Question

Consider the following elements. Hf \([Z = 72]\) ; Yb \([Z = 70]\) ; Lu \([Z = 71]\) Gd \([Z = 64]\) ; Eu \([Z = 63]\) ; Tm \([Z = 69]\) Dy \([Z = 66]\) ; Er \([Z = 68]\) A : [Hf, Yb] B : [Gd, Eu] C : [Lu, Tm] D : [Er, Dy] The pair which contains same number of \(f\) electrons.

• A. A and B
• B. B and C
• C. C and D
• D. A and D

Answer 1

The Correct Option is A

Concept: Lanthanides involve filling of \(4f\) orbitals. To determine the number of \(f\)-electrons, write the electronic configuration of each element.
Step 1: Write electronic configurations \[ Hf : [Xe]\,4f^{14}\,5d^{2}\,6s^{2} \] \[ Yb : [Xe]\,4f^{14}\,6s^{2} \] \[ Lu : [Xe]\,4f^{14}\,5d^{1}\,6s^{2} \] \[ Gd : [Xe]\,4f^{7}\,5d^{1}\,6s^{2} \] \[ Eu : [Xe]\,4f^{7}\,6s^{2} \] \[ Tm : [Xe]\,4f^{13}\,6s^{2} \] \[ Dy : [Xe]\,4f^{10}\,6s^{2} \] \[ Er : [Xe]\,4f^{12}\,6s^{2} \]
Step 2: Count number of \(f\)-electrons A : Hf \(=4f^{14}\) Yb \(=4f^{14}\) \[ \Rightarrow \text{Same number of } f \text{ electrons} \] B : Gd \(=4f^{7}\) Eu \(=4f^{7}\) \[ \Rightarrow \text{Same number of } f \text{ electrons} \] C : Lu \(=4f^{14}\) Tm \(=4f^{13}\) \[ \Rightarrow \text{Not same} \] D : Er \(=4f^{12}\) Dy \(=4f^{10}\) \[ \Rightarrow \text{Not same} \]
Step 3: Final comparison Pairs having same number of \(f\)-electrons: \[ A \text{ and } B \]