In which of the following pairs are the ions isoelectronic ?

Options

A. `Mg^(2+),Ar`
B. `Na^(+),O^(2-)`
C. `AI^(3+),CI^(-)`
D. `K^(+),Ne`

Explanation

To determine which pairs of ions are isoelectronic, we need to find out how many electrons each ion has. Isoelectronic ions have the same number of electrons.

### Step-by-Step Solution:

1. **Identify the Ions**: Write down the pairs of ions given in the question.

2. **Determine the Number of Electrons**:
- For each ion, find the atomic number (number of protons) and adjust for the charge to find the number of electrons.
- **Example**:
- For Na⁺ (Sodium ion), the atomic number is 11. Since it has a +1 charge, it has 11 - 1 = 10 electrons.
- For O²⁻ (Oxide ion), the atomic number is 8. Since it has a -2 charge, it has 8 + 2 = 10 electrons.

3. **Compare the Number of Electrons**:
- Check if the number of electrons in the ions from each pair is the same.
- If they are the same, the ions are isoelectronic.

4. **Repeat for All Pairs**: Continue this process for all pairs of ions provided in the question.

5. **Identify the Isoelectronic Pair**: Once you have compared all pairs, identify which pairs have the same number of electrons.

### Example Calculation:
- **Pair 1**: Na⁺ and O²⁻
- Na⁺: 11 - 1 = 10 electrons
- O²⁻: 8 + 2 = 10 electrons
- **Conclusion**: Isoelectronic (both have 10 electrons).

- **Pair 2**: Al³⁺ and Cl⁻
- Al³⁺: 13 - 3 = 10 electrons
- Cl⁻: 17 + 1 = 18 electrons
- **Conclusion**: Not isoelectronic.

- **Pair 3**: K⁺ and Ne
- K⁺: 19 - 1 = 18 electrons
- Ne: 10 electrons
- **Conclusion**: Not isoelectronic.

### Final Answer:
From the calculations, the only isoelectronic pair is Na⁺ and O²⁻.