Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Molecule) List II (Bond angle) 1\. \(\mathrm{BF}_{3}\) (i) \(88^{\circ}\) 2\. \(\mathrm{NF}_{3}\) (ii) \(96^{\circ}\) 3\. \(\mathrm{PF}_{3}\) (iii) \(103^{\circ}\) 4\. \(\mathrm{ClF}_{3}\) (iv) \(110^{\circ}\) (v) \(120^{\circ}\) The correct matching is: \(1 \quad 2\) \(\begin{array}{ll}3 & 4\end{array}\) (a) (iv) (iii) (ii) (i) (b) (v) (iii) (ii) (i) (c) (iv) (iii) (v) (ii) (d) (iii) (iv) (i) (ii)

Step-by-Step Solution

Answer

Option (b) is correct: BF₃ matches with (v), NF₃ with (iii), PF₃ with (ii), and ClF₃ with (i).

Step 1: Identify Bond Angle of Each Molecule

To solve this, we identify the typical bond angles for each molecule. \(\mathrm{BF}_{3}\) is a trigonal planar molecule, so it usually has a bond angle of \(120^{\circ}\). \(\mathrm{NF}_{3}\) is pyramidal, similar to \(\mathrm{NH}_{3}\), with a bond angle around \(103^{\circ}\). \(\mathrm{PF}_{3}\) is also pyramidal, typically having a bond angle near \(96^{\circ}\). \(\mathrm{ClF}_{3}\) is T-shaped or seesaw depending on electron pair geometry, often resulting in bond angles significantly different from the others, typically around \(88^{\circ}\).

Step 2: Match the Bond Angles with the List Values

Now we match each molecule with its corresponding typical bond angle: 1. \(\mathrm{BF}_{3}\) matches with \(v \rightarrow 120^{\circ}\).2. \(\mathrm{NF}_{3}\) matches with \(iii \rightarrow 103^{\circ}\).3. \(\mathrm{PF}_{3}\) matches with \(ii \rightarrow 96^{\circ}\).4. \(\mathrm{ClF}_{3}\) matches with \(i \rightarrow 88^{\circ}\).

Step 3: Choose the Correct Matching Option

According to the matches:- \(1. \text{BF}_3 \rightarrow v\)- \(2. \text{NF}_3 \rightarrow iii\)- \(3. \text{PF}_3 \rightarrow ii\)- \(4. \text{ClF}_3 \rightarrow i\)By looking at the options, (b) matches with this configuration (\(v\), \(iii\), \(ii\), \(i\)).

Key Concepts

Trigonal Planar Molecules

Trigonal planar molecules are those that have a central atom bonded to three other atoms, with all four atoms lying on the same plane. A classic example is \(\mathrm{BF}_{3}\), or boron trifluoride.
What makes trigonal planar geometry unique is the symmetry and flatness of its structure.

• Central atom forms three bonds equally spaced at \(120^{\circ}\).
• Lacks any lone pairs on the central atom.

This structure forms due to the repulsion between electron pairs trying to be as far apart as possible according to the VSEPR theory. The symmetry results in balanced electron distribution, and due to zero lone pairs, the bond angle remains consistent across the molecule. Understanding trigonal planar geometry provides excellent insights into molecular symmetry and chemical behavior.

Pyramidal Molecules

Pyramidal molecules, such as \(\mathrm{NF}_{3}\) and \(\mathrm{PF}_{3}\), have a distinct three-sided pyramid shape with a lone pair on top of the central atom.
This lone pair causes more repulsion than the bonded pairs, leading to a decrease in bond angles compared to a tetrahedral arrangement.
The general characteristics include:

• Centered at one atom with three bonds directed towards the base corners.
• Possesses a lone pair of electrons at the apex.
• Bond angles slightly reduce due to lone pair-bond pair repulsion.
• Typical bond angle is around \(103^{\circ}\) for \(\mathrm{NF}_{3}\) and \(96^{\circ}\) for \(\mathrm{PF}_{3}\).

In pyramidal molecules, the presence of the lone pair not only affects bond angles but also can impact the molecule's polarity, making it an essential point in the study of molecular interactions.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is greatly affected by the number of bonding and lone electron pairs around the central atom.
These arrangements determine the spatial orientation of different atoms and influence molecular properties such as polarity, reactivity, and phase.

• Based on the VSEPR theory, molecules attain shapes minimizing electron pair repulsions.
• Different shapes include linear, trigonal planar, tetrahedral, pyramidal, bent, and others.
• Lone pairs influence molecular shapes by pushing bonded electrons closer together.

Understanding molecular geometry helps predict molecular behavior, chemical reactions, and physical properties. Knowledge of molecular geometry is foundational to chemistry and is crucial for better grasping structures like that of \(\mathrm{BF}_{3}\), \(\mathrm{NF}_{3}\), and \(\mathrm{PF}_{3}\).

T-Shaped Molecules

T-shaped molecular geometry is a fascinating configuration often seen in molecules such as \(\mathrm{ClF}_{3}\). This shape features a central atom bonded to three others, forming a 'T' shape when visualized in two dimensions.
This unusual form arises due to the presence of two lone pairs on the central atom, which force the bonded atoms apart, creating narrow bond angles.
Key characteristics include:

• Three bonded atoms and two lone pairs on the central atom.
• Results in bond angles typically around \(88^{\circ}\).
• Strong lone pair-bond pair repulsion affecting geometry.

T-shaped molecules are quite rare and serve as excellent examples of how extra electron pairs can drastically alter molecular shape. The angle in a T-shaped molecule is considerably acute compared to other geometries, showcasing the variability of molecular structures influenced by lone pairs.