The `BCl_(3)` is a planar molecule whereas `NCI_(3)` is pyramidal because

Options

A. B-Cl bond is more polar than N-Cl bond
B. N-Cl bond is more covalent than B-Cl bond
C. nitrogen atom is smaller than boron atom
D. `BCI_(3)` has no lone pair but `NCI_(3)` has a lone pair of
electrons

Explanation

To understand why \( \text{BCl}_3 \) is a planar molecule while \( \text{NCl}_3 \) is pyramidal, we can analyze the hybridization and geometry of both molecules step by step.

### Step 1: Determine the Hybridization of \( \text{BCl}_3 \)

1. **Count the Sigma Bonds**: In \( \text{BCl}_3 \), boron (B) forms three sigma bonds with three chlorine (Cl) atoms. There are no lone pairs on the boron atom.

\[ \text{Number of Sigma Bonds} = 3 \]

2. **Calculate the Steric Number**: The steric number is calculated as the number of sigma bonds plus the number of lone pairs. Since there are no lone pairs on boron:

\[ \text{Steric Number} = 3 + 0 = 3 \]

3. **Determine Hybridization**: A steric number of 3 corresponds to \( \text{sp}^2 \) hybridization.

### Step 2: Determine the Geometry of \( \text{BCl}_3 \)

1. **Geometry from Hybridization**: The \( \text{sp}^2 \) hybridization indicates a trigonal planar geometry. Therefore, \( \text{BCl}_3 \) has a planar structure.

### Step 3: Determine the Hybridization of \( \text{NCl}_3 \)

1. **Count the Sigma Bonds**: In \( \text{NCl}_3 \), nitrogen (N) forms three sigma bonds with three chlorine (Cl) atoms. However, nitrogen has one lone pair of electrons.

\[ \text{Number of Sigma Bonds} = 3 \]

2. **Calculate the Steric Number**: The steric number for nitrogen is calculated as the number of sigma bonds plus the number of lone pairs:

\[ \text{Steric Number} = 3 + 1 = 4 \]

3. **Determine Hybridization**: A steric number of 4 corresponds to \( \text{sp}^3 \) hybridization.

### Step 4: Determine the Geometry of \( \text{NCl}_3 \)

1. **Geometry from Hybridization**: The \( \text{sp}^3 \) hybridization indicates a tetrahedral geometry. However, due to the presence of one lone pair, the molecular shape is trigonal pyramidal.

### Conclusion

- **\( \text{BCl}_3 \)** is planar because it has no lone pairs and exhibits \( \text{sp}^2 \) hybridization resulting in a trigonal planar geometry.
- **\( \text{NCl}_3 \)** is pyramidal because it has one lone pair and exhibits \( \text{sp}^3 \) hybridization resulting in a trigonal pyramidal shape.

### Final Answer

The reason \( \text{BCl}_3 \) is planar while \( \text{NCl}_3 \) is pyramidal is due to the presence of lone pairs in \( \text{NCl}_3 \), which affects its geometry.

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