`HCN` and `HNC` molcules are formed by the same atoms.
Which of the following properties are identical for `HCN` abd `HNC` molecules ?

Options

A. The number of `sigma` bonds.
B. The number of `pi` bonds.
C. The number of lone pairs.
D. Formal charge on `H` atom.

Explanation

To determine which properties are identical for the molecules HCN (Hydrogen Cyanide) and HNC (Hydrogen Isocyanide), we will analyze their structures and properties step by step.

### Step 1: Draw the Structures of HCN and HNC
- **HCN Structure**: H-C≡N
- In HCN, the carbon (C) is triple-bonded to nitrogen (N) and single-bonded to hydrogen (H).

- **HNC Structure**: H-N≡C
- In HNC, the nitrogen (N) is triple-bonded to carbon (C) and single-bonded to hydrogen (H).

### Step 2: Analyze the Number of Sigma Bonds
- **HCN**:
- There is 1 sigma bond between H and C, and 1 sigma bond between C and N (the triple bond consists of 1 sigma and 2 pi bonds).
- Total sigma bonds = 2.

- **HNC**:
- There is 1 sigma bond between H and N, and 1 sigma bond between N and C (the triple bond consists of 1 sigma and 2 pi bonds).
- Total sigma bonds = 2.

**Conclusion**: The number of sigma bonds in both HCN and HNC is identical (2).

### Step 3: Analyze the Number of Pi Bonds
- **HCN**:
- The triple bond between C and N consists of 2 pi bonds.

- **HNC**:
- The triple bond between N and C also consists of 2 pi bonds.

**Conclusion**: The number of pi bonds in both HCN and HNC is identical (2).

### Step 4: Analyze the Number of Lone Pairs
- **HCN**:
- The nitrogen atom has 1 lone pair (since it is bonded to one carbon and has a triple bond).

- **HNC**:
- The nitrogen atom has 1 lone pair, and the carbon atom carries a negative charge which can be considered as a lone pair of electrons.

**Conclusion**: The number of lone pairs in both HCN and HNC is identical (1 lone pair on nitrogen).

### Step 5: Analyze the Formal Charges
- **HCN**:
- Formal charge on H = 0, on C = 0, on N = 0.

- **HNC**:
- Formal charge on H = 0, on N = +1, on C = -1.

**Conclusion**: The formal charges are not identical, as HNC has a formal charge distribution that differs from HCN.

### Final Conclusion
The properties that are identical for HCN and HNC are:
1. Number of sigma bonds: 2 (identical)
2. Number of pi bonds: 2 (identical)
3. Number of lone pairs: 1 (identical)

However, the formal charges are not identical.

### Summary of Identical Properties
- Number of Sigma Bonds: Identical
- Number of Pi Bonds: Identical
- Number of Lone Pairs: Identical