Which of the following statements is not true ?

Options

A. Silicon carbide is a covalent crystal
B. Molecular crystals are soft in nature
C. In calcium fluoride structure coordination number of `Ca^(2+) " is " 4`
D. Increase in radius ratio results in increase in coordination number

Answer

To solve the question, we need to evaluate each of the provided statements and determine which one is not true.

### Step-by-Step Solution:

1. **Evaluate Statement 1: Silicon carbide is a covalent crystal.**
- Silicon carbide (SiC) is formed by the covalent bonding between silicon and carbon atoms. Both elements share their electrons, resulting in a strong covalent network structure. Therefore, this statement is **true**.

2. **Evaluate Statement 2: Molecular crystals are soft in nature.**
- Molecular crystals are composed of molecules held together by relatively weak intermolecular forces (like Van der Waals forces). This results in a softer structure compared to ionic or covalent crystals. Therefore, this statement is **true**.

3. **Evaluate Statement 3: In calcium chloride structure, the coordination number of Ca²⁺ is 4.**
- Calcium chloride (CaCl₂) has a structure where Ca²⁺ ions are surrounded by chloride ions. In the crystal lattice of CaCl₂, the coordination number of Ca²⁺ is actually 8, not 4. This is because each Ca²⁺ ion is surrounded by 8 Cl⁻ ions in a face-centered cubic arrangement. Therefore, this statement is **false**.

4. **Evaluate Statement 4: Increase in radius ratio results in an increase in coordination number.**
- The radius ratio of cations to anions is indeed directly related to the coordination number. As the radius ratio increases, the coordination number tends to increase as well. Therefore, this statement is **true**.

### Conclusion:
The statement that is not true is the third one: "In calcium chloride structure, the coordination number of Ca²⁺ is 4."

Thus, the correct answer is **Option 3**.

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