The order of decreasing basicity in the four halide ions is :

Options

A. `I^(-) gt Br^(-) gt Cl^(-) gt F^(-)`
B. `Cl^(-) gt Br^(-) gt I^(-) gt F^(-)`
C. `F^(-) gt Cl^(-) gt Br^(-) gt I^(-)`
D. `Cl^(-) gt F^(-) gt Br^(-) gt I^(-)`

Answer

To determine the order of decreasing basicity among the four halide ions (F⁻, Cl⁻, Br⁻, and I⁻), we can follow these steps:

### Step-by-Step Solution:

1. **Understand Acid-Base Relationships**:
Basicity is the ability of a species to accept protons (H⁺). The strength of a base is inversely related to the strength of its conjugate acid. This means that if an acid is strong, its conjugate base will be weak.

2. **Identify the Conjugate Acids**:
The halides correspond to the following acids:
- HF (hydrofluoric acid)
- HCl (hydrochloric acid)
- HBr (hydrobromic acid)
- HI (hydroiodic acid)

3. **Order of Acid Strength**:
The strength of these acids can be arranged as follows:
- HF < HCl < HBr < HI
This means HF is the weakest acid and HI is the strongest acid.

4. **Determine the Conjugate Bases**:
The conjugate bases of these acids are:
- F⁻ (from HF)
- Cl⁻ (from HCl)
- Br⁻ (from HBr)
- I⁻ (from HI)

5. **Order of Basicity**:
Since the conjugate base of a strong acid is weak, we can deduce the order of basicity from the order of acid strength:
- HI (strongest acid) → I⁻ (weakest base)
- HBr → Br⁻
- HCl → Cl⁻
- HF (weakest acid) → F⁻ (strongest base)

Therefore, the order of decreasing basicity is:
F⁻ > Cl⁻ > Br⁻ > I⁻

6. **Final Answer**:
The correct order of decreasing basicity among the halide ions is:
F⁻ > Cl⁻ > Br⁻ > I⁻