In which of the following pairs, the first member has higher first ionization energy ?

Options

A. `N,O`
B. `B,Be`
C. `Al,Ga`
D. `Cl,F`

Explanation

To determine which of the following pairs has the first member with a higher first ionization energy, we will analyze each pair based on their electronic configurations and the stability of their orbitals.

### Step-by-Step Solution:

1. **Analyze the first pair: Nitrogen (N) and Oxygen (O)**
- **Electronic Configuration:**
- Nitrogen: 1s² 2s² 2p³
- Oxygen: 1s² 2s² 2p⁴
- **Stability of Orbitals:**
- Nitrogen has a half-filled p orbital (2p³), which is more stable than the partially filled p orbital of oxygen (2p⁴).
- **Conclusion:**
- Since nitrogen is more stable, it has a higher first ionization energy than oxygen.

2. **Analyze the second pair: Boron (B) and Beryllium (Be)**
- **Electronic Configuration:**
- Boron: 1s² 2s² 2p¹
- Beryllium: 1s² 2s²
- **Stability of Orbitals:**
- Beryllium has a fully filled s orbital (2s²), which is more stable than the partially filled p orbital of boron.
- **Conclusion:**
- Therefore, beryllium has a higher first ionization energy than boron.

3. **Analyze the third pair: Aluminium (Al) and Gallium (Ga)**
- **Electronic Configuration:**
- Aluminium: 1s² 2s² 2p⁶ 3s² 3p¹
- Gallium: 1s² 2s² 2p⁶ 3s² 3p¹ 3d¹⁰
- **Stability of Orbitals:**
- Gallium has d orbitals, which provide poor shielding. This results in a smaller atomic size for gallium compared to aluminium.
- **Conclusion:**
- A smaller atomic size leads to a higher ionization energy; thus, gallium has a higher first ionization energy than aluminium.

4. **Analyze the fourth pair: Chlorine (Cl) and Fluorine (F)**
- **Electronic Configuration:**
- Chlorine: 1s² 2s² 2p⁶ 3s² 3p⁵
- Fluorine: 1s² 2s² 2p⁵
- **Stability of Orbitals:**
- As we move down the group in the periodic table, atomic size increases, leading to a decrease in ionization energy. Fluorine is smaller than chlorine.
- **Conclusion:**
- Therefore, fluorine has a higher first ionization energy than chlorine.

### Final Conclusion:
The only pair where the first member has a higher first ionization energy is the first pair: **Nitrogen (N) and Oxygen (O)**.