Which of the following set is having correct statements?
1. Molecular crystals are hard in nature
2. Silicon carbide is a covalent crystal
3. Increase in radius ratio result in increase in coordination number
4. In calcium fluoride structure, coordination number of `Ca^(2+)` is 4

Options

A. 1,2
B. 2,3
C. 1,2,3
D. 2,3,4

Answer

To determine which of the statements provided in the question are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Molecular crystals are hard in nature.**
- Molecular crystals are typically formed by molecules held together by weak intermolecular forces such as Van der Waals forces or hydrogen bonds. These forces are generally weaker than the ionic or covalent bonds found in other types of crystals (like ionic or covalent crystals). Therefore, molecular crystals are not necessarily hard; many are soft and brittle.
- **Conclusion:** This statement is **incorrect**.

2. **Statement 2: Silicon carbide is a covalent crystal.**
- Silicon carbide (SiC) is indeed a covalent network crystal. It consists of silicon and carbon atoms that are bonded together by strong covalent bonds in a three-dimensional network. This structure gives silicon carbide its hardness and high melting point.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: Increase in radius ratio results in an increase in coordination number.**
- The radius ratio is defined as the ratio of the radius of the cation to the radius of the anion. As the radius ratio increases, the coordination number typically increases as well. For example, a radius ratio of 0.225 to 0.414 corresponds to a coordination number of 4, 0.414 to 0.732 corresponds to a coordination number of 6, and a radius ratio approaching 1 corresponds to a coordination number of 8.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: In calcium fluoride structure, the coordination number of Ca²⁺ is 4.**
- In the structure of calcium fluoride (CaF₂), each calcium ion (Ca²⁺) is surrounded by 8 fluoride ions (F⁻), which means that the coordination number of Ca²⁺ is 8, not 4. The fluoride ions are arranged in a cubic lattice around the calcium ions.
- **Conclusion:** This statement is **incorrect**.

### Final Evaluation:
- **Correct Statements:**
- Statement 2: Silicon carbide is a covalent crystal. (Correct)
- Statement 3: Increase in radius ratio results in an increase in coordination number. (Correct)

- **Incorrect Statements:**
- Statement 1: Molecular crystals are hard in nature. (Incorrect)
- Statement 4: In calcium fluoride structure, coordination number of Ca²⁺ is 4. (Incorrect)

### Answer:
The correct set of statements is: **2 and 3**.

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