Assertion: The first ionisation energy of aluminium is lower than that of magnesium.
Reason: The ionic radius of aluminium is smaller than that of magnesium.

Options

A. If Both Assertion & Reason are true and the reason is the correct explanation of the assertion then mark (1) .
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion then mark (2)
C. If Assertion is true statement but Reason is false then mark (3)
D. If both Assertion and Reason are false statements then mark (4)

Answer

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 first ionisation energy of aluminium is lower than that of magnesium."

**Explanation:**
- Ionization energy is the energy required to remove an electron from an atom in the gaseous state.
- Magnesium (Mg) has an atomic number of 12 and an electronic configuration of 1s² 2s² 2p⁶ 3s². This means it has a fully filled outer shell (3s²).
- Aluminium (Al) has an atomic number of 13 and an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p¹. This means it has a partially filled outer shell (3s² 3p¹).

**Conclusion for Step 1:**
- Since magnesium has a fully filled outer shell, it is more stable and requires more energy to remove an electron compared to aluminium, which has a partially filled outer shell. Thus, the assertion is correct.

### Step 2: Understand the Reason
The reason states that "The ionic radius of aluminium is smaller than that of magnesium."

**Explanation:**
- The ionic radius refers to the size of an ion in a crystal lattice.
- As we move from left to right across a period in the periodic table, the ionic radius generally decreases due to an increase in nuclear charge, which pulls the electrons closer to the nucleus.
- Since aluminium comes after magnesium in the periodic table, its ionic radius is indeed smaller than that of magnesium.

**Conclusion for Step 2:**
- The reason is also true, but it does not correctly explain why the first ionization energy of aluminium is lower than that of magnesium.

### Final Conclusion
- Both the assertion and the reason are true, but the reason does not provide a correct explanation for the assertion. Therefore, the correct answer is that both the assertion and reason are true, but the reason is not a correct explanation of the assertion.

### Final Answer
The correct option is **B**: Both assertion and reason are true, but the reason is not a correct explanation of the assertion.

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