Hund's rule (the rule of maximum multiplicity) states that when electrons fill a set of degenerate orbitals, they occupy them singly, with parallel spins, before any pairing begins. This arrangement minimises the repulsion between electrons and so gives the lowest-energy configuration. For example, the three 2p orbitals of nitrogen each hold one electron before any of them is doubly occupied.

洪特规则(最大多重度规则)指出,电子在填充一组简并轨道时,会先以平行自旋单独占据各个轨道,然后才开始配对。这种排布使电子间的排斥最小,从而给出能量最低的组态。例如,氮的三个 2p 轨道在任何一个被双重占据之前都各占一个电子。