Which of the following ions is smallest in size?

Options

A. `Cl^(-)`
B. `Na^(+)`
C. `Mg^(2+)`
D. `S^(2-)`

Answer

To determine which of the given ions is the smallest in size, we will analyze the ions provided: Cl⁻, Na⁺, Mg²⁺, and S²⁻.

### Step-by-Step Solution:

1. **Identify the Ions and Their Charges**:
- Cl⁻ (Chloride ion)
- Na⁺ (Sodium ion)
- Mg²⁺ (Magnesium ion)
- S²⁻ (Sulfide ion)

2. **Determine the Isoelectronic Species**:
- Isoelectronic species are ions or atoms that have the same number of electrons.
- Cl⁻, Na⁺, and Mg²⁺ all have 10 electrons (same as Ne), while S²⁻ has 18 electrons (same as Ar).

3. **Understand the Trend in Ionic Size**:
- Anions (negatively charged ions) are generally larger than their neutral atoms because they have gained electrons, which increases electron-electron repulsion.
- Cations (positively charged ions) are smaller than their neutral atoms because they have lost electrons, resulting in a stronger attraction between the remaining electrons and the nucleus.

4. **Compare the Sizes of Cations**:
- Among the cations, Mg²⁺ has a +2 charge, while Na⁺ has a +1 charge.
- The greater the positive charge, the smaller the ionic radius because the effective nuclear charge experienced by the remaining electrons increases, pulling them closer to the nucleus.

5. **Conclusion**:
- Since Mg²⁺ has a higher positive charge than Na⁺, it will be smaller in size.
- Therefore, the smallest ion among the given options is **Mg²⁺**.

### Final Answer:
The smallest ion in size is **Mg²⁺**.