In which of the following molecules, central atom is not `sp^3` hybridized?

Options

A. `NH_3`
B. `CH_4`
C. `H_2O`
D. `CO_2`

Answer

To determine which molecule has a central atom that is not `sp^3` hybridized, we will analyze the hybridization of the central atom in each of the given molecules.

### Step-by-Step Solution:

1. **Analyze Ammonia (NH₃)**:
- Ammonia has one nitrogen atom as the central atom.
- It has 3 hydrogen atoms bonded to it and 1 lone pair.
- The steric number (number of bonds + lone pairs) is 3 (from H) + 1 (lone pair) = 4.
- The hybridization corresponding to a steric number of 4 is `sp^3`.
- **Conclusion**: Nitrogen in NH₃ is `sp^3` hybridized.

2. **Analyze Methane (CH₄)**:
- Methane has one carbon atom as the central atom.
- It has 4 hydrogen atoms bonded to it and no lone pairs.
- The steric number is 4 (from H) + 0 (lone pairs) = 4.
- The hybridization corresponding to a steric number of 4 is `sp^3`.
- **Conclusion**: Carbon in CH₄ is `sp^3` hybridized.

3. **Analyze Water (H₂O)**:
- Water has one oxygen atom as the central atom.
- It has 2 hydrogen atoms bonded to it and 2 lone pairs.
- The steric number is 2 (from H) + 2 (lone pairs) = 4.
- The hybridization corresponding to a steric number of 4 is `sp^3`.
- **Conclusion**: Oxygen in H₂O is `sp^3` hybridized.

4. **Analyze Carbon Dioxide (CO₂)**:
- Carbon dioxide has one carbon atom as the central atom.
- It has 2 oxygen atoms bonded to it and no lone pairs.
- The steric number is 2 (from O) + 0 (lone pairs) = 2.
- The hybridization corresponding to a steric number of 2 is `sp`.
- **Conclusion**: Carbon in CO₂ is not `sp^3` hybridized; it is `sp` hybridized.

### Final Answer:
The molecule in which the central atom is not `sp^3` hybridized is **Carbon Dioxide (CO₂)**.