Isoelectronic Species

Atoms and ions that happen to contain exactly the same TOTAL number of electrons are called isoelectronic species, and — because electronic configuration is entirely determined by electron count for a neutral or ionic species being filled by the Aufbau principle — all isoelectronic species share the identical electronic configuration, even though they are chemically different species with different nuclear charges. Potassium, K (Z=19, 19 electrons), loses one electron to form K⁺, which then has $19-1=18$ electrons; species such as argon, Ar (Z=18, 18 electrons neutrally) and Ca²⁺ (Z=20, losing 2 electrons to also reach 18) are therefore isoelectronic with K⁺, and all of them — Ar, K⁺ and Ca²⁺ alike — share the electronic configuration $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$. The same logic works for anions formed by gaining electrons: for instance, neon (Z=10) is isoelectronic with O²⁻ (an oxygen atom, Z=8, that has ga …