Which of the following pairs would have a smaller size. Explain.
(a) `Na^(o+) or Mg^(2+)` , (b) `O^(2-) or F^(ɵ)`
(c ) `P or As`

Explanation

To determine which of the following pairs has a smaller size, we will analyze each pair step-by-step based on the principles of ionic and atomic sizes.

### Step 1: Compare `Na^(o+)` and `Mg^(2+)`
1. **Identify the charges**:
- Sodium ion (Na⁺) has a +1 charge.
- Magnesium ion (Mg²⁺) has a +2 charge.

2. **Understand the trend in ionic size**:
- Cations with higher positive charges are smaller in size because they have a greater effective nuclear charge pulling the electrons closer to the nucleus.

3. **Conclusion**:
- Since Mg²⁺ has a higher positive charge than Na⁺, Mg²⁺ will be smaller in size.
- Therefore, **Mg²⁺ is smaller than Na⁺**.

### Step 2: Compare `O^(2-)` and `F^(ɵ)`
1. **Identify the charges**:
- Oxide ion (O²⁻) has a -2 charge.
- Fluoride ion (F⁻) has a -1 charge.

2. **Understand the trend in ionic size**:
- Anions with higher negative charges are larger in size because they have more electrons repelling each other.

3. **Conclusion**:
- Since O²⁻ has a higher negative charge than F⁻, O²⁻ will be larger in size.
- Therefore, **F⁻ is smaller than O²⁻**.

### Step 3: Compare `P` and `As`
1. **Identify the elements**:
- Phosphorus (P) is above Arsenic (As) in the periodic table.

2. **Understand the trend in atomic size**:
- As you move down a group in the periodic table, the atomic size increases due to the addition of electron shells.

3. **Conclusion**:
- Since As is below P in the periodic table, P will be smaller in size.
- Therefore, **P is smaller than As**.

### Final Summary of Sizes:
- (a) **Mg²⁺ is smaller than Na⁺**
- (b) **F⁻ is smaller than O²⁻**
- (c) **P is smaller than As**