The compound is covalent in gaseous state but ionic in solid state is.

Options

A. `PCl_5`
B. `PCl_3`
C. `C Cl_4`
D. `NH_3`

Explanation

To solve the question "The compound is covalent in gaseous state but ionic in solid state is," we will analyze the given options one by one.

### Step 1: Analyze the Options
We have four options to consider:
1. PCl5
2. PCl3
3. CCl4
4. NH3

### Step 2: Examine PCl5
- **Gaseous State**: In the gaseous state, PCl5 exists as discrete molecules. It has a trigonal bipyramidal structure due to sp3d hybridization, and it forms covalent bonds between phosphorus (P) and chlorine (Cl) atoms.
- **Solid State**: In the solid state, PCl5 does not remain as discrete molecules. Instead, it exists as ions: PCl4+ (tetrahedral cation) and PCl6- (octahedral anion). This indicates that it behaves ionically in the solid state.

### Step 3: Examine PCl3
- **Gaseous State**: PCl3 exists as covalent molecules in the gaseous state with a trigonal pyramidal structure.
- **Solid State**: PCl3 remains covalent in the solid state as well, so it does not fit the criteria.

### Step 4: Examine CCl4
- **Gaseous State**: CCl4 is a covalent compound in the gaseous state with a tetrahedral structure.
- **Solid State**: CCl4 remains covalent in the solid state, so it does not fit the criteria.

### Step 5: Examine NH3
- **Gaseous State**: NH3 is a covalent molecule in the gaseous state with a trigonal pyramidal structure.
- **Solid State**: NH3 remains covalent in the solid state (as solid NH3 forms hydrogen bonds), so it does not fit the criteria.

### Conclusion
After analyzing all the options, we find that **PCl5** is the only compound that is covalent in the gaseous state but ionic in the solid state.

### Final Answer
The compound that is covalent in the gaseous state but ionic in the solid state is **PCl5**.

---