Question

Both \(\mathrm{BF}_{3}\) and \(\mathrm{NF}_{3}\) are covalent but \(\mathrm{BF}_{3}\) molecule is non-polar while \(\mathrm{NF}_{3}\) is polar because: (a) Atomic size of boron is smaller than nitrogen (b) \(\mathrm{BF}_{3}\) is planar but \(\mathrm{NF}_{3}\) is pyramidal (c) Boron is a metal while nitrogen is gas (d) BF bond has no dipole moment while NF bond has dipole

Short answer

The correct reason is (b): BF₃ is planar and NF₃ is pyramidal.

Step-by-step solution

Understanding Molecular Structures

Begin by understanding the molecular structures of both BF₃ and NF₃. The BF₃ molecule has a trigonal planar structure with the boron atom at the center and fluorine atoms forming an equilateral triangle around it. In contrast, NF₃ has a trigonal pyramidal structure, with nitrogen at the center and the fluorine atoms forming the base of the pyramid.

Review Symmetry and Dipole Moments

In a trigonal planar molecule like BF₃, the symmetry causes the dipole moments of the bonds (BF) to cancel each other out, rendering the molecule non-polar. In NF₃, the trigonal pyramidal structure is asymmetric, so the dipole moments do not cancel each other, making it polar.

Evaluate the Options

Option (a) mentions atomic size - this isn't relevant to polarity. Option (c) states boron is a metal - incorrect as boron is a metalloid, and nitrogen being a gas is irrelevant. Option (d) insists that BF has no dipole moment - while the bonds themselves have moments, the symmetry negates the molecular dipole. Hence, option (b) correctly identifies the difference based on molecular geometry.

Key concepts

covalent bonding

Covalent bonding is a fundamental concept in chemistry. It refers to the sharing of electrons between atoms, allowing them to attain stable electron configurations. This kind of bonding typically occurs between non-metal atoms due to their similar electronegativities. When atoms share electron pairs, they form a covalent bond. This sharing can be equal, resulting in a non-polar covalent bond, or unequal, leading to a polar covalent bond.
Typically, elements like boron, nitrogen, and fluorine come together through covalent bonds. In molecules like \(\mathrm{BF}_{3}\) and \(\mathrm{NF}_{3}\), covalent bonds create strong connections between the central atom (boron or nitrogen) and the surrounding fluorine atoms. Understanding how these bonds work helps explain the molecule's shape, stability, and behavior under various conditions.

trigonal planar

The term trigonal planar describes a molecular geometry where a central atom is bonded to three peripheral atoms. These peripheral atoms are arranged around the central atom in a flat or planar triangular shape.
One important characteristic is that all atoms in a trigonal planar molecule lie in the same plane. In the case of \(\mathrm{BF}_{3}\), the molecule adopts this shape, looking like an equilateral triangle when viewed from above. This arrangement allows for the symmetrical distribution of electron density, contributing to its non-polarity.
This symmetry also means that individual bond dipoles in a trigonal planar structure can effectively cancel each other out, resulting in no net dipole moment for the molecule.

trigonal pyramidal

The trigonal pyramidal structure is distinct from the trigonal planar configuration. It occurs when a central atom is bonded to three atoms or groups, forming a base that looks like a triangle when viewed from above. This triangle is topped by the central atom, positioned above the plane formed by the peripheral atoms, giving it a three-dimensional shape.
In \(\mathrm{NF}_{3}\), nitrogen is at the center of this shape with three fluorine atoms forming the base. One key difference from the trigonal planar arrangement is the presence of a lone pair of electrons on the central atom. This lone pair pushes down the bonded atoms due to electron repulsion, resulting in a less symmetric shape.
This asymmetry in a trigonal pyramidal molecule means that the dipole moments do not cancel each other out, thus creating an overall dipole moment, making the molecule polar.

dipole moment cancellation

Dipole moment cancellation is a principle where the individual dipole moments in a molecule offset each other due to symmetry. When a molecule has geometric or spatial symmetry, the vector sum of these dipole moments can add up to zero.
This concept is crucial in understanding why some molecules are non-polar despite having polar covalent bonds. In \(\mathrm{BF}_{3}\), for instance, the molecule's trigonal planar shape means that the bond dipoles point towards the corners of an equilateral triangle. They balance each other out, resulting in a net dipole moment of zero.
In contrast, \(\mathrm{NF}_{3}\) lacks this symmetry because of its trigonal pyramidal shape. The bond dipoles do not cancel due to the presence of a lone pair on nitrogen, leading to a molecular dipole moment. Dipole moment cancellation helps predict and explain the polarity of molecules, which in turn influences their physical and chemical properties.