Which is not a pyramidal molecule ?

Options

A. `NH_3`
B. `PCI_3`
C. `SOCI_2`
D. `ICI_3`

Answer

To determine which molecule is not pyramidal, we will analyze the hybridization and steric number of each given molecule using the formula:

**Steric Number = (1/2) * (Valence Electrons of Central Atom + Monovalent Atoms - Charge of Cation + Charge of Anion)**

### Step-by-Step Solution:

1. **Identify the molecules and their central atoms:**
- NH3 (Ammonia) - Central atom: Nitrogen (N)
- PCl3 (Phosphorus Trichloride) - Central atom: Phosphorus (P)
- SOCl2 (Thionyl Chloride) - Central atom: Sulfur (S)
- ICl3 (Iodine Trichloride) - Central atom: Iodine (I)

2. **Calculate the steric number for each molecule:**

**For NH3:**
- Valence electrons of N = 5
- Monovalent atoms (H) = 3
- Steric Number = (1/2) * (5 + 3) = (1/2) * 8 = 4
- NH3 has 3 bond pairs and 1 lone pair, making it pyramidal.

**For PCl3:**
- Valence electrons of P = 5
- Monovalent atoms (Cl) = 3
- Steric Number = (1/2) * (5 + 3) = (1/2) * 8 = 4
- PCl3 has 3 bond pairs and 1 lone pair, making it pyramidal.

**For SOCl2:**
- Valence electrons of S = 6
- Monovalent atoms (Cl) = 2
- Steric Number = (1/2) * (6 + 2) = (1/2) * 8 = 4
- SOCl2 has 3 bond pairs and 1 lone pair, making it pyramidal.

**For ICl3:**
- Valence electrons of I = 7
- Monovalent atoms (Cl) = 3
- Steric Number = (1/2) * (7 + 3) = (1/2) * 10 = 5
- ICl3 has 3 bond pairs and 2 lone pairs, resulting in a T-shaped geometry, which is not pyramidal.

3. **Conclusion:**
- The molecule that is not pyramidal is **ICl3**.

### Final Answer:
The molecule that is not a pyramidal molecule is **ICl3**.