Aufbau principle

The word Aufbau is German for "building up," and the principle captures exactly that idea: in constructing the ground-state electron configuration of an atom, orbitals are filled strictly in order of their increasing energy - electrons always occupy the lowest-energy orbital that still has room, and only once every lower orbital is completely filled do electrons begin entering the next one up.

Because the actual energy ordering of orbitals in a multi-electron atom follows the (n+l) rule developed in section 2.5.2 rather than simple increasing $n$, the practical filling sequence has some orbitals from a higher shell filling before orbitals from a lower one (most famously, 4s fills before 3d). Figure 2.12 gives the standard visual shortcut for this: a grid of orbital boxes with n running down the rows and subshell type (s, p, d, f) across the columns, with a series of diagonal arrows swept across it. Reading the orbitals in the order the arrows sweep through them reproduces the (n+l) f …