Answer the question on the basis of passage given below:
In the periodic table, the d-block contains the elements of group 3 to 12. The d-orbitals are progressively filled in each of the four long periods. The elements of d-block referred as transition metals have partly filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and show paramagnetic behaviour.
The two inner transition metals series 4f and 5f are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition elements in any series.
For the four successive transition elements given below, the stability of +2 oxidation state will be
(A) Cr (Atomic number, Z = 24)
(B) Mn (Z = 25)
(C) Fe (Z = 26)
(D) Co (Z = 27)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Mn > Fe > Cr > Co
Note: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (4) Co > Mn > Fe > Cr, as explained below.
The stability of the +2 state relative to the +3 state is measured by the standard electrode potential of the M3+/M2+ couple. A large positive E° means M3+ is readily reduced to M2+, so the +2 state is stable; a negative E° means M2+ is readily oxidised to M3+, so the +2 state is unstable.
Cr (Z = 24): [Ar]3d54s1; \( \ce{Cr^2+} \) is 3d4. E° = −0.41 V: \( \ce{Cr^2+} \) is a strong reducing agent and changes to \( \ce{Cr^3+} \) (3d3, half-filled t2g) — last.
Mn (Z = 25): [Ar]3d54s2; \( \ce{Mn^2+} \) is 3d5, half-filled. E° = +1.57 V: \( \ce{Mn^2+} \) is very stable — second.
Fe (Z = 26): [Ar]3d64s2; \( \ce{Fe^2+} \) is 3d6. E° = +0.77 V: comparatively low, because \( \ce{Fe^3+} \) (3d5) is extra stable — third.
Co (Z = 27): [Ar]3d74s2; \( \ce{Co^2+} \) is 3d7. E° = +1.97 V: \( \ce{Co^3+} \) is a strong oxidant in water, so \( \ce{Co^2+} \) is the stable ion — first.
Option (2) Mn > Fe > Cr > Co — The answer given by NTA, but incorrect. \( \ce{Mn^2+} \) is very stable, but \( \ce{Co^2+} \) resists oxidation even more (E° +1.97 V against +1.57 V), and \( \ce{Cr^2+} \), with a negative E°, is the least stable of the four, not more stable than \( \ce{Co^2+} \).
Option (4) Co > Mn > Fe > Cr — Correct as per NCERT. It follows the E°(M3+/M2+) values of Table 4.2: +1.97 > +1.57 > +0.77 > −0.41 V.
NCERT (2026-27 edition):
Trends in the M3+/M2+ Standard Electrode Potentials – …
