Question

The electronic configurations of Eu(Atomic No. 63),
Gd(Atomic No. 64) and Tb (Atomic No. 65) are [2016]
(a) [Xe]4f⁷6s², [Xe]4f⁸ 6s² and [Xe]4f⁸5d¹6s²
(b) [Xe]4f⁷5d¹6s², [Xe]4f⁷ 5d¹ 6s² and [Xe]4f⁹6s²
(c) [Xe]4f⁶5d¹6s², [Xe]4f⁷5d¹6s² and [Xe]4f⁸5d¹6s²
(d) [Xe]4f⁷6s², [Xe]4f⁷5d¹6s² and [Xe]4f⁹6s²

Answer 1

answer : option (D) [Xe]4f⁷6s², [Xe]4f⁷5d¹6s² and [Xe]4f⁹6s²

here we have to use Aufbau Principe of electronic configuration.

for Eu ⇒ atomic No = 63

electronic configuration would be , [Xe]4f⁷6s²

because atomic number of Xe = 54 now after then we have to fill 9 electrons.

here, 4f⁷ is half filled orbital. and we know, half filled or full filled orbital is stable that's why we prefer it.

now for GD ⇒atomic no = 64

so, electronic configuration = [Xe]4f⁷5d¹6s²

54 electrons are filled by [Xe] and then 10 electrons filled by 6s, 4f and 5d. i.e., 7 + 1 + 2 = 10

it is also correct.

for Tb ⇒atomic no = 65

so, electronic configuration = [Xe]4f⁹6s²

it is because when 54 (Xe) + 11 = 65 and 11 electrons are filled by 4f and 6s , you see.

all above configurations only in option (D) so option (D) is correct choice.