CI- is not isoelectronic with

Isoelectronic species are atoms or ions that possess the same number of electrons and identical electron configurations [1]. The chloride ion (Cl-) is formed when a chlorine atom (atomic number 17) gains one electron, resulting in 18 electrons, which is isoelectronic with the noble gas Argon (Ar) [1]. Potassium (K+) loses one electron from its 19, leaving 18 electrons. Sulfide (S2-) gains two electrons to its 16, and Phosphide (P3-) gains three electrons to its 15, both reaching 18 electrons. In contrast, Magnesium (Mg) has an atomic number of 12; the Mg2+ ion is formed by losing two electrons, resulting in only 10 electrons. Therefore, Mg2+ is isoelectronic with Neon (Ne) rather than Argon, making it the species that is not isoelectronic with Cl-.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 47