Read the passage given and answer the question.
The transition elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example, exhibits all the oxidation states from +2 to +7. The lesser number of oxidation states at extreme ends stems from either too few electrons to lose or share (Sc, Ti) or too many d-electrons (hence fewer orbitals available in which to share electrons with others) for higher valence (Cu, Zn). Thus, early in the series scandium (II) is virtually unknown and titanium (IV) is more stable than Ti(III) or Ti(II). At the other end, the only oxidation state of zinc is +2 (no d electrons are involved). The maximum oxidation states of reasonable stability correspond in value to the sum of s- and d-electrons upto manganese \((Ti^{IV}O_2, V^VO_2^+, Cr^{VI}O_4^{2-}, Mn^{VII}O_4^- )\) followed by a rather abrupt decrease in stability of higher oxidation states, so that the typical species to follow are \(Fe^{II, III}\), \(Co^{II, III}\), \(Cu^{I, II}\), \(Zn^{II}\).
The variability of oxidation states, a characteristic of transition elements, arises out of incomplete filling of d-orbitals in such a way that their oxidation states differ from each other by unity, e.g., \(V^{II}\), \(V^{III}\), \(V^{IV}\), \(V^V\). This is in contrast with the variability of oxidation states of non transition elements where oxidation states normally differ by a unit or two.
An interesting feature in the variability of oxidation states of the d-block elements is noticed among the groups (groups 4 through 10). Although in the p-block, the lower oxidation states are favoured by the heavier members (due to inert pair effect), the opposite is true in the groups of d-block. For example, in group 6, \(Mo^{(VI)}\) and \(W^{(VI)}\) are found to be more stable than \(Cr^{(VI)}\). The \(Cr^{(VI)}\) in the form of dichromate in acidic medium is a strong oxidising agent, whereas \(MoO_3\) and \(WO_3\) are not.
Low oxidation states are found when a complex compound has ligands capable of \(\pi \)-acceptor character in addition to the \(\sigma \)-bonding. For example, in \(Ni(CO)_4\) and \(Fe(CO)_5\), the oxidation state of nickel and iron is zero.
Most acidic species among the following is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Mn2O7
For the oxides of one metal, the higher the oxidation state, the more covalent and the more acidic the oxide. The oxidation states of manganese in the given oxides are:
\( \ce{MnO} \): +2 (basic)
\( \ce{Mn2O3} \): +3 (basic)
\( \ce{MnO2} \): +4 (amphoteric)
\( \ce{Mn2O7} \): +7 (acidic)
\( \ce{Mn2O7} \), with manganese in its highest state (+7), is a covalent green oil and is strongly acidic: with water it gives permanganic acid.
\( \ce{Mn2O7 + H2O -> 2HMnO4} \)
Why the other options are incorrect
Option (2) MnO2 — Incorrect. Manganese is only +4; the oxide is amphoteric, not strongly acidic.
Option (3) MnO — Incorrect. Manganese is +2, the lowest state here; MnO is an ionic, basic oxide.
Option (4) Mn2O3 — Incorrect. Manganese is +3; the oxide is basic, far less acidic than \( \ce{Mn2O7} \).
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… As the oxidation number of a metal increases, ionic character decreases. In the case of Mn, Mn2O7 is a covalent green oil. Even CrO3 and V2O5 have low melting points. In these higher oxides, the acidic character is predominant.”