Question

The number of 4f electrons in the ground state electronic configuration of \(Gd^{2+}\) is _________. [Atomic number of Gd = 64] (Integer answer)

Hint:

Gadolinium is a classic example in exams. Remember the "4f7 5d1 6s2" rule for Gd and "4f14 5d1 6s2" for Lu. Electrons are always removed from 6s first.

Answer 1

Correct Answer: 7

Step 1: Understanding the Concept:
Electronic configuration of lanthanides involves filling the 4f subshell. Gadolinium (Gd) is unique because it exhibits a half-filled stability in its 4f subshell.
Step 2: Key Formula or Approach:
First, write the ground state configuration of neutral Gd (Z=64), then remove electrons starting from the outermost subshell (6s, then 5d or 4f).
Step 3: Detailed Explanation:
1. Ground state configuration of Gd (Z=64):
\[ [Xe] 4f^7 5d^1 6s^2 \]
This configuration is stable because the 4f subshell is exactly half-filled.
2. Formation of \(Gd^{2+}\):
To form a 2+ ion, two electrons are removed from the outermost subshell, which is 6s.
Configuration of \(Gd^{2+}\):
\[ [Xe] 4f^7 5d^1 6s^0 \]
3. Counting 4f electrons:
There are 7 electrons in the 4f subshell.
Step 4: Final Answer:
The number of 4f electrons is 7.