Electron shells and subshells

An s subshell holds two electrons, p holds six and d holds ten. Fill 1s,2s,2p,3s,3p,4s,3d,4p and 5s for the simple sequence through element 38, checking that exponents total the electron count. Noble-gas shorthand replaces a filled inner core: [Ne]3s2 represents twelve electrons.

Remember the observed exceptions chromium, [Ar]3d54s1, and copper, [Ar]3d104s1. Strontium is [Kr]5s2. For main-group atoms, outer electrons help determine group and bonding. When transition-metal cations form, remove outer 4s electrons before 3d electrons. Calculate an ion’s electron count from its charge before checking the configuration.

Watch out: Chromium and copper have experimentally established configurations that differ from the simplest prediction.