Hundszabály

Hundszabály, known in English as Hund’s rule, is a principle in atomic and quantum chemistry named after the German physicist Friedrich Hund. It concerns how electrons populate orbitals of the same energy, or degenerate orbitals, within an atom.

The first Hund’s rule states that for a given electron configuration, the term with the greatest total spin S (equivalently the highest multiplicity 2S+1) has the lowest energy. In other words, electrons prefer to occupy separate degenerate orbitals with parallel spins before pairing up. This maximizes exchange interactions and lowers the atom’s energy.

The second Hund’s rule says that, for a fixed multiplicity, the term with the largest total orbital angular momentum L is the lowest in energy. Together, the rules help determine the ground-state term and the ordering of energy levels in atoms, especially for light and mid-range elements where spin-orbit coupling is weaker than electron–electron repulsion.

A simple illustration is carbon with the electron configuration 1s^2 2s^2 2p^2. The two 2p electrons occupy separate p orbitals with parallel spins, yielding a triplet state and the ground term 3P. Hund’s rules also underlie many spectroscopic interpretations by predicting term symbols and relative line intensities.

Exceptions and limitations exist. In atoms with strong spin-orbit coupling, heavy elements, or substantial configuration interaction, Hund’s rules may be modified or less predictive. In such cases more comprehensive quantum-mechanical methods are used.

Hundszabály remains a foundational guideline for understanding atomic structure and spectroscopy, widely taught and applied in interpreting electronic configurations and spectral data.