Which of the following ion is the largest in size ?

Options

A. `K^(+)`
B. `Ca^(2+)`
C. `CI^(-)`
D. `S^(-2)`

Answer

To determine which of the given ions is the largest in size, we can follow these steps:

### Step 1: Identify the ions and their electron configurations
The ions given are:
- K⁺ (Potassium ion)
- Ca²⁺ (Calcium ion)
- Cl⁻ (Chloride ion)
- S²⁻ (Sulfide ion)

### Step 2: Determine the number of protons and electrons in each ion
1. **K⁺**:
- Atomic number of K = 19
- Protons = 19
- Electrons = 19 - 1 = 18

2. **Ca²⁺**:
- Atomic number of Ca = 20
- Protons = 20
- Electrons = 20 - 2 = 18

3. **Cl⁻**:
- Atomic number of Cl = 17
- Protons = 17
- Electrons = 17 + 1 = 18

4. **S²⁻**:
- Atomic number of S = 16
- Protons = 16
- Electrons = 16 + 2 = 18

### Step 3: Compare the effective nuclear charge
The effective nuclear charge (Z_eff) can be understood as the net positive charge experienced by electrons in an atom. It can be calculated as:
\[ Z_{\text{eff}} = \text{Number of protons} - \text{Number of inner shell electrons} \]

In this case, since all ions have 18 electrons, we will compare the number of protons:
- **K⁺**: 19 protons
- **Ca²⁺**: 20 protons
- **Cl⁻**: 17 protons
- **S²⁻**: 16 protons

### Step 4: Analyze the attraction between protons and electrons
The greater the number of protons, the stronger the attraction to the electrons. Thus:
- **Ca²⁺** (20 protons) will have the strongest attraction to its 18 electrons, making it the smallest.
- **K⁺** (19 protons) will have a strong attraction to its 18 electrons, making it smaller than Cl⁻ and S²⁻.
- **Cl⁻** (17 protons) will have a weaker attraction than K⁺.
- **S²⁻** (16 protons) will have the weakest attraction to its 18 electrons, making it the largest.

### Conclusion
Among the ions K⁺, Ca²⁺, Cl⁻, and S²⁻, the largest ion is **S²⁻**.