Arrange the following d-block elements in increasing order of number of their valence shell d-electrons:
(A) Pt
(B) Mo
(C) Ni
(D) Pd
Choose correct answer from options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B), (C), (A), (D)
The number of valence shell d electrons comes from the ground state configuration. Normally (n−1)d fills after ns2, but some elements shift electrons into d because half-filled (d5) and completely filled (d10) subshells, or nearly filled ones, have extra stability. Mo, Pt and Pd are such exceptions.
(B) Mo (Z = 42): [Kr] 4d5 5s1: 5 d electrons (half-filled, like Cr).
(C) Ni (Z = 28): [Ar] 3d8 4s2: 8 d electrons.
(A) Pt (Z = 78): [Xe] 4f14 5d9 6s1: 9 d electrons.
(D) Pd (Z = 46): [Kr] 4d10 5s0: 10 d electrons (both 5s electrons move into 4d).
Shortcut: Options (1) and (2) both start with Mo (B) and end with Pd (D). Ni (8) has fewer d electrons than Pt (9), so (C) comes before (A): Option (1).