Assertion (A): Chromium(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO3 and WO3 are not.
Reason (R): In the p-block, the lower oxidation states are favoured by the heavier members of a group because of the inert pair effect.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): Chromium(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO3 and WO3 are not. — True. Cr(VI) readily takes electrons and goes to the more stable Cr(III), so acidified dichromate is a strong oxidant.
Molybdenum and tungsten are in the same group (group 6), but their +6 state is stable, so their oxides do not act as strong oxidants.
Reason (R): In the p-block, the lower oxidation states are favoured by the heavier members of a group because of the inert pair effect. — True. This is the usual trend in the p-block: going down a group, the lower oxidation state becomes more stable.
Why R does not explain A?
Reason (R) describes the p-block, while Assertion (A) is about group 6 of the d-block, where the trend is the opposite.
The correct reason for A is that in the d-block groups the heavier members favour the higher oxidation states: Mo(VI) and W(VI) are more stable than Cr(VI), so only Cr(VI) is eager to be reduced.
NCERT (2026-27 edition):
4.3.4 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.”