Friedrich Hund: A Century of Quantum Science

Introduction

Friedrich Hermann Hund (1896-1997) was a German physicist who lived for 101 years and took part in the whole story of quantum mechanics, from the early days of the old quantum theory to the era of computers and molecular modelling. He is remembered above all for Hund's rules, which tell us how electrons fill orbitals, for the discovery of quantum tunnelling in molecules, and for helping to create molecular orbital theory with Robert Mulliken. His legacy in one line: every student who draws electrons as parallel arrows in separate orbitals is using his rule.

Early Life and Family

Hund was born on 4 February 1896 in Karlsruhe, in the German state of Baden. He showed an early talent for mathematics and science at school. The First World War broke out when he was a young man, and his studies were affected by the events of the time.

Education and Mentors

Hund studied mathematics, physics and geography at the University of Marburg and then at the University of Göttingen, which at that time was one of the world's greatest centres of mathematics and physics. At Göttingen he became a student of Max Born and also learned from James Franck and the mathematicians around David Hilbert. He received his doctorate in 1922 and became Born's assistant. His early work dealt with the structure of atomic spectra. In 1926-1927 he spent time in Copenhagen with Niels Bohr, which brought him into the centre of the new quantum mechanics.

Career Timeline

Major Scientific Contributions

• Hund's rules (1925-27): rules for the lowest-energy term of an atomic configuration, based on spin multiplicity, orbital angular momentum and spin-orbit coupling.
• Quantum tunnelling (1927): in the study of molecules with a double-well potential, he showed how a particle can pass through a barrier. He used this to discuss the stability of chiral molecules and the inversion of ammonia.
• Molecular orbital theory: with Mulliken, he developed the way of classifying electrons in diatomic molecules through quantum numbers and symmetry. The approach is sometimes called the Hund-Mulliken theory.
• Hund's coupling cases: a scheme for how electronic and rotational angular momenta combine in diatomic molecules, still used in spectroscopy.
• History of physics: a book on the history of quantum theory, which became a respected reference.

The details of these topics are available in our companion post on Hund's rule.

Awards, Honours and Nobel Prize

Hund did not receive a Nobel Prize, although many physicists and chemists have said that his ideas deserved wide recognition. His honours included the Max Planck Medal (1943) of the German Physical Society and membership of several scientific academies. Honorary doctorates from German and foreign universities came late in his long life.

Personality, Struggles and Lesser-Known Facts

• Remarkably long career. He stayed intellectually active into extreme old age and kept writing on physics and its history long after retirement.
• Scientific nomad. Between the 1920s and 1950s he held posts in Rostock, Leipzig, Jena, Frankfurt and Göttingen, in a career shaped by the turbulent decades of twentieth-century Germany.
• Quiet research temperament. He did not seek publicity. His rules came from careful work with spectra, and he often said that his role was to find patterns in the data.
• Contributor to understanding spin. Alongside Pauli, Heisenberg and others, he helped the community understand how electron spin controls atomic structure.
• Family. He married Ingeborg Seynsche in 1931, and they had several children.
• Tunnelling in everyday science. The same tunnelling idea he found in 1927 was later used in alpha-decay theory by Gamow and in modern devices such as the scanning tunnelling microscope.

Legacy and Influence on Modern Chemistry

Hund's name is mentioned in every general chemistry course and in advanced courses on spectroscopy and magnetism. His tunnelling idea is a basis for understanding hydrogen transfer in enzymes, proton tunnelling in chemical reactions, and the chiral stability of molecules. His coupling cases are still used to interpret molecular spectra in laboratories, from the study of interstellar molecules to analysis of diatomic radicals in flames. Molecular orbital theory, which he helped to found, is now the standard method of computational chemistry.

Lessons for Students

1. Look for patterns in data. Hund's rules came from sorting out spectra.
2. Move between atoms and molecules. Ideas learned for one often help the other.
3. Do not wait for a prize. A rule used by millions is a reward in itself.
4. Stay curious for life. He worked and wrote for decades after retirement.
5. Understand the history of your subject. It gives context to the formulas.

FAQ

What is Hund's rule in simple words?

Electrons fill degenerate orbitals one at a time with parallel spins, and only then begin to pair up.

Who was Hund's teacher?

Max Born at Göttingen, and he also worked with Niels Bohr in Copenhagen.

How is Hund linked to Mulliken?

The two developed the molecular orbital approach, and the Hund-Mulliken theory is a name for it.

How long did Hund live?

He lived for 101 years, from 1896 to 1997.

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