The horizontal quantum number for the last electron in sodium atom is

Options

A. 1
B. 2
C. 0
D. 3

Answer

To find the horizontal quantum number (which refers to the azimuthal quantum number, \( l \)) for the last electron in a sodium atom, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Atomic Number of Sodium**:
Sodium has an atomic number of 11. This means it has 11 electrons.

2. **Determine the Electronic Configuration**:
The electronic configuration of sodium can be written as:
\[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^1 \]
This shows that the first 10 electrons fill the 1s, 2s, and 2p orbitals, and the last electron (the 11th) is in the 3s orbital.

3. **Identify the Last Electron's Shell and Subshell**:
The last electron is in the 3s subshell. Here, the principal quantum number \( n \) is 3 (indicating the third shell).

4. **Determine the Azimuthal Quantum Number \( l \)**:
The azimuthal quantum number \( l \) is determined by the type of subshell:
- For s subshell, \( l = 0 \)
- For p subshell, \( l = 1 \)
- For d subshell, \( l = 2 \)
- For f subshell, \( l = 3 \)

Since the last electron is in the 3s subshell, we have:
\[ l = 0 \]

5. **Conclusion**:
Therefore, the horizontal quantum number (azimuthal quantum number) for the last electron in the sodium atom is \( l = 0 \).

### Final Answer:
The horizontal quantum number for the last electron in a sodium atom is \( 0 \).

---