Atomic numbers of few elements are given below which of the pairs belongs to s-block?

Options

A. 7,4
B. 3,20
C. 8,15
D. 9,17

Explanation

To determine which pairs of elements belong to the s-block, we need to analyze the electronic configurations of the elements based on their atomic numbers. The s-block elements are those whose last electron enters the s-orbital.

### Step-by-Step Solution:

1. **Identify the Atomic Numbers and Write Electronic Configurations**:
- For each pair of atomic numbers given in the options, we will write the electronic configurations.

2. **Option A: Atomic Numbers 7 and 4**:
- Atomic number 7 (Nitrogen):
- Electronic configuration: 1s² 2s² 2p³ (last electron in p-orbital)
- Atomic number 4 (Beryllium):
- Electronic configuration: 1s² 2s² (last electron in s-orbital)
- **Conclusion**: This pair does not belong to the s-block.

3. **Option B: Atomic Numbers 3 and 20**:
- Atomic number 3 (Lithium):
- Electronic configuration: 1s² 2s¹ (last electron in s-orbital)
- Atomic number 20 (Calcium):
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² (last electron in s-orbital)
- **Conclusion**: This pair belongs to the s-block.

4. **Option C: Atomic Numbers 8 and 15**:
- Atomic number 8 (Oxygen):
- Electronic configuration: 1s² 2s² 2p⁴ (last electron in p-orbital)
- Atomic number 15 (Phosphorus):
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p³ (last electron in p-orbital)
- **Conclusion**: This pair does not belong to the s-block.

5. **Option D: Atomic Numbers 9 and 17**:
- Atomic number 9 (Fluorine):
- Electronic configuration: 1s² 2s² 2p⁵ (last electron in p-orbital)
- Atomic number 17 (Chlorine):
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁵ (last electron in p-orbital)
- **Conclusion**: This pair does not belong to the s-block.

### Final Answer:
The only pair that belongs to the s-block is **Option B: Atomic numbers 3 and 20**.