In which of the following compounds cation and anion size ratio is minimum?

Options

A. `CsF`
B. `LiI`
C. `LiF`
D. `CsI`

Explanation

To solve the problem of determining which compound has the minimum cation to anion size ratio, we need to analyze the sizes of the cations and anions in each of the given compounds.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Ions:**
- Option 1: Cesium Fluoride (CsF) → Cation: Cs⁺, Anion: F⁻
- Option 2: Lithium Iodide (LiI) → Cation: Li⁺, Anion: I⁻
- Option 3: Lithium Fluoride (LiF) → Cation: Li⁺, Anion: F⁻
- Option 4: Cesium Iodide (CsI) → Cation: Cs⁺, Anion: I⁻

2. **Determine the Sizes of Cations:**
- **Cs⁺ (Cesium ion)**: Larger size due to being lower in the periodic table.
- **Li⁺ (Lithium ion)**: Smaller size as it is higher in the periodic table.

3. **Determine the Sizes of Anions:**
- **F⁻ (Fluoride ion)**: Smaller size.
- **I⁻ (Iodide ion)**: Larger size due to being lower in the periodic table.

4. **Calculate the Size Ratios:**
- For **CsF**: Size ratio = Size of Cs⁺ / Size of F⁻ (larger cation, smaller anion)
- For **LiI**: Size ratio = Size of Li⁺ / Size of I⁻ (smaller cation, larger anion)
- For **LiF**: Size ratio = Size of Li⁺ / Size of F⁻ (smaller cation, smaller anion)
- For **CsI**: Size ratio = Size of Cs⁺ / Size of I⁻ (larger cation, larger anion)

5. **Compare the Ratios:**
- **CsF**: Larger cation and smaller anion → larger ratio.
- **LiI**: Smaller cation and larger anion → smaller ratio.
- **LiF**: Smaller cation and smaller anion → moderate ratio.
- **CsI**: Larger cation and larger anion → larger ratio.

6. **Conclusion:**
- The compound with the minimum cation to anion size ratio is **Lithium Iodide (LiI)**, as it has the smallest cation (Li⁺) and the largest anion (I⁻) compared to the other options.

### Final Answer:
The correct option is **Option B: Lithium Iodide (LiI)**.