Assertion: Ionisation potential of `Be` (atomic no.4) is less than `B` (atomic no.5).
Reason: The first electron released from `Be` is of p-orbital but that from `B` is of s-orbital.

Options

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

Explanation

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

**Step 1: Understand the assertion.**
The assertion states that the ionization potential of beryllium (Be) is less than that of boron (B).

- Beryllium (atomic number 4) has the electronic configuration: 1s² 2s².
- Boron (atomic number 5) has the electronic configuration: 1s² 2s² 2p¹.

**Step 2: Understand the concept of ionization potential.**
Ionization potential (or ionization energy) is the energy required to remove an electron from an atom in its gaseous state. Generally, the more stable an electron configuration is (like a filled or half-filled subshell), the higher the ionization potential.

**Step 3: Analyze the electronic configurations.**
- In beryllium, the outermost shell (n=2) is fully filled with 2 electrons in the 2s orbital.
- In boron, the outermost shell has 2 electrons in the 2s orbital and 1 electron in the 2p orbital.

**Step 4: Compare the stability of the configurations.**
- Beryllium's configuration is stable due to the fully filled 2s orbital.
- Boron's configuration is less stable because it has an unpaired electron in the 2p orbital, making it more likely to lose that electron to achieve a stable configuration similar to beryllium.

**Step 5: Conclusion about the assertion.**
Since boron has an unpaired electron and is more likely to lose it, it will have a lower ionization potential compared to beryllium. Thus, the assertion is true.

**Step 6: Understand the reason.**
The reason states that the first electron released from beryllium is from a p-orbital, while that from boron is from an s-orbital.

- In reality, the first electron removed from beryllium is from the 2s orbital (not p), and the first electron removed from boron is from the 2p orbital.

**Step 7: Conclusion about the reason.**
Since the reason is incorrect (the assertion about the orbitals is wrong), we conclude that the reason is false.

**Final Conclusion:**
- The assertion is true: Ionization potential of Be is less than that of B.
- The reason is false: The first electron released from Be is from the s-orbital, not the p-orbital.

Thus, the correct answer is that the assertion is true and the reason is false.