Question

What is the observed electronic configuration of Cu ?

• A. \([\text{Ar}] 3d^9 4s^2\)
• B. \([\text{Ar}] 3d^{10} 4s^1\)
• C. \([\text{Ar}] 3d^5 4s^2\)
• D. \([\text{Ar}] 3d^6 4s^2\)

Hint:

Copper takes a completely filled 3d subshell by moving one 4s electron into 3d.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Filling by the Aufbau order would give \([\text{Ar}]\,3d^9 4s^2\) for copper (Z = 29). But a completely filled d subshell has extra stability from symmetry and exchange energy.

Step 2: Detailed Explanation:
One electron from 4s moves into 3d. This makes the 3d subshell full (\(3d^{10}\)) and leaves 4s half filled (\(4s^1\)).
The observed configuration is therefore \([\text{Ar}]\,3d^{10}4s^1\).
Option (A) is the expected Aufbau configuration, not the observed one. Options (C) and (D) do not add up to 29 electrons in total: \(18+5+2 = 25\) and \(18+6+2 = 26\) belong to Mn and Fe.

Step 3: Check:
Total electrons in option (B): \(18 + 10 + 1 = 29\), which matches Cu.

Final Answer:
Copper is an exception with a fully filled 3d subshell. \[ \boxed{\text{(B) }[\text{Ar}]\,3d^{10}4s^1} \]