Properties of transition metals
In general, the electronic configuration of outer orbitals of transition elements is (n-1)d1– 10 ns1–2. However, the electronic configurations of outer orbitals of Zn, Cd, Hg, and Cn are represented by the general formula (n-1)d1– 10 ns2. These metals are very hard and have low volatility due to high enthalpy of atomization. The more unpaired electrons, the higher the enthalpy of atomization. The stability of the half-filled d subshell in Mn2+ and the completely filled d10 configuration in Zn2+ are related to their E° values, whereas E° for Ni is related to the highest negative ΔhydH°.
Identify the metal that is a d-block element but not a transition metal.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Cd
All elements of groups 3 to 12 are d-block elements, because their last electron enters a d orbital. A transition element is defined more narrowly: it must have an incompletely filled d subshell either in its ground state or in one of its common oxidation states.
Zinc, cadmium and mercury (group 12) have a completely filled d subshell in the ground state and in their common +2 state:
\( \displaystyle \ce{Cd}\ (Z=48):\ [\ce{Kr}]\,4d^{10}\,5s^2 \)
\( \displaystyle \ce{Cd^2+}:\ [\ce{Kr}]\,4d^{10}\quad(\text{the two 5s electrons removed}) \)
Cadmium never has a partly filled d subshell, so it is a d-block element but not a transition metal. This is also why cadmium compounds are colourless and diamagnetic.
Why the other options are incorrect
Option (2) Ag — Incorrect. Ag (Z = 47) is [Kr]4d105s1, but silver also forms \( \ce{Ag^{2+}} \): removing the 5s electron and one 4d electron gives 4d9, which has an incompletely filled d subshell, so silver is a transition metal.
Option (3) Au — Incorrect. Au (Z = 79) is [Xe]4f145d106s1; gold commonly forms \( \ce{Au^{3+}} \), and removing the 6s electron and two 5d electrons gives 5d8, with an incompletely filled d subshell.
Option (4) Pt — Incorrect. Platinum (Z = 78) is [Xe]4f145d96s1 in the ground state itself, so its d subshell is incomplete.
Note: It seems that there is a mistake in the original NTA paper, which prints the outer electronic configuration of Zn, Cd, Hg and Cn as (n−1)d1–10ns2 when it should read (n−1)d10ns2, because these metals have a completely filled d subshell. The next sentence, “These metals are very hard and have low volatility due to high enthalpy of atomization”, reads as if it describes Zn, Cd and Hg, but it is true of transition metals in general. Zn, Cd and Hg are soft and fairly volatile (mercury is a liquid) and have the lowest enthalpies of atomisation, because they have no unpaired d electrons for metallic bonding.
NCERT (2026-27 edition):
Electronic Configurations of the d-Block Elements – …
