Question

Which of the following isoelectronic ions have lowest ionization energy?

• A. $S^{2-}$
• B. $C{l}^{-}$
• C. $C{a}^{2+}$
• D. $K^{+}$

Answer 1

The correct option is A $S^{2-}$

Ionization energy is the minimum energy that an electron in a gaseous atom or ion has to absorb to be ejected.
Larger the size of the atom, lower be the ionization energy
Since, in $S^{2-}$ number of electrons will be more so it will have lowest ionization energy and $Z_{eff}$ is least.