Answer the question on the basis of passage given below:
All transition elements display typical metallic properties like high melting point, boiling point, tensile strength, high malleability and ductility, lustre, etc. Their metallic character increases down the group and elements of 4d and 5d series exhibit higher metallic characters. They show variable oxidation states and form a number of coloured compounds due to the presence of unpaired electrons which show d-d transitions and make them paramagnetic.
The two strongest oxidising agents among the oxides will be -
A. \(MoO_3\)
B. \(CrO_3\)
C. \(Mn_2O_7\)
D. \(WO_3\)
E. \(VO_2\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → B and C only
An oxide is a strong oxidising agent when its metal sits in a high oxidation state that is unstable, so that the metal readily takes up electrons and drops to a lower state. In the d-block the higher oxidation states become more stable down a group, the opposite of the p-block trend.
A. MoO3 — Incorrect. Mo is +6 (x + 3(−2) = 0), but Mo(VI) is stable, so \( \ce{MoO3} \) is not a strong oxidant.
B. CrO3 — Correct. Cr is +6. Cr(VI) is much less stable than Mo(VI) and W(VI) and is readily reduced to Cr(III), which is why chromium(VI) compounds such as dichromate are strong oxidising agents.
C. Mn2O7 — Correct. 2x + 7(−2) = 0 gives Mn = +7, its highest state (the group number). It is very easily reduced, so \( \ce{Mn2O7} \) is a powerful, even explosive, oxidant.
D. WO3 — Incorrect. W(VI) is the most stable +6 state of group 6, so \( \ce{WO3} \) is a weak oxidant.
E. VO2 — Incorrect. V is only +4 here, below its group maximum of +5. V(IV) is a common, stable state, not a high unstable one, so \( \ce{VO2} \) has little tendency to take up electrons.
NCERT (2026-27 edition):
Oxidation States – “… 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). Thus Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO3 and WO3 are not.”