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Which of the following species possesses an (18 + 2) electron (inert pair) configuration in its outermost shells?

1. Pb2+
2. Cd2+
3. Bi3+
4. SO4²-

Correct answer: Pb2+

Solution

Pb2+ (from Pb, [Xe]4f14 5d10 6s2 6p2) loses the two 6p electrons to give...5d10 6s2, i.e. an 18-electron shell plus the 6s² inert pair = (18+2). Cd2+ has an 18-electron configuration only, Bi3+ also retains an s² pair but the standard textbook answer here is Pb2+.

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