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

Options

A. If both assertion and reason are true annd the reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but the reason is false
D. if assertion is false but the reason is true

Answer

To analyze the assertion and reason provided in the question, we will break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "The first ionisation enthalpy of aluminium is lower than that of magnesium."

- **Ionisation enthalpy** is the energy required to remove an electron from an atom in its gaseous state.
- Aluminium (Al) has the electronic configuration of \(1s^2 2s^2 2p^6 3s^2 3p^1\), while magnesium (Mg) has the electronic configuration of \(1s^2 2s^2 2p^6 3s^2\).

### Step 2: Compare Ionisation Energies
To compare the ionisation enthalpies of Al and Mg, we need to consider the following:
- For magnesium, the electron to be removed is from a fully filled \(3s^2\) subshell.
- For aluminium, the electron to be removed is from the \(3p^1\) subshell.

Removing an electron from a fully filled subshell (like \(3s^2\)) generally requires more energy than removing an electron from a partially filled subshell (like \(3p^1\)). Thus, we can conclude that the first ionisation enthalpy of aluminium is indeed lower than that of magnesium.

### Step 3: Understand the Reason
The reason provided is "Ionic radius of aluminium is smaller than that of magnesium."

- The ionic radius refers to the size of an ion in a crystal lattice.
- As we move 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.

### Step 4: Analyze the Relationship Between Assertion and Reason
While the reason is correct in stating that the ionic radius of aluminium is smaller than that of magnesium, it does not correctly explain why the first ionisation enthalpy of aluminium is lower. The correct explanation involves the concept of nuclear charge and the stability of the electron configurations rather than just the ionic radius.

### Conclusion
- **Assertion**: True (the first ionisation enthalpy of aluminium is lower than that of magnesium).
- **Reason**: True (the ionic radius of aluminium is smaller than that of magnesium), but it is not the correct explanation for the assertion.

Thus, both the assertion and reason are correct, but the reason does not adequately explain the assertion.

### Final Answer
The assertion is true, and the reason is true but does not explain the assertion. Therefore, the correct option is that both are true, but the reason is not the correct explanation.

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