Question

The electronic configuration of the element with atomic number 78 is :

• A. \( [Xe]4f^{14}5d^9 6s^1 \)
• B. \( [Kr]4f^{14}5d^8 6s^2 \)
• C. \( [Xe]4f^{14}5d^{10} \)
• D. \( [Kr]4f^{14}5d^{10} \)

Hint:

Transition elements often show exceptions in electronic configuration due to stability of half-filled and fully filled orbitals.

Answer 1

The Correct Option is A

Step 1: Identify the element.
Atomic number 78 corresponds to Platinum (Pt).

Step 2: Write expected configuration.
Using Aufbau principle:
\[ [Xe]\, 4f^{14}\, 5d^8\, 6s^2 \]

Step 3: Consider stability exception.
Platinum shows anomalous electronic configuration due to extra stability of partially filled \( d \)-orbitals.
One electron from \( 6s \) shifts to \( 5d \):
\[ [Xe]\, 4f^{14}\, 5d^9\, 6s^1 \]

Step 4: Conclusion.
Thus, the correct configuration is:
\[ \boxed{[Xe]4f^{14}5d^9 6s^1} \]