The `BCl_3` is a planar molecule whereas `NCl_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 atoms
D. `BCl_(3)` has no lone pair but `NCl_(3)` has a lone pair of electrons.

Explanation

To understand why \( BCl_3 \) is a planar molecule while \( NCl_3 \) is pyramidal, we need to analyze the molecular structures and the electron configurations of boron and nitrogen.

### Step-by-Step Solution:

1. **Determine the Valence Electrons:**
- **Boron (\( B \)):** Atomic number 5. Its electronic configuration is \( 1s^2 2s^2 2p^1 \). Therefore, boron has 3 valence electrons.
- **Nitrogen (\( N \)):** Atomic number 7. Its electronic configuration is \( 1s^2 2s^2 2p^3 \). Therefore, nitrogen has 5 valence electrons.

2. **Bonding in \( BCl_3 \):**
- Boron uses its 3 valence electrons to form bonds with 3 chlorine atoms.
- The structure of \( BCl_3 \) has no lone pairs of electrons on boron, resulting in a trigonal planar geometry.
- The bond angles in \( BCl_3 \) are \( 120^\circ \), making it a planar molecule.

3. **Bonding in \( NCl_3 \):**
- Nitrogen uses 3 of its 5 valence electrons to bond with 3 chlorine atoms.
- The remaining 2 electrons form a lone pair on nitrogen.
- The presence of this lone pair causes the molecular geometry to be pyramidal due to lone pair-bond pair repulsion.
- The structure of \( NCl_3 \) is thus trigonal pyramidal, with bond angles less than \( 109.5^\circ \).

4. **Conclusion:**
- The key difference between \( BCl_3 \) and \( NCl_3 \) is the presence of lone pairs. \( BCl_3 \) has no lone pairs, resulting in a planar structure, while \( NCl_3 \) has one lone pair, resulting in a pyramidal structure.

### Final Answer:
\( BCl_3 \) is a planar molecule because it has no lone pairs of electrons, while \( NCl_3 \) is pyramidal due to the presence of one lone pair of electrons on nitrogen.