JEE Main 2026 Apr 4 Shift 1, Chemistry Q10: f – Block Elements

A. Down group $6$, higher oxidation states become more stable, so Mo(VI) and W(VI) are more stable than Cr(VI). Incorrect.

B. For lanthanoids, the $+3$ state is the most stable. Hence $+4$ ions ($\mathrm{C}{\mathrm{e}}^{4+},\mathrm{T}{\mathrm{b}}^{4+}$) tend to gain an electron and act as oxidants, while $+2$ ions ($\mathrm{E}{\mathrm{u}}^{2+},\mathrm{Y}{\mathrm{b}}^{2+}$) tend to lose an electron and act as reductants. Correct.

C. Configurations: Cm is $[\mathrm{Rn}]5f^{7}6d^{1}7s^2$, giving $7+1=8$ unpaired electrons; Am is $[\mathrm{Rn}]5f^{7}7s^2$, giving $7$ unpaired electrons. So Cm does not have seven. Incorrect.

D. Actinoid contraction is greater per element than lanthanoid contraction because $5f$ orbitals shield the nuclear charge less effectively than $4f$. Correct.

Hence only B and D are correct.