Assertion: ionisation energy of sulphur is less than that of phosphorus.
Reason: Half filled p-orbitals have extra stability.

Options

A. Both A and B true and R is the correct explanation A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that the ionization energy of sulfur is less than that of phosphorus.

- **Ionization Energy**: This is the energy required to remove an electron from an atom in its gaseous state. Generally, ionization energy increases across a period and decreases down a group in the periodic table.

### Step 2: Compare the Electron Configurations
- **Phosphorus (P)** has the electron configuration: \(1s^2 2s^2 2p^6 3s^2 3p^3\)
- **Sulfur (S)** has the electron configuration: \(1s^2 2s^2 2p^6 3s^2 3p^4\)

### Step 3: Analyze the Stability of Electron Configurations
- Phosphorus has three electrons in the 3p sublevel, which means it has a half-filled p subshell (3p^3).
- Sulfur has four electrons in the 3p sublevel (3p^4), which means it has one more electron than phosphorus.

### Step 4: Stability of Half-Filled Orbitals
- Half-filled orbitals (like in phosphorus) are known to be more stable due to symmetry and exchange energy. This extra stability means that it is harder to remove an electron from phosphorus compared to sulfur.

### Step 5: Conclusion on Ionization Energies
Since phosphorus has a half-filled p subshell, it is more stable and requires more energy to remove an electron compared to sulfur, which has a less stable configuration. Therefore, the assertion that the ionization energy of sulfur is less than that of phosphorus is **true**.

### Step 6: Evaluate the Reason
The reason states that half-filled p-orbitals have extra stability. This is also **true** as explained in the previous steps.

### Final Conclusion
Both the assertion and reason are true, and the reason correctly explains the assertion.

### Summary
- **Assertion**: True (Ionization energy of sulfur is less than that of phosphorus)
- **Reason**: True (Half-filled p-orbitals have extra stability)