The transition elements exhibit variable oxidation states because (n−1)d electrons can also participate in bonding, as the energy difference between (n−1)d and ns orbitals is very small. The maximum oxidation state that can be exhibited by a transition element is equal to the total number of electrons present in both ns and (n−1)d orbitals. The elements in the middle exhibit more oxidation states, e.g., manganese exhibits all oxidation states from +2 to +7. In the second series, a stable maximum oxidation state is exhibited by technetium, and an unstable maximum oxidation state of +8 is exhibited by ruthenium. In the third series, a stable maximum oxidation state of +8 is exhibited by osmium. Transition metal ions having a completely filled or an exactly half-filled d subshell are stable. The Fe3+ ion is more stable than the Fe2+ ion because of the stable half-filled 3d5 electronic configuration of Fe3+.
Which of the following statements is false?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The most stable oxidation state of chromium is +6.
In group 6 the higher oxidation states become more stable going down the group, so Cr(VI) is the least stable of Cr(VI), Mo(VI) and W(VI).
Cr(VI), as dichromate in acid, is a strong oxidising agent: it is readily reduced to \( \ce{Cr^3+} \). At the other side, \( \ce{Cr^2+} \) is a strong reducing agent and is readily oxidised to \( \ce{Cr^3+} \).
Both pass readily to \( \ce{Cr^3+} \), so +6 is not the most stable state of chromium; +3 is more stable than both +6 and +2.
Option (1) The most stable oxidation state of chromium is +6. — A false statement. Cr(VI) is a strong oxidising agent and passes readily to the more stable +3 state.
Option (2) Most elements of the 3d series show the +2 oxidation state; scandium is the exception. — A true statement. Scandium (3d14s2) shows only +3; scandium(II) is virtually unknown, while the other 3d elements form \( \ce{M^2+} \) ions.
Option (3) Ruthenium and osmium show a maximum oxidation state of +8. — A true statement. As the passage states, ruthenium shows an unstable +8 state and osmium a stable +8 state.
Option (4) Zinc does not show variable oxidation states. — A true statement. Zinc (3d104s2) loses only its two 4s electrons; its only oxidation state is +2.
NCERT (2026-27 edition):
4.3.4 Oxidation States – “… For example, in group 6, Mo(VI) and W(VI) are found to be more stable than Cr(VI). Thus Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO3 and WO3 are not.”