According to Fajans'rules, the percentage of covalent character in an ionic compound increase if the cation is highly charged or small in size and the anion is large or cation has pseudoinert gas configuration. As a result of the increased covalent character, solubility in less polar solvent increases and the melting point decreases.
Which of the following has highest melting point?

Options

A. `LiCl`
B. `NaCl`
C. `KCl`
D. `RbCl`

Explanation

To determine which compound has the highest melting point based on Fajans' rules, we can follow these steps:

### Step 1: Understand Fajans' Rules
According to Fajans' rules, the covalent character of an ionic compound increases if:
- The cation is highly charged or small in size.
- The anion is large or the cation has a pseudoinert gas configuration.

### Step 2: Relate Covalent Character to Melting Point
- As the covalent character of an ionic compound increases, the melting point decreases.
- Therefore, to find the compound with the highest melting point, we need to look for the one with the lowest covalent character.

### Step 3: Analyze the Cations
We are considering alkali metal chlorides (LiCl, NaCl, KCl, RbCl, CsCl). The size of the cations increases as we move down the group in the periodic table:
- Lithium (Li) < Sodium (Na) < Potassium (K) < Rubidium (Rb) < Cesium (Cs)

### Step 4: Determine the Effect of Cation Size
- The larger the cation, the lower the covalent character. Therefore, the melting point will be highest for the largest cation.
- Among the alkali metals, cesium (Cs) is the largest, followed by rubidium (Rb).

### Step 5: Conclusion
- Since rubidium (Rb) is larger than lithium (Li), sodium (Na), and potassium (K), it will have the lowest covalent character among the chlorides of these alkali metals.
- Therefore, rubidium chloride (RbCl) will have the highest melting point.

### Final Answer
Rubidium chloride (RbCl) has the highest melting point among the given options.

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