Writing an Electron Configuration

Writing an atom's ground-state electron configuration means filling its orbitals in Aufbau order, respecting the Pauli exclusion principle's two-electron limit per orbital and Hund's rule for any degenerate subshell, until every electron matching that element's atomic number is placed.

Building phosphorus, atomic number 15, fills 1s² then 2s² then 2p⁶, using up ten electrons, then 3s² and finally 3p³, placing the three remaining electrons singly across the three 3p orbitals in accordance with Hund's rule. The full configuration reads 1s²2s²2p⁶3s²3p³, or [Ne]3s²3p³ in core-abbreviated form, and the superscripts sum to 15, matching phosphorus's atomic number.