Question

\(\mathrm{BCl}_{3}\) is a planar molecule, whercas \(\mathrm{NCl}_{3}\) is pyramidal because (1) \(\mathrm{BCl}_{3}\) has no lonc pair of electrons but \(\mathrm{NCl}_{3}\) has a lone pair of clectrons. (2) B-Cl bond is more polar than N-Cl bond. (3) Nitrogen atom is smaller than boron atom. (4) N-Cl bond is more covalcnt than B-Cl bond.

Short answer

The reason is (1). \(\text{BCl}_{3}\) has no lone pair while \(\text{NCl}_{3}\) has one lone pair, affecting their shape.

Step-by-step solution

Understand Molecular Geometry

Review the concepts of molecular geometry. A planar molecule such as \(\text{BCl}_{3}\) means it has a flat, triangular structure, whereas a pyramidal molecule like \(\text{NCl}_{3}\) has a three-dimensional shape with one atom at the apex.

Compare Lone Pairs

Identify the presence of lone pairs in the central atom. \(\text{BCl}_{3}\) has boron as the central atom, which has no lone pairs of electrons. \(\text{NCl}_{3}\) has nitrogen as the central atom, which has one lone pair of electrons.

Evaluate Given Statements

Evaluate each of the given statements to determine which factor contributes to the molecular geometry.

Determine the Key Factor

The key difference is the presence of lone pairs. \(\text{BCl}_{3}\) being planar is due to the absence of lone pairs making its geometry trigonal planar. \(\text{NCl}_{3}\) has a lone pair which pushes the chlorine atoms downward, making the molecule pyramidal.

Key concepts

Planar Molecules

Planar molecules are those that lie flat in a single plane. They often have a triangular or square structure, depending on the number of atoms involved.
For instance, \(\text{BCl}_{3}\) (boron trichloride) is a classic example of a planar molecule. This means all three chlorine atoms are positioned around the central boron atom in the same plane, forming a trigonal planar structure.
This happens because boron does not have any lone pairs of electrons. Without lone pairs to repel the bonding pairs of electrons, the molecule remains flat. As a result, its bond angles are typically 120 degrees. Understanding this flat, triangular structure helps in predicting the behavior and reactivity of such molecules.

Pyramidal Molecules

Pyramidal molecules, on the other hand, have a three-dimensional shape where one atom sits at the apex, and the other atoms form the base.
Take \(\text{NCl}_{3}\) (nitrogen trichloride) as an example. In this molecule, the central nitrogen atom forms a pyramid-like shape with the three chlorine atoms.
This pyramid shape forms because of the lone pair of electrons on the nitrogen atom. The lone pair exerts a repulsive force on the bonding pairs, pushing them downwards and out of the plane, creating a three-dimensional structure.
This changes the bond angles to less than 109.5 degrees, which is smaller than you would find in a tetrahedral structure. As a result, these molecules often exhibit different chemical and physical properties compared to their planar counterparts.

Lone Pairs

Lone pairs are pairs of valence electrons not involved in bonding.
They are found in the outer shells of atoms and have significant effects on the shape of molecules. For example, consider \(\text{NCl}_{3}\).
The nitrogen atom has one lone pair of electrons. This lone pair repels the bonded pairs of electrons more strongly than bonding pairs repel each other. This repulsion causes the nitrogen-chlorine bonds to move away from the lone pair, resulting in a pyramidal structure.
In contrast, \(\text{BCl}_{3}\) has no lone pairs. Thus, the electron pairs are equally spaced around the central boron atom, making the molecule planar. Understanding lone pairs' role is crucial for predicting the molecular geometry and reactivity of molecules.

Boron Trichloride (\text{BCl}_{3})

Boron trichloride (\text{BCl}_{3}) is a planar molecule where the boron atom is surrounded by three chlorine atoms in a flat trigonal planar structure.
This structure is due to the lack of lone pairs on the boron atom. Without lone pairs, there are no additional repulsive forces to distort the geometry.
The bond angles are approximately 120 degrees, making it an essential example for understanding trigonal planar molecules.
This flat geometry affects how \(\text{BCl}_{3}\) interacts with other molecules and its reactivity in chemical reactions.

Nitrogen Trichloride (\text{NCl}_{3})

Nitrogen trichloride (\text{NCl}_{3}) is a pyramidal molecule where the central nitrogen atom is at the apex with three chlorine atoms forming a base.
The pyramidal shape is due to the lone pair of electrons on the nitrogen atom. This lone pair causes the bonded pairs to repel more strongly, pushing them downward.
The bond angles in \(\text{NCl}_{3}\) are less than 109.5 degrees, resulting in its three-dimensional shape. Understanding this geometry helps in predicting the molecule's characteristics and behavior in different environments.