Which of the following is/are not considered as transition elements?
Ag, Mo, Zn, Hg, W
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Hg and Zn
By the IUPAC definition, a transition element has an incomplete d subshell either in the neutral atom or in one of its common ions. Zinc, cadmium and mercury (group 12) have a completely filled d subshell, \( \mathrm{(n-1)d^{10}ns^{2}} \), in the atom and in their common ions, so they are not regarded as transition elements.
Zn (Z = 30): \( \mathrm{[Ar]\,3d^{10}\,4s^{2}} \); \( \ce{Zn^2+} \) is \( \mathrm{3d^{10}} \). Not a transition element.
Hg (Z = 80): \( \mathrm{[Xe]\,4f^{14}\,5d^{10}\,6s^{2}} \); \( \ce{Hg^2+} \) is \( \mathrm{5d^{10}} \). Not a transition element.
Ag (Z = 47): \( \mathrm{[Kr]\,4d^{10}\,5s^{1}} \); in the +2 state (e.g. \( \ce{AgF2} \)) it is \( \mathrm{4d^{9}} \), an incomplete d subshell. A transition element.
Mo (Z = 42): \( \mathrm{[Kr]\,4d^{5}\,5s^{1}} \) and W (Z = 74): \( \mathrm{[Xe]\,4f^{14}\,5d^{4}\,6s^{2}} \); both have incomplete d subshells in the atom. Transition elements.
Option (1) Ag and Zn — Not both outside the transition elements. Zn is not a transition element, but silver has an incomplete d subshell in its +2 state, so it counts as one.
Option (2) Mo and Zn — Not both outside the transition elements. Molybdenum has a half-filled 4d subshell even in the atom.
Option (3) Hg and Zn — Neither is a transition element. Both have a completely filled d subshell in the atom and in their common ions.
Option (4) Hg and Mo — Not both outside the transition elements. Hg is not a transition element, but Mo is.
NCERT (2026-27 edition):
The d- and f-Block Elements – “… Originally the name transition metals was derived from the fact that their chemical properties were transitional between those of s and p-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.”