Electron Configuration

The line spectra of atoms reveal that electrons occupy fixed energy levels, but a full description of where electrons reside requires a more detailed model. The principal energy levels, labeled by the principal quantum number $n = 1, 2, 3, \dots$, describe the main shells, with higher $n$ meaning higher energy and greater average distance from the nucleus. Each principal level is divided into sublevels, also called subshells, designated $s$, $p$, $d$, and $f$. The first level contains only an $s$ sublevel; the second contains $s$ and $p$; the third adds $d$; and the fourth adds $f$. Within a given principal level, the sublevels differ slightly in energy, following the order $s < p < d < f$.

Each sublevel is made of one or more orbitals, regions of space where there is a high probability of