Along the period `(rarr)` atomic`//`ionic radii and metallic character decreases while `IE`, `EN` , non-metallic character and oxidising power increases. Down the group `(darr)`, atomic`//`ionic radii, metallic character and reducing character increase while `IE` and `EN` decrease. However, `Delta_(eg)H^(ɵ)` becomes less negative down a group but more negative along a period.
In which of the following pairs, both species have nearly the same size ? (more than one correct)

Options

A. `Mg^(2+), Al^(3+)`
B. `K^(o+)`, `F^(ɵ)`
C. `Li^(o+)`, `Mg^(2+)`
D. `Rb^(o+), O^(2-)`

Explanation

To determine which pairs of ions have nearly the same size, we will analyze the ionic radii of the given ions. The pairs provided in the question are:

1. Magnesium ion (Mg²⁺) and Aluminum ion (Al³⁺)
2. Potassium ion (K⁺) and Fluoride ion (F⁻)
3. Lithium ion (Li⁺) and Magnesium ion (Mg²⁺)
4. Rubidium ion (Rb⁺) and Oxide ion (O²⁻)

We will look at the ionic radii of each ion:

1. **Magnesium ion (Mg²⁺)**: 72 pm
2. **Aluminum ion (Al³⁺)**: 53.5 pm
3. **Potassium ion (K⁺)**: 138 pm
4. **Fluoride ion (F⁻)**: 133 pm
5. **Lithium ion (Li⁺)**: 76 pm
6. **Rubidium ion (Rb⁺)**: 152 pm
7. **Oxide ion (O²⁻)**: 140 pm

### Step-by-Step Analysis:

1. **Compare Mg²⁺ and Al³⁺**:
- Mg²⁺: 72 pm
- Al³⁺: 53.5 pm
- **Conclusion**: These ions have significantly different sizes.

2. **Compare K⁺ and F⁻**:
- K⁺: 138 pm
- F⁻: 133 pm
- **Conclusion**: These ions have nearly the same size (difference of 5 pm).

3. **Compare Li⁺ and Mg²⁺**:
- Li⁺: 76 pm
- Mg²⁺: 72 pm
- **Conclusion**: These ions have a small difference in size (4 pm), but they are not nearly the same size.

4. **Compare Rb⁺ and O²⁻**:
- Rb⁺: 152 pm
- O²⁻: 140 pm
- **Conclusion**: These ions have a significant difference in size.

### Final Answer:
The pairs of ions that have nearly the same size are:
- **Potassium ion (K⁺) and Fluoride ion (F⁻)** (Option 2)
- **Lithium ion (Li⁺) and Magnesium ion (Mg²⁺)** (Option 3)

Therefore, the correct options are **2 and 3**.