Which of the following has highest ionisation enthalpy ?

Options

A. Nitrogen
B. Phosphorus
C. Oxygen
D. Sulphur

Explanation

To determine which of the given elements has the highest ionization enthalpy, we will analyze the elements: nitrogen, phosphorus, oxygen, and salt water (which is not an element but we will consider the oxygen component).

### Step-by-Step Solution:

1. **Understanding Ionization Enthalpy**:
- Ionization enthalpy (or ionization energy) is the minimum amount of energy required to remove the most loosely bound electron from a gaseous atom.
- Generally, ionization energy increases across a period and decreases down a group in the periodic table.

2. **Identify the Elements**:
- The elements provided are:
- Nitrogen (N) - Atomic number 7
- Phosphorus (P) - Atomic number 15
- Oxygen (O) - Atomic number 8
- Salt water (H₂O) - Contains oxygen, so we will consider the oxygen part.

3. **Electron Configuration**:
- **Nitrogen (N)**: 1s² 2s² 2p³
- **Oxygen (O)**: 1s² 2s² 2p⁴
- **Phosphorus (P)**: 1s² 2s² 2p⁶ 3s² 3p³

4. **Comparing Ionization Energies**:
- **Oxygen vs. Nitrogen**: Oxygen has a higher atomic number than nitrogen and is further to the right in the periodic table, which generally means it has a higher ionization energy. However, oxygen has a half-filled p subshell (2p⁴) which makes it slightly less stable than nitrogen (2p³).
- **Nitrogen vs. Phosphorus**: Nitrogen is higher up in the periodic table than phosphorus, which means nitrogen will have a higher ionization energy than phosphorus.

5. **Conclusion**:
- Between nitrogen and oxygen, nitrogen has a higher ionization energy due to its smaller size and half-filled p subshell stability.
- Phosphorus has a lower ionization energy than both nitrogen and oxygen due to its larger atomic size and additional electron shells.

Thus, the element with the highest ionization enthalpy among the given options is **Nitrogen (N)**.

### Final Answer:
**Option 1: Nitrogen**