Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct increasing bond angle among \(\mathrm{BF}_{3}, \mathrm{PF}_{3}\) and \(\mathrm{C} 1 \mathrm{~F}_{3}\) follows the order (a) \(\mathrm{BF}_{3}<\mathrm{PF}_{3}<\mathrm{CIF}_{3}\) (b) \(\mathrm{PF}_{3}<\mathrm{BF}_{3}<\mathrm{CIF}_{3}\) (c) \(\mathrm{CIF}_{3}<\mathrm{PF}_{3}<\mathrm{BF}_{3}\) (d) \(\mathrm{BF}_{3}<\mathrm{PF}_{3}<\mathrm{CIF}_{3}\)

Step-by-Step Solution

Answer

The correct order is (c) \(\mathrm{CIF}_3 < \mathrm{PF}_3 < \mathrm{BF}_3\).

Step 1: Analyze the Geometry

Begin by determining the molecular geometry of each compound. \(\mathrm{BF}_3\) has a trigonal planar structure with a bond angle of 120 degrees. \(\mathrm{PF}_3\) forms a trigonal pyramidal geometry, influenced by the presence of a lone pair, leading to bond angles slightly less than 109.5 degrees. \(\mathrm{CIF}_3\) adopts a T-shaped geometry due to the presence of two lone pairs on chlorine, resulting in bond angles of about 90 degrees.

Step 2: Compare the Bond Angles

Now compare the bond angles of each compound. The bond angle of \(\mathrm{CIF}_3\) (approximately 90 degrees) is less than that of \(\mathrm{PF}_3\) (slightly less than 109.5 degrees), which is itself less than that of \(\mathrm{BF}_3\) (120 degrees). Therefore, the correct order of increasing bond angles is \(\mathrm{CIF}_3 < \mathrm{PF}_3 < \mathrm{BF}_3\).

Step 3: Identify the Correct Answer Option

Find the answer choice that matches the increasing order of bond angles. Option (c) \(\mathrm{CIF}_3 < \mathrm{PF}_3 < \mathrm{BF}_3\) matches the correct order.

Key Concepts

Bond Angle Comparison

Understanding bond angles is key in recognizing how the shapes of different molecules influence their geometry. Bonds exist at different angles depending on the arrangement of atoms around a central atom. The angle measures the space between two bonds originating from the same atom.
In molecules like \(\mathrm{BF}_3\), \(\mathrm{PF}_3\), and \(\mathrm{CIF}_3\), each has different molecular geometries, which directly influence the bond angles.

• \(\mathrm{BF}_3\) has a bond angle of 120 degrees due to its trigonal planar structure.
• \(\mathrm{PF}_3\) has smaller bond angles, slightly less than 109.5 degrees, due to its trigonal pyramidal geometry.
• \(\mathrm{CIF}_3\) has bond angles close to 90 degrees because of its T-shaped geometry.

To compare the bond angles: \(\mathrm{CIF}_3 < \mathrm{PF}_3 < \mathrm{BF}_3\), as lower bond angles correspond to more tightly packed structures due to lone pair repulsion.

Trigonal Planar Structure

A trigonal planar structure is a form of molecular geometry where three bonds are evenly spaced around a central atom, lying on the same plane. It forms a flat shape and is noted for its symmetry.
\(\mathrm{BF}_3\) is a classic example of a molecule with a trigonal planar structure.

• The boron atom is central, with three fluorine atoms evenly spaced around it.
• This configuration results in bond angles that are exactly 120 degrees.

The even distribution of electron pairs around the central atom minimizes repulsion, allowing the molecule to maintain its planar state. This setup not only determines the angle size but also dictates much of the stability and reactivity of the molecule.

Lone Pairs and Bond Angles

Lone pairs are non-bonding pairs of electrons that reside on atoms. These pairs can exert greater repulsion than bonding pairs, affecting bond angles in a molecule.
In \(\mathrm{PF}_3\), a lone pair on phosphorus reduces the bond angle from the typical tetrahedral angle of 109.5 degrees to slightly less than this value.

• The presence of a lone pair distorts the geometry by pushing the bonded electron pairs closer together.

In \(\mathrm{CIF}_3\), two lone pairs on the chlorine atom create significant repulsion. These interactions result in a T-shaped geometry and bond angles approximately 90 degrees.

• Lone pairs prevent bonded atoms from achieving angles of 120 or 180 degrees because they occupy more space.

Thus, understanding the impact of lone pairs is crucial when predicting and comparing bond angles in molecules.