Hund's rule of maximum multiplicity

Hund's rule states that electrons fill degenerate orbitals singly with parallel spins before pairing. This arrangement maximizes total spin and, among configurations of the same subshell, favors the greatest spin multiplicity.

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Electron configuration: The distribution of an atom's electrons among shells, subshells, and orbitals. Hund's rule supplies one of the ordering principles used to write ground-state configurations.
Pauli exclusion principle: No two identical fermions in one system can occupy the same quantum state. It prevents two electrons in one orbital from having the same spin state.
Ground state: The lowest-energy state available to a physical system. Hund's rule helps predict ground-state electron arrangements in partially filled subshells.
Hund's rule of maximum orbital angular momentum: A rule ranking atomic terms of the same spin multiplicity by their total orbital angular momentum. It ranks terms after maximum spin is considered, rather than specifying orbital-by-orbital filling.
Atomic orbital: A quantum state describing an electron's spatial distribution in an atom. The rule applies to orbitals within the same subshell.
Aufbau principle: The principle that electrons occupy available atomic states in order of increasing energy. It determines which subshell is considered before Hund's rule resolves occupancy within it.
Term symbol: A compact notation identifying an atom or ion's total spin, orbital angular momentum, and total angular momentum. Its spin multiplicity often reflects the maximum-spin arrangement favored by the rule.
Hund's rule of maximum total angular momentum: A rule selecting the lowest-energy total angular momentum among terms with the same spin and orbital angular momenta. It addresses fine-structure term ordering, not the initial filling of degenerate orbitals.
Orbital degeneracy: The equality of energy between two or more distinct quantum states. Only equal-energy orbitals are filled singly before pairing under the rule.
Quantum number: A value that specifies a quantized property of a quantum state. Orbital and spin quantum numbers distinguish the states being filled.