The CORRECT order of first ionisation enthalpy is :

Options

A. `Mg lt S lt Al lt P `
B. `Mg lt Al lt S lt P`
C. `Al lt Mg lt S lt P`
D. `Mg lt Al lt P lt S`

Answer

To determine the correct order of first ionization enthalpy for the given elements (Magnesium, Aluminum, Phosphorus, and Sulfur), we will analyze their electronic configurations and apply the principles of ionization energy.

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers**:
- Magnesium (Mg) - Atomic Number 12
- Aluminum (Al) - Atomic Number 13
- Phosphorus (P) - Atomic Number 15
- Sulfur (S) - Atomic Number 16

2. **Write the Electronic Configurations**:
- Magnesium (Mg): 1s² 2s² 2p⁶ 3s²
- Aluminum (Al): 1s² 2s² 2p⁶ 3s² 3p¹
- Phosphorus (P): 1s² 2s² 2p⁶ 3s² 3p³
- Sulfur (S): 1s² 2s² 2p⁶ 3s² 3p⁴

3. **Analyze the Trends in Ionization Energy**:
- Ionization energy generally increases across a period from left to right due to increasing nuclear charge and effective nuclear charge (Z_eff).
- Magnesium has a fully filled 3s subshell, while Aluminum has one electron in the 3p subshell. Removing an electron from a fully filled subshell (Mg) is generally easier than removing from a partially filled subshell (Al).
- Phosphorus has a half-filled 3p subshell (3p³), which is more stable than the 3p⁴ configuration of Sulfur. Therefore, it requires more energy to remove an electron from Phosphorus than from Sulfur.

4. **Establish the Order Based on Stability**:
- The stability order based on the electronic configurations and the principles of ionization energy is:
- Phosphorus (most stable, highest ionization energy)
- Sulfur
- Magnesium
- Aluminum (least stable, lowest ionization energy)

5. **Final Order of First Ionization Enthalpy**:
- The correct order of first ionization enthalpy is:
- P > S > Mg > Al

### Conclusion:
The correct order of first ionization enthalpy for the elements is **Phosphorus (P) > Sulfur (S) > Magnesium (Mg) > Aluminum (Al)**.