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 \(\mathrm{ClF}_{3}\) 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: Determine Molecular Structures

Identify the molecular structures of \(\mathrm{BF}_3\), \(\mathrm{PF}_3\), and \(\mathrm{ClF}_3\). \(\mathrm{BF}_3\) has a trigonal planar structure, \(\mathrm{PF}_3\) has a trigonal pyramidal structure due to a lone pair on phosphorus, and \(\mathrm{ClF}_3\) has a T-shaped structure due to two lone pairs on chlorine.

Step 2: Bond Angles in Molecular Geometries

In \(\mathrm{BF}_3\), being trigonal planar, the bond angle is approximately \(120^\circ\). In \(\mathrm{PF}_3\), the trigonal pyramidal shape leads to bond angles slightly less than \(109.5^\circ\) due to the lone pair. \(\mathrm{ClF}_3\)'s T-shaped structure results in bond angles less than \(90^\circ\).

Step 3: Compare Bond Angles

Compare the bond angles: \(\mathrm{BF}_3\) with \(\approx 120^\circ\), \(\mathrm{PF}_3\) with slightly less than \(109.5^\circ\), and \(\mathrm{ClF}_3\) with less than \(90^\circ\). Thus, the correct increasing order of bond angles is \(\mathrm{ClF}_3 < \mathrm{PF}_3 < \mathrm{BF}_3\).

Key Concepts

Molecular Geometry

Molecular geometry is like the 3D shape of a molecule. Imagine atoms as tiny balls and bonds as sticks connecting them, forming a complex structure. Knowing the geometry helps us predict how a molecule behaves in space. For instance, - **Trigonal Planar**: This shape looks like a flat triangle with one atom in the center and three surrounding it, similar to c\(\mathrm{BF}_3\) d. The atoms are spread out evenly, giving a bond angle of about \(120^\circ\).- **Trigonal Pyramidal**: This is more like a pyramid with the central atom on top, seen in \(\mathrm{PF}_3\). There's a lone pair on the center atom, like an invisible balloon pushing the other atoms slightly closer.- **T-Shaped**: Seen in \(\mathrm{ClF}_3\), it looks like a letter T with three atoms at the bottom and two lone pairs at the sides altering the angles, resulting in bond angles less than \(90^\circ\).

Understanding these shapes is crucial because they affect how molecules interact with others. They determine not just angles, but reactivity, polarity, and even color!

Chemical Bonding

Chemical bonding is about atoms sticking together to form molecules. Think of it as a glue that holds them in place. There are different types of bonding, but most pertinent here are:- **Covalent Bonds**: These bonds form when atoms share electrons to achieve stability, filling up their outer shell. In \(\mathrm{BF}_3\), \(\mathrm{PF}_3\), and \(\mathrm{ClF}_3\), shared electrons form the strong links we call bonds.- **Lone Pairs**: Sometimes, electrons are not shared. In \(\mathrm{PF}_3\) and \(\mathrm{ClF}_3\), you find these unshared electrons chilling out on the central atom. They don't bond but they push away other bonds, affecting the molecule's geometry.- **Electronegativity**: This explains the strength each atom pulls on electrons within the bond. For instance, in \(\mathrm{ClF}_3\), chlorine's strength creates a skewed electron cloud, impacting geometry and bond angles.

Understanding chemical bonding is key to predicting how a molecule behaves and interacts with others. It affects strength, angle, and even physical properties like boiling points.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

The VSEPR theory helps predict the shape of a molecule by focusing on electron pairs around the central atom. Here's how it works:- **Repulsion is Key**: Imagine the electron pairs as repelling each other like magnets. They arrange in a way that minimizes repulsion, spreading out as far as possible.- **Role of Lone Pairs**: Lone pairs, like in \(\mathrm{PF}_3\) and \(\mathrm{ClF}_3\), repel more strongly than bonding pairs, causing squashing and altering bond angles. For example, the lone pair in \(\mathrm{PF}_3\) makes the bond angles smaller than the typical \(109.5^\circ\).- **Predicting Geometries**: By counting the bonding and lone pairs, and using repulsion rules, you can predict the geometry. A useful tool for visualizing the 3D arrangement.

The VSEPR theory is practical and vital for figuring out molecular structures without complex equipment! Understanding it aids greatly in mastering chemistry concepts.