Question

Which one of the following pairs of elements has the second element with greater or almost equal first ionization energy ?

• A. K, Na
• B. P, As
• C. Al, Ga
• D. S, Se

Answer 1

Answer: (A), (C)

The correct options are
A K, Na
C Al, Ga

(A) K < Na.
On moving down the group, the ionization energy decreases.
Hence, the ionization energy of Na is higher than that of K.

(C) Al (577) < Ga (579).
The ionization energy of Ga is greater than the ionization energy of Al due to poor shielding effect of d electrons due to which the effective nuclear charge increases.