Question
In which of the following arrangements, the order is not according to the property indicated against it?
A.
$$A{l^{3 + }} < M{g^{2 + }} < N{a^ + } < {F^ - }: $$ Ionic size
B.
$$B < C < N < O:$$ First ionization enthalpy
C.
$$I < Br < F < Cl:$$ Electron gain enthalpy with negative sign
D.
$$Li < Na < K < Rb:$$ Metallic radius
Answer :
$$B < C < N < O:$$ First ionization enthalpy
Solution :
(A) All of these are isoelectronic ions with decreasing nuclear charge and increasing ionic size.
$$A{l^{3 + }}\left( {Z = 13} \right) < M{g^{2 + }}\left( {Z = 12} \right)$$ $$ < N{a^ + }\left( {Z = 11} \right) < {F^ - }\left( {Z = 9} \right)$$
(B) $$I{E_1}$$ of these elements are in the order : $$B < C < O < N.$$ $$I{E_1}$$ of $$N\left( {1{s^2}2{s^2}2{p^3}} \right)$$ is more than $$O\left( {1{s^2}2{s^2}2{p^4}} \right)$$ due to extra stable half-filled $$2p$$ -subshell in the valence shell. Thus, the given order is not correct.
(C) Electron gain enthalpies of these elements are $$I\left( { - 295\,kJ\,mo{l^{ - 1}}} \right) < $$ $$Br\left( { - 325\,kJ\,mo{l^{ - 1}}} \right) < $$ $$F\left( { - 328\,kJ\,mo{l^{ - 1}}} \right) < $$ $$Cl\left( { - 349\,kJ\,mo{l^{ - 1}}} \right)$$
(D) Metallic radius increases down the group in Ist group.