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Which of the following sequences does not show the correct increasing order of the property mentioned alongside it?

1. V2+ < Cr2+ < Mn2+ < Fe2+: paramagnetic nature
2. Ni2+ < Co2+ < Fe2+ < Mn2+: ionic radius
3. Co3+ < Fe3+ < Cr3+ < Sc3+: stability in aqueous solution
4. Sc < Ti < Cr < Mn: number of oxidation states

Correct answer: V2+ < Cr2+ < Mn2+ < Fe2+: paramagnetic nature

Solution

Paramagnetism follows unpaired electrons: V2+(d3)=3, Cr2+(d4)=4, Mn2+(d5)=5, Fe2+(d6)=4. So the order V2+ < Cr2+ < Mn2+ < Fe2+ is wrong because Fe2+ (4) is less paramagnetic than Mn2+ (5). The other three orders (ionic radius, +3 aqueous stability, number of oxidation states) are correct.

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