Electron Configurations of Ions

Adding electrons continues the normal aufbau order. Chlorine, [Ne] 3s² 3p⁵, gains one electron to become Cl⁻ with [Ne] 3s² 3p⁶, matching argon. Most main-group anions end up isoelectronic with the nearest noble gas.

Use N(electrons) = Z − q, where q is the signed charge in units of the elementary charge. For S²⁻, Z = 16 and q = −2, so N = 18; for Al³⁺, N = 13 − 3 = 10. A positive ion has fewer electrons, not more protons. Ion formation leaves the element's proton count and identity unchanged. After counting, write the occupied subshells and verify the sum; a familiar noble-gas label is a result to check, not a substitute for counting.

Magnesium, [Ne] 3s², loses both 3s electrons to give Mg²⁺ = [Ne]. Aluminium loses three to reach [Ne]. The number of electrons lost is normally the group''s valence count, which is why group 1 forms 1+ and group 2 forms 2+ ions.

Removing electrons from a main-group atom starts at the highest occupied principal shell and, within that shell, the higher-energy occupied subshell. Aluminum is [Ne] 3s² 3p¹: Al⁺ loses 3p¹, Al²⁺ loses one 3s electron, and Al³⁺ is [Ne]. Chlorine instead gains one electron to complete 3p⁶. Group patterns predict common monatomic charges such as Na⁺, Mg²⁺ and Cl⁻, but not every element forms a stable ion by simply pursuing a noble-gas count. In particular, transition metals commonly retain partly filled d subshells.

A configuration describes a specified electron count; it does not establish that an ion is common or stable in every environment. Fe²⁺ and Fe³⁺ both occur even though neither has a noble-gas configuration. Removing electrons beyond an outer shell becomes much more costly, which helps explain why Na⁺ is common and Na²⁺ is not a usual aqueous ion.

Practice the two checks independently. O²⁻ and Mg²⁺ each have ten electrons and both are [Ne], but their nuclei contain eight and twelve protons respectively. They are isoelectronic, not the same species. Cu⁺ and Cu²⁺ are [Ar] 3d¹⁰ and [Ar] 3d⁹: after the single 4s electron is removed, the next removal must come from 3d. Never subtract two 4s electrons from copper's one-electron 4s population.