Which of the following statements is not correct about the electronic configuration of gaseou chromium atom

Options

A. It has 5 electrons is 3d and one electron in 4s orbitals
B. The principal quantum numbers of its valence electrons are 3 and 4
C. It has 6 electrons in 3d orbital
D. Its valance electrons have quantum number I = 0

Answer

To solve the question regarding the electronic configuration of a gaseous chromium atom, we will follow these steps:

### Step 1: Understand the Atomic Structure of Chromium
Chromium (Cr) has an atomic number of 24, which means it has 24 electrons in its neutral gaseous state.

### Step 2: Write the Expected Electronic Configuration
The expected electronic configuration follows the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to the highest. The general order of filling is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

For chromium, the expected filling would be:
- 1s²
- 2s²
- 2p⁶
- 3s²
- 3p⁶
- 4s²
- 3d⁴

However, chromium is an exception to this rule due to stability considerations.

### Step 3: Apply the Stability Rule
Chromium prefers to have a half-filled d subshell for stability. Therefore, instead of filling the 4s orbital completely, one electron from the 4s orbital is used to fill the 3d orbital. Thus, the actual electronic configuration of chromium is:
- 1s²
- 2s²
- 2p⁶
- 3s²
- 3p⁶
- 4s¹
- 3d⁵

This can be summarized as:
\[ \text{Cr: } [\text{Ar}] 3d^5 4s^1 \]

### Step 4: Analyze the Given Statements
Now, we need to evaluate the statements provided in the question to identify which one is incorrect:
- **Option A**: "It has 5 electrons in 3d and 1 electron in 4s." (This is correct)
- **Option B**: "The principal quantum number of its valence electrons is 3 and 4." (This is also correct)
- **Option C**: "It has 6 electrons in 3D orbital." (This is incorrect)
- **Option D**: "Its valence electrons have quantum number l = 0." (This is correct, as the 4s orbital has l = 0)

### Conclusion
The statement that is not correct about the electronic configuration of a gaseous chromium atom is **Option C**.