JEE Main · 2026 · 04 Apr 2026 · Shift 2 · d and f Block Elements

Counting the 4f electrons from the electronic configurations: Eu(63) is $[\mathrm{Xe}]4f^{7}6s^2$ and Gd(64) is $[\mathrm{Xe}]4f^{7}5d^{1}6s^2$, both with seven 4f electrons; Dy(66) is $4f^{10}$ and Ho(67) is $4f^{11}$, which differ; Yb(70) is $4f^{14}6s^2$ and Hf(72) is $[\mathrm{Xe}]4f^{14}5d^{2}6s^2$, both with fourteen; Lu(71) is $4f^{14}5d^{1}6s^2$ and Tm(69) is $4f^{13}6s^2$, which differ. The pairs having the same number of 4f electrons are (Eu and Gd) and (Yb and Hf).