Classification of Elements Part 2

Concept:The second ionisation enthalpy depends on the stability of the ion after the first electron removal. In an isoelectronic series, ionic radius decreases as nuclear charge increases.Explanation:• Statement I: Electronic configuration of Na is $1s^2 2s^2 2p^6 3s^1$. After removing one electron, $\text{Na}^+$ attains a stable noble gas core ($1s^2 2s^2 2p^6$).Removing a second electron from this stable core requires extremely high energy.• Electronic configuration of Mg is $1s^2 2s^2 2p^6 3s^2$. After first ionisation, $\text{Mg}^+$ is $1s^2 2s^2 2p^6 3s^1$. Removing the remaining valence $3s$ electron needs less energy.Therefore, $IE_2(\text{Na}) > IE_2(\text{Mg})$. Statement I is true.• Statement II: $\text{O}^{2-}$ (8+2=10 electrons) and $\text{F}^{-}$ (9+1=10 electrons) are isoelectronic. In an isoelectronic series, higher nuclear charge pulls electrons more strongly, leading to a smaller radius. Nuclear charge of O is 8, F is 9. Hence, $\text{O}^{2-}$ is larger than $\text{F}^{-}$. Statement II is true.Answer:Both Statement I and Statement II are true. Option C.