Which of the following is a transition element?

The correct answer is Option (3) → Silver

A transition element has an incompletely filled d subshell in its neutral atom or in its ions. Zinc, cadmium and mercury have a completely filled d subshell both in the ground state and in their common +2 state, so they are not regarded as transition elements, even though they are placed in the d-block.

• Zinc (Z = 30): [Ar]3d¹⁰4s²; losing the two 4s electrons gives \( \ce{Zn^{2+}} \): 3d¹⁰.

• Cadmium (Z = 48): [Kr]4d¹⁰5s²; losing the two 5s electrons gives \( \ce{Cd^{2+}} \): 4d¹⁰.

• Mercury (Z = 80): [Xe]4f¹⁴5d¹⁰6s²; losing the two 6s electrons gives \( \ce{Hg^{2+}} \): 5d¹⁰.

• Silver (Z = 47): [Kr]4d¹⁰5s¹; in the +2 state (as in \( \ce{AgF2} \)) it loses the 5s electron and one 4d electron, giving 4d⁹, an incompletely filled d subshell.

So silver is a transition element.

Why the other options are incorrect

• Option (1) Zinc — Incorrect. Its 3d subshell is full in the atom and in \( \ce{Zn^{2+}} \).

• Option (2) Mercury — Incorrect. Its 5d subshell is full in the atom and in \( \ce{Hg^{2+}} \).

• Option (4) Cadmium — Incorrect. Its 4d subshell is full in the atom and in \( \ce{Cd^{2+}} \).

NCERT (2026-27 edition):
The d- and f-Block Elements – “… Now according to IUPAC, transition metals are defined as metals which have incomplete d subshell either in neutral atom or in their ions. Zinc, cadmium and mercury of group 12 have full d¹⁰ configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals.”