Which of the following can not change its magnetic moment on excitation ?

Options

A. H
B. Be
C. B
D. O

Answer

To solve the question "Which of the following can not change its magnetic moment on excitation?", we need to analyze the magnetic moment of the given elements based on their unpaired electrons before and after excitation. The magnetic moment (μ) can be calculated using the formula:

\[ \mu = \sqrt{n(n + 2)} \]

where \( n \) is the number of unpaired electrons.

### Step-by-Step Solution:

1. **Identify the Elements**: Let's consider the elements mentioned in the video: Hydrogen (H), Beryllium (Be), Boron (B), and Oxygen (O).

2. **Calculate Unpaired Electrons for Each Element**:
- **Hydrogen (H)**:
- Atomic Number (Z) = 1
- Electronic Configuration = 1s¹
- Number of unpaired electrons (n) = 1
- On excitation, the configuration may change (1s to 2s), but the number of unpaired electrons remains 1.
- Thus, magnetic moment remains unchanged.

- **Beryllium (Be)**:
- Atomic Number (Z) = 4
- Electronic Configuration = 1s² 2s²
- Number of unpaired electrons (n) = 0
- On excitation, one electron can be promoted from 2s to 2p, resulting in n = 2.
- Thus, magnetic moment changes from 0 to a positive value.

- **Boron (B)**:
- Atomic Number (Z) = 5
- Electronic Configuration = 1s² 2s² 2p¹
- Number of unpaired electrons (n) = 1
- On excitation, one of the 2s electrons can be promoted to 2p, resulting in n = 3.
- Thus, magnetic moment changes.

- **Oxygen (O)**:
- Atomic Number (Z) = 8
- Electronic Configuration = 1s² 2s² 2p⁴
- Number of unpaired electrons (n) = 2
- On excitation, electrons can be promoted to higher energy levels, resulting in an increase in unpaired electrons.
- Thus, magnetic moment changes.

3. **Conclusion**:
- The only element that does not change its magnetic moment upon excitation is **Hydrogen (H)**, as it has only one unpaired electron that remains unchanged regardless of excitation.

### Final Answer:
**Hydrogen (H) cannot change its magnetic moment on excitation.**