(solved)Question 3.17 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{first IE}}}$

Na [Z = 11] is $\displaystyle 1s^22s^22p^63s^1$

Mg [Z = 12] is $\displaystyle 1s^22s^22p^63s^2$

3s electron has to be removed for both.

(1) the 3s electrons in both cases are equally shielded by the inner electrons.

(2) however, because of higher nuclear charge, the 3s electrons of magnesium are more tightly bound.

This causes the first IE of Mg to be higher than that of Na.

$\displaystyle \underline{\underline{\text{second IE}}}$

Na⁺ is $\displaystyle 1s^22s^22p^6$

Mg⁺ is $\displaystyle 1s^22s^22p^63s^1$

(1) Mg⁺ has a losely held 3s electron which is more shielded by completely filled inner shells, and can therefore be easily removed.

(2) Na⁺ has a stable neon-like configuration which makes it difficult to remove its electron.

Hence, the 2nd IE of Na⁺ is higher.