Choose the pair in which after filling of electron , energy sequence is reversed

Options

A. `2p gt 2s`
B. `3d gt 4s`
C. ` 4p lt 5s`
D. `5p lt 6s`

Answer

To solve the question, we need to analyze the energy levels of the orbitals and determine which pair shows a reversal in energy sequence after the filling of electrons.

### Step-by-Step Solution:

1. **Understand the n + l Rule**:
The energy of an orbital is determined by the n + l rule, where 'n' is the principal quantum number and 'l' is the azimuthal quantum number. The lower the n + l value, the lower the energy of the orbital.

2. **List the Energy Order of Orbitals**:
According to the n + l rule, the order of filling is:
- 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s, etc.

3. **Analyze Each Option**:
- **Option A**: 2p > 2s
- This is true, but after filling, the order does not reverse. So, this option is not correct.
- **Option B**: 3d > 4s
- Initially, 4s has lower energy than 3d. However, after filling, the electrons in 3d repel the electrons in 4s, causing 4s to have a higher energy than 3d. Thus, this option shows a reversal in energy sequence.
- **Option C**: 4p < 5s
- This is true, and after filling, the order remains the same. So, this option is not correct.
- **Option D**: 5p < 6s
- This is true as well, and after filling, the order remains the same. So, this option is not correct.

4. **Conclusion**:
The only pair where the energy sequence is reversed after filling is **Option B: 3d > 4s**.

### Final Answer:
**Option B: 3d > 4s**

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