Aufbau Principle

'Aufbau' is German for 'building up', and the Aufbau principle describes how the electrons of a ground-state atom are filled into its orbitals, one at a time, building up the atom's full electronic configuration. It rests on three combined rules: (i) the increasing order of orbital energies, fixed by the $(n+l)$ rule covered in section 4.7.4 — this decides WHICH orbital an electron goes into next, following the sequence $1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s\ldots$ (often remembered via a simple diagonal-arrow mnemonic drawn across a table of subshells arranged by shell); (ii) Pauli's exclusion principle, which caps how many electrons any one orbital can actually hold; and (iii) Hund's rule of maximum multiplicity, which decides the order in which electrons fill a set of orbitals that are all …