Classification of Elements and Periodicity in Properties

Ans. (2,3)
(1) For isoelectronic series
ionic size $\propto \frac{1}{Z_{eff}} \propto \frac{-ve charge}{+ve charge}$ $Al^{+3} (2) I $^{st}$ IP order
$\begin{array}{c c c c} \text { B } & \text { C } & \text { N } & \text { O } \\ 2 p ^ {1} & 2 p ^ {2} & \boxed {2 p ^ {3}} & 2 p ^ {4} \\ & & \longrightarrow & \text { Half filled stable configuration } \end{array}$
\mathrm{B} < \mathrm{C} < \mathrm{O} < \mathrm{N}
(3) Electron gain enthalpy:
EA of $3^{rd}$ period > EA of $2^{nd}$ period
Correct order = I < Br < F < Cl
(4) Down the group metallic radius increases. Li < Na < K < Rb