If first ionisation enthalpy $\left(\Delta_{i} H\right)$ values of $\mathrm{Na}, \mathrm{Mg}$ and Si are respectively 496,737 and $786 \mathrm{~kJ} \mathrm{~mol}^{-1}$, the first ionisation enthalpy value of $\mathrm{Al}\left(\mathrm{in} \mathrm{kJ} \mathrm{mol}^{-1}\right.$ ) will be

The set containing the elements with positive electron gain enthalpies is

Assertion (A) : The ionic radii of $\mathrm{Na}^{+}$and $\mathrm{F}^{-}$are same.

Reason (R) : Both $\mathrm{Na}^{+}$and $\mathrm{F}^{-}$are isoelectronic species.

The correct answer is

In which of the following ionic pairs, second ion is smaller in size than the first ion?