Which of the following statements is`//are` correct ?

Options

A. An anion is larger than a cation if they are isoelectronic.
B. Out of `Na^(o+)` and `Al^(3+)`, `Na^(o+)` has the largest size.
C. The ionic radii of trivalent lanthanides `[La^(3+), Ce^(3+), Pr^(3+), ….]` decreases with increasing atomic number.
D. Out of `P^(3-), S^(2-)`, and `Cl^(ɵ), Cl^(ɵ)` ion has the largest size.

Explanation

To determine which statements are correct, we will analyze each statement step by step.

### Step 1: Analyze Statement 1
**Statement:** An anion is larger than a cation if they are isoelectronic.

**Explanation:**
- Isoelectronic species have the same number of electrons.
- For example, consider Mg²⁺ (12 protons, 10 electrons) and F⁻ (9 protons, 10 electrons).
- The ionic radius of Mg²⁺ is 72 pm, while that of F⁻ is 133 pm.
- Since the effective nuclear charge (ENC) is higher for Mg²⁺ (12/10 = 1.2) compared to F⁻ (9/10 = 0.9), the size of the cation (Mg²⁺) is smaller than that of the anion (F⁻).

**Conclusion:** This statement is correct.

### Step 2: Analyze Statement 2
**Statement:** Out of sodium ion (Na⁺) and aluminum ion (Al³⁺), sodium ion has the largest size.

**Explanation:**
- The ionic radius of Na⁺ is approximately 102 pm, while that of Al³⁺ is about 53.5 pm.
- Since Na⁺ has a lower effective nuclear charge (11 protons and 10 electrons gives ENC = 1.1) compared to Al³⁺ (13 protons and 10 electrons gives ENC = 1.2), Na⁺ is indeed larger.

**Conclusion:** This statement is correct.

### Step 3: Analyze Statement 3
**Statement:** The ionic radii of trivalent lanthanides decrease with increasing atomic number.

**Explanation:**
- As we move from left to right in the lanthanide series, the ionic radii decrease due to the increase in effective nuclear charge.
- For example, La³⁺ (106 pm), Ce³⁺ (103 pm), and Pr³⁺ (101 pm) show a decreasing trend in size.

**Conclusion:** This statement is correct.

### Step 4: Analyze Statement 4
**Statement:** Out of phosphide (P³⁻), sulfide (S²⁻), and chloride (Cl⁻), chloride has the largest size.

**Explanation:**
- All three ions are isoelectronic with 18 electrons.
- The effective nuclear charge increases from P³⁻ (15 protons) to S²⁻ (16 protons) to Cl⁻ (17 protons).
- Therefore, the size decreases in the order: P³⁻ > S²⁻ > Cl⁻.
- Thus, chloride does not have the largest size; phosphide does.

**Conclusion:** This statement is incorrect.

### Final Answer
The correct statements are:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Correct
- Statement 4: Incorrect

Thus, the correct statements are **1, 2, and 3**.

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