Friedrich Hund (1896–1997) was a titan of 20th-century physics whose life spanned the entirety of the quantum revolution. While perhaps less of a household name than his contemporaries like Heisenberg or Bohr, Hund’s work provides the literal "rules" by which we understand the electronic structure of atoms and the formation of chemical bonds. His career was defined by an extraordinary longevity, both in his physical life—living to 101—and in the enduring relevance of his scientific contributions.
1. Biography: A Century of Science
Friedrich Hermann Hund was born on February 4, 1896, in Karlsruhe, Germany. His academic journey began at the universities of Marburg and Göttingen, where he studied mathematics, physics, and geography.
Formative Years
At Göttingen, Hund studied under the legendary Max Born. He completed his doctorate in 1922 with a dissertation on the anomalous Zeeman effect.
The Quantum Hub
In the mid-1920s, Hund served as Born’s assistant, placing him at the epicenter of the development of quantum mechanics. He spent significant time at Niels Bohr’s Institute in Copenhagen (1926), where the "Copenhagen Interpretation" was being forged.
Academic Trajectory
Hund held a series of prestigious professorships across Germany:
- Rostock (1927): Where he was appointed professor at the young age of 31.
- Leipzig (1929–1946): Collaborating closely with Werner Heisenberg.
- Jena (1946–1951): Serving in the post-war Soviet occupation zone.
- Frankfurt (1951–1956): Moving to West Germany during the height of the Cold War.
- Göttingen (1957–1997): Returning to his roots, where he remained an emeritus professor until his death.
2. Major Contributions: The Rules of the Atom
Hund’s contributions are fundamental to both physics and chemistry, bridging the gap between abstract quantum equations and the tangible behavior of elements.
- Hund’s Rules (Atomic Spectroscopy): Every chemistry student encounters "Hund’s Rules," which describe how electrons fill atomic orbitals. The most famous, the Rule of Maximum Multiplicity, states that electrons will occupy empty orbitals of the same energy singly (with parallel spins) before they start pairing up. This minimizes electron-electron repulsion and explains the magnetic properties of many elements.
- Molecular Orbital (MO) Theory: Along with Robert Mulliken, Hund developed the Hund-Mulliken theory, now known as MO theory. Unlike the earlier "Valence Bond" theory, MO theory posits that electrons are not tied to individual bonds but move under the influence of the nuclei in the whole molecule. This remains the dominant framework for computational chemistry today.
- Quantum Tunneling: In 1927, Hund was the first to describe the phenomenon of quantum tunneling—the ability of a particle to pass through a potential energy barrier that it classically should not be able to cross. He discovered this while studying the "inversion" of the ammonia molecule.
- Hund’s Cases: He classified the various ways in which the angular momenta (spin and orbital) of electrons in a diatomic molecule can couple together. These five "Hund’s Cases" are essential for interpreting molecular spectra.
3. Notable Publications
Hund was a prolific writer, producing over 250 papers and several influential books that shaped the pedagogical landscape of physics.
- Linienspektren und periodisches System der Elemente (Line Spectra and the Periodic System of the Elements, 1927): This seminal work applied the new quantum mechanics to the periodic table, providing a theoretical foundation for chemical periodicity.
- Zur Deutung der Molekelspektren (On the Interpretation of Molecular Spectra, 1927–1930): A series of papers that laid the groundwork for molecular orbital theory.
- Geschichte der Quantentheorie (History of Quantum Theory, 1967): Written in his later years, this remains a definitive historical account of the quantum revolution by someone who witnessed it firsthand.
4. Awards and Recognition
Despite his foundational contributions, Hund never received the Nobel Prize—an omission often cited by historians of science (his collaborator Robert Mulliken received the Nobel in Chemistry in 1966 for their joint work on MO theory). However, his accolades were numerous:
- Max Planck Medal (1943): The highest award of the German Physical Society.
- Otto Hahn Prize for Chemistry and Physics (1974).
- Honorary Degrees: Awarded by the universities of Frankfurt, Uppsala, and Cologne.
- Honorary Citizenship: He was made an honorary citizen of Jena on his 100th birthday in 1996.
5. Impact and Legacy
Friedrich Hund is often called the "Father of Molecular Orbital Theory." His legacy is embedded in the very language of modern science:
- Chemistry Education: His rules for electron configuration are a cornerstone of secondary and tertiary science education worldwide.
- Computational Chemistry: Modern drug discovery and materials science rely on Molecular Orbital calculations, a field Hund pioneered.
- The "Hund-Mulliken" vs. "Heitler-London" Debate: His work sparked the great intellectual rivalry between Molecular Orbital theory and Valence Bond theory, a tension that drove chemical physics forward for decades.
6. Collaborations
Hund’s career was a "who’s who" of 20th-century physics:
- Max Born: His mentor and supervisor, who taught him the rigorous mathematical framework of quantum mechanics.
- Robert S. Mulliken: His most significant collaborator. Though they worked on different continents (Mulliken in the US, Hund in Germany), their independent and joint efforts created the MO model.
- Werner Heisenberg: During their time together in Leipzig, they made the city a global center for theoretical physics, attracting students from around the world.
- Niels Bohr: Hund’s time in Copenhagen allowed him to refine his ideas on the "building-up principle" (Aufbauprinzip) of the periodic table.
7. Lesser-Known Facts
- The Centenarian Scientist: Hund remained intellectually active well into his 100s. He famously gave a lecture on the history of physics at the age of 100, standing for the entire hour.
- The "Hund’s Paradox": He was the first to point out a paradox in quantum mechanics regarding chiral molecules (molecules that are mirror images of each other). He wondered why some molecules stay in a specific "handedness" rather than tunneling into their mirror image—a problem that touches on the very foundations of how the macroscopic world emerges from quantum laws.
- A Bridge Across the Iron Curtain: After WWII, Hund chose to stay in Jena (East Germany) to help rebuild the university, despite his lack of affinity for the socialist regime. He eventually moved to West Germany in 1951 only after the political pressure on academic freedom became untenable.
- The "Hund’s Rule" for Life: When asked about the secret to his longevity, he often attributed it to
"moderate work, moderate food, and moderate exercise,"
though his colleagues suspected it was simply his unrelenting curiosity.
Friedrich Hund’s life was a bridge between the classical physics of the 19th century and the high-tech quantum world of the 21st. His "rules" remain as vital today as they were in 1927.