In which of the following pairs, the ionisation energy of the first species is less than of the second

Options

A. `O^(+)` , O
B. S, P
C. N, P
D. `Be^(+)` , Be

Answer

To determine which pair has the first species with a lower ionization energy than the second, we can analyze the ionization energies of the given species based on their electronic configurations and general trends in the periodic table.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom or ion. Generally, ionization energy increases across a period and decreases down a group in the periodic table.

2. **Analyzing the Given Pairs**:
- We need to compare the ionization energies of the first species with that of the second in each pair.

3. **Pair Analysis**:
- **Pair 1: Beryllium (Be) and Beryllium Ion (Be⁺)**:
- Beryllium (Be) has a neutral configuration of 1s² 2s².
- Beryllium ion (Be⁺) has a configuration of 1s².
- Removing an electron from Be⁺ requires more energy than removing one from neutral Be.
- Therefore, ionization energy of Be < ionization energy of Be⁺.

- **Pair 2: Nitrogen (N) and Oxygen (O)**:
- Nitrogen (N) has a configuration of 1s² 2s² 2p³.
- Oxygen (O) has a configuration of 1s² 2s² 2p⁴.
- Due to the half-filled stability of nitrogen, it has a higher ionization energy than oxygen.
- Therefore, ionization energy of N > ionization energy of O.

- **Pair 3: Phosphorus (P) and Sulfur (S)**:
- Phosphorus (P) has a configuration of 1s² 2s² 2p⁶ 3s² 3p³.
- Sulfur (S) has a configuration of 1s² 2s² 2p⁶ 3s² 3p⁴.
- Similar to nitrogen and oxygen, phosphorus has a half-filled p subshell, which makes it more stable and thus has a higher ionization energy than sulfur.
- Therefore, ionization energy of P > ionization energy of S.

4. **Conclusion**:
- From the analysis, we find that the only pair where the first species has a lower ionization energy than the second is:
- **Sulfur (S) and Phosphorus (P)**: ionization energy of S < ionization energy of P.
- Thus, the correct answer is **Option 2**.