定义Definition
电子亚层是电子层中能量相同（单电子原子中）的一组轨道，用 s、p、d、f 表示。s 亚层有 1 个轨道，最多容纳 2 个电子；p 有 3 个轨道，6 个电子；d 有 5 个轨道，10 个电子；f 有 7 个轨道，14 个电子。
A subshell is a set of orbitals within a shell that share the same energy (in a one-electron atom), labelled s, p, d and f. An s subshell has 1 orbital holding up to 2 electrons; p has 3 orbitals and 6 electrons; d has 5 orbitals and 10 electrons; f has 7 orbitals and 14 electrons.
填充顺序Filling order
电子按能量由低到高填充：1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p…，4s 比 3d 先填充。同一亚层中电子先分占不同轨道且自旋平行（洪特规则），每个轨道最多两个自旋相反的电子（泡利不相容原理）。
Electrons fill subshells in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p…, with 4s before 3d. Within a subshell electrons occupy separate orbitals with parallel spins first (Hund's rule), and each orbital holds at most two electrons of opposite spin (Pauli exclusion principle).
例外与离子Exceptions and ions
铬和铜的基态分别为 [Ar] 3d⁵ 4s¹ 和 [Ar] 3d¹⁰ 4s¹，半满和全满的 d 亚层较稳定。过渡金属形成离子时先失去 4s 电子，如 Fe²⁺ 为 [Ar] 3d⁶。
Chromium and copper are [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹, because half-filled and filled d subshells are stabilised. Transition metals lose their 4s electrons first when forming ions: Fe²⁺ is [Ar] 3d⁶.