The outer electronic configuration of chromium atom is `3d^4 4s^2`

Answer

To determine the correct outer electronic configuration of a chromium atom, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Atomic Number of Chromium:**
- Chromium (Cr) has an atomic number of 24. This means it has 24 electrons.

2. **Write the Electronic Configuration:**
- The electronic configuration can be built up using the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to the highest. The order of filling is:
- 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.
- For chromium, the electronic configuration would initially be:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴
- This configuration can be summarized as:
- [Ar] 4s² 3d⁴ (where [Ar] represents the electron configuration of Argon, which accounts for the first 18 electrons).

3. **Recognize the Exception in Chromium's Configuration:**
- However, chromium is known to be an exception to the expected electronic configuration. Instead of having 4 electrons in the 3d subshell, one electron from the 4s subshell is promoted to the 3d subshell to achieve a more stable half-filled configuration.
- This results in the configuration:
- [Ar] 4s¹ 3d⁵

4. **Conclusion:**
- Therefore, the correct outer electronic configuration of a chromium atom is [Ar] 4s¹ 3d⁵, not [Ar] 4s² 3d⁴.

### Final Answer:
The outer electronic configuration of chromium is [Ar] 4s¹ 3d⁵.

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