Which of the following sets contain only isoelectronic ions?
(i) $Z{n}^{2+},C{a}^{2+},G{a}^{3+},A{l}^{3+}$

(ii) $K^{+},C{a}^{2+},S{c}^{3+},C{l}^{-}$

(iii) $P^{3-},S^{2-},C{l}^{-},K^{+}$

(iv) $T{i}^{4+},Ar,C{r}^{3+},v^{5+}$

Answers:

(ii) $K^{+},C{a}^{2+},S{c}^{3+},C{l}^{-}$ and

(iii) $P^{3-},S^{2-},C{l}^{-},K^{+}$

(i) $Z{n}^{2+}(30-2=28),C{a}^{2+}(20-2=18),G{a}^{3+}(31-3=28),A{l}^{3+}(13-3=10)$
These species hae different number of electrons and hence are not isoelectronic ions.
(ii) $K^{+}(19-1=18),C{a}^{2+}(20-2=18),S{c}^{3+}(21-3=18),C{l}^{-}(17+1=18)$
These are all isoelectronic ions since each one of them has 18 electrons.

(iii) $P^{3-}(15+3=18),S^{2-}(16+2=18),C{l}^{-}(17+1=18),K^{+}(19-1=18)$
These are all isoelectronic ions since each one of them has 18 electrons.

(iv) $T{i}^{4+}(22-4=18),Ar=\left(18\right),C{r}^{3+}(24-3=21),V^{5+}(23-5=18).$
These have different number of electrons and hence are not isoelectronic ions.