Question

With respect to periodic properties, the correct statement is:

• A. The order of electron affinities is $F>O>Cl$
• B. The order of first ionisation energies is $Al>Mg>K$
• C. The order of atomic radii is $N>P>As$
• D. The order of ionic radii is $K^{+}>C{a}^{2+}>M{g}^{2+}$

Answer 1

The correct option is D The order of ionic radii is $K^{+}>C{a}^{2+}>M{g}^{2+}$

(a) Electron affinity is defined as the amount of energy released when an electron is added to a neutral gaseous atom. The correct order of electron affinities will be $O<F<Cl$. Though electron affinity decreases down the group, chlorine has higher electron affinity than flourine. This is due to the smaller size of flourine, which results in more repulsions between the existing and the incoming electrons.

(b) The correct order of ionisation energies is $K<Al<Mg$. Across the period, ionisation enthalpy increases, but $Al$ has less ionisation enthalpy than $Mg$. This can be explained on the basis of their electronic configuration.
$Mg=1s^{2}2s^{2}2p^{6}3s^{2}Al=1s^{2}2s^{2}2p^{6}3s^{2}3p^1$
As $Mg$ has a completely filled orbital, it will be more stable. Hence, $Mg$ will have a higher ionisation energy than $Al$. Down the group, ionisation energy decreases. So, $K$ has the least ionisation energy among the given elements.

(c) Down the group, atomic radius increases. Therefore, the correct order of atomic radii of group 15 elements is $N<P<As$.

(d) For isoelectronic species, as the positive charge increases, ionic radius decreases. Also, down the group, ionic radii increases. So, the correct order of ionic radii is $K^{+}>C{a}^{2+}>M{g}^{2+}$