In each of the following questions a Statement of Assertion (A ) is given followed by a corresponding statement of Reason (R ) Just below it . Of the statement mark the correct answer as
Assertion `Na^(+)` and `A1^(3+)` are isoelectronic but the magnitude of the ionic radius of `A1^(3+)` is less than that of `Na^(+)`
Reason . The magnitude of effective nuclear charge of the outer shell electrons in `A1^(3+)` is greater than in `Na^(+)`

Explanation

To solve the question, we need to analyze both the Assertion (A) and the Reason (R) statements provided.

### Step 1: Understand the Assertion
The assertion states that `Na^(+)` and `Al^(3+)` are isoelectronic, meaning they have the same number of electrons.

- **Sodium ion (Na^+)** has lost one electron from neutral sodium (11 electrons), resulting in 10 electrons.
- **Aluminum ion (Al^(3+))** has lost three electrons from neutral aluminum (13 electrons), also resulting in 10 electrons.

Thus, both ions have 10 electrons, confirming that they are indeed isoelectronic.

### Step 2: Analyze the Ionic Radii
The assertion further claims that the ionic radius of `Al^(3+)` is less than that of `Na^(+)`.

- The ionic radius is influenced by the effective nuclear charge experienced by the outer shell electrons.
- In `Na^(+)`, the effective nuclear charge is lower compared to `Al^(3+)`, because `Na^(+)` has a nuclear charge of +11 (11 protons) acting on 10 electrons, while `Al^(3+)` has a nuclear charge of +13 (13 protons) acting on the same 10 electrons.

### Step 3: Understand the Reason
The reason states that the effective nuclear charge of the outer shell electrons in `Al^(3+)` is greater than in `Na^(+)`.

- This is true because the effective nuclear charge increases with the number of protons in the nucleus.
- Since `Al^(3+)` has more protons (13) than `Na^(+)` (11), the outer shell electrons in `Al^(3+)` experience a stronger pull towards the nucleus, resulting in a smaller ionic radius.

### Conclusion
Both the Assertion (A) and the Reason (R) are correct. The reason correctly explains why the assertion is true.

### Final Answer
Both the Assertion and Reason are correct, and the Reason correctly explains the Assertion.

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