Which one of the following halide has the highest melting point ?

Options

A. (a) KCl
B. (b) KBr
C. (c) KF
D. (d) KI

Answer

To determine which halide has the highest melting point among the given options, we can follow these steps:

### Step 1: Understand the Nature of Halides
Halides can be ionic or covalent in nature. Ionic compounds generally have higher melting points due to strong electrostatic forces between the ions, while covalent compounds have lower melting points due to weaker intermolecular forces.

**Hint:** Remember that ionic compounds tend to have higher melting points than covalent compounds.

### Step 2: Identify the Cation and Anion
In this case, we are comparing halides of potassium (K). The cation is the same (K⁺), so we need to focus on the anions (the halides: F⁻, Cl⁻, Br⁻, I⁻).

**Hint:** Keep in mind that the size of the anion affects the ionic character of the compound.

### Step 3: Apply Fajans' Rule
According to Fajans' rule, the ionic character increases with:
- Smaller cations and larger anions favor covalent character.
- Larger cations and smaller anions favor ionic character.

Since K⁺ is a larger cation, we need to consider the size of the anions. The halide ions in order of increasing size are: F⁻ < Cl⁻ < Br⁻ < I⁻.

**Hint:** Smaller anions lead to higher ionic character and thus higher melting points.

### Step 4: Compare the Anions
Among the halides, F⁻ is the smallest anion, which means K⁺F⁻ (KF) will have the highest ionic character compared to KCl, KBr, and KI. Therefore, KF will have the highest melting point.

**Hint:** The smaller the anion, the higher the ionic character and melting point.

### Step 5: Conclusion
Based on the analysis, potassium fluoride (KF) has the highest melting point among the halides due to its strong ionic character.

**Final Answer:** The halide with the highest melting point is potassium fluoride (KF).