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

Explanation

To determine which statement is not true, we will analyze each option provided in the question.

### Step 1: Analyze the first statement
**Statement:** Silicon carbide is a covalent crystal.
**Explanation:** Silicon (Si) and carbon (C) both belong to group 14 of the periodic table. They tend to form covalent bonds by sharing electrons rather than forming ionic bonds, as forming ions would require a significant amount of energy. Therefore, this statement is **true**.

### Step 2: Analyze the second statement
**Statement:** Molecular crystals are soft in nature.
**Explanation:** Molecular crystals are held together by weak intermolecular forces, such as London dispersion forces. This results in them being soft compared to ionic or covalent crystals, which are held together by stronger forces. Thus, this statement is also **true**.

### Step 3: Analyze the third statement
**Statement:** In calcium fluoride structure, the coordination number of Ca²⁺ is 4.
**Explanation:** In the calcium fluoride (CaF₂) structure, calcium ions (Ca²⁺) are coordinated by 8 fluoride ions (F⁻) in a face-centered cubic (FCC) arrangement, while each fluoride ion is surrounded by 4 calcium ions. Therefore, the coordination number of Ca²⁺ is **not** 4; it is actually 8. This statement is **false**.

### Step 4: Analyze the fourth statement
**Statement:** Increase in radius ratio results in an increase in coordination number.
**Explanation:** The radius ratio (the ratio of the cation radius to the anion radius) does influence the coordination number. As the radius ratio increases, the coordination number typically increases as well. For example, a radius ratio of 0.414 to 0.732 corresponds to a coordination number of 6. Therefore, this statement is **true**.

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

**Final Answer:** The correct answer is **C**.

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