To understand why the electron configurations of chromium (Cr) with atomic number 24 and copper (Cu) with atomic number 29 are considered abnormal, we need to analyze their expected and actual electron configurations.
### Step 1: Write the Expected Electron Configurations
- For Chromium (Cr, atomic number 24):
- Following the Aufbau principle and the n + l rule, the expected electron configuration would be:
\[
\text{Cr: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^4
\]
- For Copper (Cu, atomic number 29):
- Similarly, the expected electron configuration would be:
\[
\text{Cu: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^9
\]
### Step 2: Write the Actual Electron Configurations
- For Chromium (Cr):
- The actual electron configuration is:
\[
\text{Cr: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^1 \, 3d^5
\]
- For Copper (Cu):
- The actual electron configuration is:
\[
\text{Cu: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^1 \, 3d^{10}
\]
### Step 3: Analyze the Stability of the Configurations
- The reason for the deviation from the expected configurations is due to the stability associated with half-filled and fully filled subshells:
- **Chromium**: The half-filled 3d subshell (3d^5) is more stable than having 4 electrons in the d subshell (3d^4). Thus, one electron from the 4s subshell is used to achieve a half-filled d subshell.
- **Copper**: The fully filled 3d subshell (3d^{10}) is more stable than having 9 electrons in the d subshell (3d^9). Therefore, one electron from the 4s subshell is used to achieve a fully filled d subshell.
### Conclusion
The abnormal electron configurations of chromium and copper arise from the extra stability provided by half-filled (for Cr) and fully filled (for Cu) d subshells.
### Final Answer
The electron configurations of Cr and Cu are abnormal due to the extra stability of half-filled and fully filled subshells.
---
