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 solve the question, we will analyze each statement one by one to determine which ones are correct.

### Step 1: Analyze Statement 1
**Statement:** Molecular crystals are hard in nature.
**Analysis:** This statement is not necessarily true. Molecular crystals can vary in hardness depending on the types of intermolecular forces present. For example, ionic compounds (like NaCl) are hard due to strong ionic bonds, while covalent molecular crystals may be softer. Therefore, this statement is **incorrect**.

### Step 2: Analyze Statement 2
**Statement:** Silicon carbide is a covalent crystal.
**Analysis:** Silicon carbide (SiC) is indeed a covalent crystal. Both silicon and carbon are from group 14 and have four valence electrons. They form strong covalent bonds by sharing electrons to complete their octets. Thus, this statement is **correct**.

### Step 3: Analyze Statement 3
**Statement:** Increase in radius ratio results in an increase in coordination number.
**Analysis:** The radius ratio is a crucial factor in determining the coordination number in ionic compounds. As the radius ratio increases, the coordination number also tends to increase. For example, a radius ratio of 0.225 to 0.414 corresponds to a coordination number of 2, while a radius ratio of 0.414 to 0.732 corresponds to a coordination number of 4, and so on. Therefore, this statement is **correct**.

### Step 4: Analyze Statement 4
**Statement:** In calcium fluoride structure, the coordination number of Ca²⁺ is 4.
**Analysis:** In the calcium fluoride (CaF₂) structure, each Ca²⁺ ion is surrounded by 8 fluoride ions (F⁻), resulting in a coordination number of 8 for calcium. Hence, this statement is **incorrect**.

### Conclusion
After analyzing all the statements:
- Statement 1: Incorrect
- Statement 2: Correct
- Statement 3: Correct
- Statement 4: Incorrect

The correct statements are 2 and 3 only. Therefore, the answer is that the correct set of statements is option 2.