Which of the following compounds has the lowest melting point ?

Options

A. `CaF_(2)`
B. `CaCl_(2)`
C. `CaBr_(2)`
D. `Cal_(2)`

Answer

To determine which of the given compounds has the lowest melting point, we will analyze the compounds based on their ionic character and the size of their constituent ions. The compounds in question are:

1. CaF2 (Calcium fluoride)
2. CaCl2 (Calcium chloride)
3. CaBr2 (Calcium bromide)
4. CaI2 (Calcium iodide)

### Step 1: Understand the Concept of Lattice Energy
Lattice energy is the energy required to separate one mole of an ionic solid into its gaseous ions. Compounds with higher lattice energy tend to have higher melting points, while those with lower lattice energy have lower melting points.

### Step 2: Apply Fajans' Rules
According to Fajans' rules, the covalent character of an ionic compound increases when:
- The size of the anion increases.
- The size of the cation decreases.

In our case, the cation (Ca²⁺) is the same for all compounds, so we will focus on the anions (F⁻, Cl⁻, Br⁻, I⁻).

### Step 3: Compare the Sizes of the Anions
The anions can be arranged in order of increasing size:
- F⁻ < Cl⁻ < Br⁻ < I⁻

As we move down the group in the periodic table, the size of the halide ions increases. Therefore, I⁻ is the largest anion among the given options.

### Step 4: Determine the Covalent Character
Since I⁻ is the largest anion, CaI2 will exhibit the most covalent character among the compounds listed. This is because larger anions can polarize the cation more effectively, leading to increased covalent character.

### Step 5: Conclude on Melting Points
Since CaI2 has the highest covalent character, it will have the lowest lattice energy and therefore the lowest melting point among the given compounds.

### Final Answer
The compound with the lowest melting point is **CaI2**.

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