Question

What is the outer electronic configuration of Gd (Gadolinium, atomic number 64)?

• A. [Xe] \( 4f^7 \, 5d^1 \, 6s^2 \)
• B. [Xe] \( 4f^8 \, 6s^2 \)
• C. [Xe] \( 4f^6 \, 5d^2 \, 6s^2 \)
• D. [Xe] \( 4f^7 \, 5d^0 \, 6s^2 \)

Hint:

For lanthanides, prioritize placing electrons to stabilize the \( 4f \) subshell. Gadolinium prefers \( 4f^7 \) over \( 4f^8 \) due to half-filled shell stability.

Answer 1

The Correct Option is A

Step 1: Atomic Number of Gadolinium
Gadolinium (Gd) has an atomic number of \( Z = 64 \), indicating 64 electrons in its neutral form.
Step 2: Noble Gas Core
The preceding noble gas is Xenon (Xe), with 54 electrons. Thus, we start with: \[ \text{[Xe]} = \text{1s}^2 \, \text{2s}^2 \, \text{2p}^6 \, \text{3s}^2 \, \text{3p}^6 \, \text{4s}^2 \, \text{3d}^{10} \, \text{4p}^6 \, \text{5s}^2 \, \text{4d}^{10} \, \text{5p}^6 \] Step 3: Electron Placement After [Xe]
We need to account for \( 64 - 54 = 10 \) additional electrons. These are placed as follows:

• \( 6s \) orbital: 2 electrons → \( 6s^2 \)
• \( 4f \) orbital: 7 electrons → \( 4f^7 \)
• \( 5d \) orbital: 1 electron → \( 5d^1 \)

Step 4: Full Configuration
\[ \boxed{\text{[Xe]} \, 4f^7 \, 5d^1 \, 6s^2} \] This configuration is most stable for Gd because the \( 4f^7 \) subshell is half-filled, contributing to extra stability.