathematician Ingeborg Seynsche and they raised six children, one of whom, Gerhard, became a chess master and the chronicler of his father’s papers. Two of his granddaughters, Barbara and Isabel, played chess at international level. The family was bookish in the German bourgeois manner, with mathematics threading through three generations.

He kept publishing into his eighties. His two-volume history of the concepts of physics, written in the 1960s and 1970s, is still in print. He became an honorary citizen of Jena, had a street and a square named after him, and saw the Institute for Theoretical Physics at Göttingen renamed Friedrich-Hund-Platz 1 in his honour. He died in Göttingen on the thirty-first of March 1997, six weeks past his hundred and first birthday, having outlived every other founder of quantum mechanics. He had personally witnessed the entire arc from Planck’s 1900 paper, which appeared when he was four years old, to the first commercial scanning tunnelling microscopes, which exploited the effect he had named.

For the quantum story he stands at the hinge where atoms became molecules. The rules that bear his name explain why iron is ferromagnetic, why your blood is red, why the carbon in your bones forms four bonds rather than two. The molecular orbital theory he built with Mulliken is the language in which every modern computational chemist describes a reaction. And the tunnelling effect he wrote down in 1927 is the mechanism by which a transistor switches, an enzyme catalyses, and a star burns.