The Quantum Architects

How Hund and Mulliken Built Modern Chemistry

Unveiling the invisible forces that hold matter together

Navigation

Introduction
Key Concepts
Key Experiment
Scientist's Toolkit
Legacy
Epilogue

Introduction: The Unseen Revolution

Friedrich Hund (1896-1997)

Robert Mulliken (1896-1986)

Quantum mechanics revolution

In the turbulent 1920s, as quantum mechanics shattered classical physics, two young scientists—Friedrich Hund in Germany and Robert Mulliken in America—began mapping an uncharted landscape: the mysterious quantum behavior of electrons in molecules. Their collaboration-before-communication forged molecular orbital (MO) theory, revolutionizing chemistry. When Mulliken received the 1966 Nobel Prize, he insisted: "I would have gladly shared it with Hund" ¹ ⁸ . Their story, chronicled in Mulliken's autobiography Life of a Scientist, reveals how a theory dismissed for decades now underpins modern chemistry, from drug design to materials science.

Key Concepts: Electrons Unleashed

1. The Core Idea: Molecular Orbitals

Traditional chemistry pictured bonds as static electron pairs between atoms. Hund and Mulliken proposed a radical alternative: electrons occupy delocalized orbitals spanning entire molecules, akin to atomic orbitals but molecule-sized. These orbitals form when atomic orbitals combine mathematically:

Bonding orbitals (e.g., σ, π): Lower energy, stabilize molecules
Antibonding orbitals (e.g., σ*, π*): Higher energy, weaken bonds
Nonbonding orbitals: Neutral energy, no bonding effect ²

Table 1: Types of Molecular Orbitals

Orbital Type	Energy vs. Atomic Orbitals	Symmetry	Role
Bonding (σ, π)	Lower	Constructive interference	Stabilizes molecule
Antibonding (σ, π)	Higher	Destructive interference	Destabilizes molecule
Nonbonding	Equal	No net interaction	Minimal bonding effect

Source: Adapted from molecular orbital theory descriptions ²

Molecular orbital diagram for hydrogen ²

π bonding in molecular orbitals

2. Hund's Quantum Rules

Parallel to MO work, Hund established rules for electron behavior:

Maximum Multiplicity: Electrons fill degenerate orbitals singly before pairing (explaining magnetism) ¹ ⁶
Hund's Paradox: Why chiral molecules (like amino acids) exist in left/right forms, not quantum mixtures—resolved later via quantum decoherence ¹

Hund's Rules Visualization

Electron filling order according to Hund's rules ⁶

3. The VB vs. MO Rivalry

MO theory initially struggled against Linus Pauling's valence bond (VB) theory, which localized bonds between atom pairs. The clash was philosophical:

Valence Bond (VB) Theory

Bonds as "handshakes" between atoms (localized)

Molecular Orbital (MO) Theory

Electrons as "social entities" in molecules (delocalized) ⁷

MO triumphed by explaining phenomena VB couldn't, like oxygen's paramagnetism (see Key Experiment below).

In-Depth Look: The Experiment That Settled the Debate

The Puzzle of Paramagnetic Oxygen

Why does liquid oxygen defy gravity to cling to magnets?

Methodology: Spectroscopy Meets Quantum Theory (1928–1931)

Hund and Mulliken's approach combined lab data with theoretical prediction:

Spectral Analysis: Examined O₂'s ultraviolet absorption bands
Orbital Modeling: Applied LCAO (Linear Combination of Atomic Orbitals) to oxygen
Electron Configuration: Calculated energies using Hund's rule ² ⁶

Liquid oxygen suspended between magnets demonstrating paramagnetism ²

Results & Analysis

The MO diagram revealed two unpaired electrons in antibonding π* orbitals:

Bond order = (8 bonding e⁻ – 4 antibonding e⁻)/2 = 2 (consistent with O=O bond)
Unpaired electrons: Explained paramagnetism (attraction to magnets) ²

Table 2: Oxygen's Molecular Orbital Configuration

Molecular Orbital	Electrons	Energy Level	Magnetic Contribution
σ₂s, σ*₂s	4	Core	None
σ₂p_z	2	Bonding	None
π₂p_x, π₂p_y	4	Bonding	None
π₂p_x, π₂p_y	2	Antibonding	2 unpaired electrons

Source: Molecular orbital theory of dioxygen ²

VB theory predicted all electrons paired—contradicting experiments showing O₂'s paramagnetism. This cemented MO theory's validity.

Table 3: Magnetic Susceptibility of Diatomic Molecules

Molecule	Unpaired Electrons	Magnetic Behavior	MO Prediction	VB Prediction
O₂	2	Paramagnetic	✅ Correct	❌ Incorrect
N₂	0	Diamagnetic	✅	✅
B₂	2	Paramagnetic	✅	❌

Data confirms MO theory's superiority for paramagnetism ² ⁶

The Scientist's Toolkit: Building MO Theory

Key resources Hund and Mulliken used to decode molecules:

Table 4: Essential Tools for Quantum Chemistry Pioneers

Tool/Concept	Function	Example Use
Spectrometers	Measure light absorbed/emitted by molecules	Detected O₂'s spectral lines revealing unpaired electrons
LCAO Approximation	Models MOs as sums of atomic orbitals	Predicted orbital energies for diatomic molecules
Variational Principle	Optimizes orbital coefficients	Minimized energy in Schrödinger equation solutions
Group Theory	Analyzes orbital symmetries	Classified σ/π orbitals via reflection symmetry
Hartree-Fock Method	Computes electron interactions	Early computational MO models (1950s) ²

Early spectrometers used in molecular analysis ²

LCAO diagram for hydrogen fluoride

Legacy: From Controversy to Cornerstone

Hund and Mulliken's work faced skepticism until the 1950s, when:

Computational chemistry made MO calculations practical
Spectroscopy consistently validated MO predictions
New materials (e.g., semiconductors) required MO-based design ⁴ ⁷

Modern Applications of MO Theory

Drug Design

Protein-ligand orbital interactions ⁴

Quantum Computing

Molecular qubits ⁴

Nanotechnology

Graphene's π-orbitals ⁴

As Mulliken wrote: "The molecule was a molecule, not just atoms held together... A superatom" ⁸ . Hund's autobiography adds: "Our theory was not a calculation tool—it was a new language for chemistry" ¹ .

Epilogue: The Human Element

Hund lived to 101 (1896–1997), mentoring generations in Göttingen. Mulliken died in 1985, his autobiography completed posthumously. Their partnership—forged across war-torn continents—proves science transcends borders. As Hund's granddaughter, chess champion Barbara Hund, reflected: "He saw molecules as cosmic puzzles... each electron a player in a quantum game" ¹ . In MO theory, every bond tells their story.

Friedrich Hund in his later years ¹

For Further Reading

Mulliken, R.S. (1989). Life of a Scientist (Springer).
Hund, F. (1974). The History of Quantum Theory (Academic Press).