An electron shell is the collection of all orbitals that share the same principal quantum number n. Electrons in the same shell lie at roughly the same distance from the nucleus and have similar energies. A shell is divided into subshells (s, p, d, f), and the outermost occupied shell is the valence shell that governs chemical behaviour.

电子壳层是具有相同主量子数 n 的所有轨道的集合。同一壳层中的电子与原子核的距离大致相同,能量也相近。一个壳层又分为若干亚层(s、p、d、f),而最外层被占据的壳层即为决定化学性质的价层。