Question

Which from following pair of elements have one electron in 5d-subshell in observed electronic configuration?

• A. Sm (Z=61) and Eu (Z=63)
• B. Gd (Z=64) and Lu (Z=71)
• C. Ce (Z=58) and Nd (Z=60)
• D. Lu (Z=57) and Dy (Z=66)

Answer 1

The Correct Option is B

we need to examine the electron configurations of the elements. 
(A) Sm (Z=61) has the electronic configuration: [Xe] 4f⁶ 6s² Eu (Z=63) has the electronic configuration: [Xe] 4f⁷ 6s² 
In both cases, the 5d-subshell is not yet occupied, so this pair does not fulfill the given condition. 
(B) Gd (Z=64) has the electronic configuration: [Xe] 4f⁷ 5d¹ 6s² Lu (Z=71) has the electronic configuration: [Xe] 4f¹⁴ 5d¹ 6s² 
In this pair, both Gd and Lu have one electron in the 5d-subshell, satisfying the condition. 
(C) Ce (Z=58) has the electronic configuration: [Xe] 4f¹ 5d¹ 6s² Nd (Z=60) has the electronic configuration: [Xe] 4f⁴ 5d⁰ 6s² 
In this pair, only Ce has one electron in the 5d-subshell, so it satisfies the condition. 
(D) Lu (Z=57) has the electronic configuration: [Xe] 4f¹⁴ 5d¹ 6s² Dy (Z=66) has the electronic configuration: [Xe] 4f¹⁰ 5d⁰ 6s² 
In this pair, only Lu has one electron in the 5d-subshell, so it satisfies the condition. 
Therefore, the correct pair of elements is: (B) Gd (Z=64) and Lu (Z=71)