Hund's rule of maximum multiplicity

Hund's Rule of Maximum Multiplicity is a principle in quantum mechanics that guides the arrangement of electrons in orbitals of an atom. It is one of the three rules proposed by Friedrich Hund in 1925, the others being the Aufbau Principle and Pauli Exclusion Principle, which together help explain the electron configuration of atoms. Hund's rule addresses the distribution of electrons among degenerate orbitals (orbitals that have the same energy level) to minimize the electron repulsion within an atom.

Overview

According to Hund's Rule, for a given electron shell, electrons will occupy degenerate orbitals singly as far as possible and with parallel spins. This arrangement is due to the electron-electron repulsion and exchange energy considerations, which leads to the most stable (lowest energy) configuration when the electrons are unpaired in degenerate orbitals with parallel spins. The rule can be summarized as: every orbital in a subshell gets filled with one electron before any one orbital gets a second electron, and all singly occupied orbitals will have electrons with the same spin direction.

Application

Hund's Rule is crucial in determining the ground state of an atom or molecule. The rule applies primarily to the building-up process where electrons are added to an atom or molecule. It is particularly important in the filling of p, d, and f orbitals. For example, in the case of carbon, with an electronic configuration of 1s² 2s² 2p², Hund's Rule explains why the two electrons in the 2p subshell occupy separate orbitals with parallel spins, rather than pairing up in a single orbital.

Implications

The implications of Hund's Rule are significant in various fields of chemistry and physics, including:

• Chemical bonding and molecular structure prediction
• Spectroscopy, where it helps explain the multiplicity of spectral lines
• Magnetic properties of atoms and molecules, as the unpaired electrons contribute to paramagnetism

Exceptions

While Hund's Rule provides a general guideline for electron configuration, there are exceptions, particularly when considering the effects of electron-electron repulsions and the specific energies of subshells in heavier elements. In such cases, the actual electron configuration may deviate from the predictions of Hund's Rule.

See Also

• Electron configuration
• Quantum mechanics
• Atomic orbital
• Aufbau Principle
• Pauli Exclusion Principle