MHTCET 22 Apr 2025 Shift 2 Paper

Concept:Isoelectronic species are atoms or ions that have the same total number of electrons.Explanation:Total electrons in an ion = atomic number $Z$ minus the charge $q$ on the ion.So, electrons $= Z - q$.Option A: Ne has $Z = 10$, so electrons $= 10$.$\mathrm{O}^{2-}$ has $Z = 8$ and $q = -2$, so electrons $= 8 - (-2) = 10$.Both have 10 electrons, hence they form an isoelectronic pair.Option B: $\mathrm{Cl}^{-}$ has $Z = 17$ and $q = -1$, so electrons $= 17 - (-1) = 18$.Ca has $Z = 20$, so electrons $= 20$.Not isoelectronic because $18 \neq 20$.Option C: Ar has $Z = 18$, so electrons $= 18$.$\mathrm{F}^{-}$ has $Z = 9$ and $q = -1$, so electrons $= 9 - (-1) = 10$.Not isoelectronic because $18 \neq 10$.Option D: $\mathrm{K}^{+}$ has $Z = 19$ and $q = +1$, so electrons $= 19 - 1 = 18$.$\mathrm{Al}^{3+}$ has $Z = 13$ and $q = +3$, so electrons $= 13 - 3 = 10$.Not isoelectronic because $18 \neq 10$.Therefore, only Option A satisfies the condition of equal electron count.Answer:Option A: Ne and $\mathrm{O}^{2-}$.