Which of the following transitions are not allowed in the normal electronic emission spectrum of an atom?

Options

A. `1srarr2s`
B. `2prarr1s`
C. `5drarr4p`
D. `5prarr3s`

Explanation

To determine which transitions are not allowed in the normal electronic emission spectrum of an atom, we need to analyze each transition based on the principles of electronic transitions and the n + l rule.

### Step-by-Step Solution:

1. **Understand the Emission Spectrum**: In an emission spectrum, electrons transition from higher energy levels (higher principal quantum number, n) to lower energy levels. Only downward transitions are allowed.

2. **Identify the Transitions**: We will analyze each transition given in the options.

- **Option 1: 1 to 2p**
- Transition: 1s (n=1, l=0) to 2p (n=2, l=1)
- This transition is from a lower energy level (1s) to a higher energy level (2p), which is not allowed in emission spectra.
- **Conclusion**: Not allowed.

- **Option 2: 2p to 1s**
- Transition: 2p (n=2, l=1) to 1s (n=1, l=0)
- This transition is from a higher energy level (2p) to a lower energy level (1s), which is allowed in emission spectra.
- **Conclusion**: Allowed.

- **Option 3: 5d to 4p**
- Transition: 5d (n=5, l=2) to 4p (n=4, l=1)
- This transition is from a higher energy level (5d) to a lower energy level (4p), which is allowed in emission spectra.
- **Conclusion**: Allowed.

- **Option 4: 5p to 3s**
- Transition: 5p (n=5, l=1) to 3s (n=3, l=0)
- This transition is from a higher energy level (5p) to a lower energy level (3s), which is allowed in emission spectra.
- **Conclusion**: Allowed.

3. **Final Answer**: Based on the analysis, the transition that is not allowed in the normal electronic emission spectrum of an atom is **Option 1: 1 to 2p**.