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°.
Arrange the following in the increasing order of their oxidizing power:
Cr2O72-, MnO4-, VO2+
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → VO2+ < Cr2O72- < MnO4-
The oxidising power of an oxoion in acid solution is measured by its standard reduction potential: the more positive the E°, the more readily the ion accepts electrons, and the stronger it is as an oxidising agent. Across the 3d series the higher oxidation states become less stable, so the oxoions become stronger oxidising agents from V to Mn.
\( \displaystyle \ce{VO2^+ + 2H^+ + e^- -> VO^2+ + H2O}\quad E^\circ=+1.00\ \mathrm{V} \)
\( \displaystyle \ce{Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O}\quad E^\circ=+1.33\ \mathrm{V} \)
\( \displaystyle \ce{MnO4^- + 8H^+ + 5e^- -> Mn^2+ + 4H2O}\quad E^\circ=+1.51\ \mathrm{V} \)
Here V is in the +5 state (group 5 maximum), Cr in +6 and Mn in +7. The E° values increase in the order \( \ce{VO2^{+}} \) < \( \ce{Cr2O7^{2-}} \) < \( \ce{MnO4^{-}} \).
Shortcut: Permanganate is the strongest oxidising agent of the three, so it must come last. Only Option (3) ends with \( \ce{MnO4^{-}} \).
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):
Trends in Stability of Higher Oxidation States – …
