Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following substances: 1\. \(\mathrm{OF}_{2}\) 2\. \(\mathrm{Cl}_{2} \mathrm{O} \quad\) 3. \(\mathrm{Br}_{2} \mathrm{O}\) The correct sequence of \(X-O=X\) bond angle is (a) \(3>2>1\) (b) \(1>2>3\) (c) \(2>1>3\) (d) \(1>3>2\)

Step-by-Step Solution

Answer

The correct sequence is (a) 3 > 2 > 1.

Step 1: Understand Bond Angle Concept

The bond angle in molecules is influenced by the size and electronegativity of the atoms involved, as well as the number of lone pairs on the central atom. Generally, larger atoms require wider angles to minimize electron pair repulsion.

Step 2: Analyze Each Molecule

1. Oxygen difluoride (
OF}_{2}) has oxygen bonded to two fluorine atoms; fluorine is highly electronegative, causing lone pairs on oxygen to repel, leading to a smaller bond angle. 2. Dichlorine monoxide (
Cl}_{2}
O) has a larger chlorine atom compared to fluorine, so bond angles may be larger due to less repulsion forces. 3. Dibromine oxide (
Br}_{2}
O) involves even larger bromine atoms, further potentially increasing the bond angle.

Step 3: Comparing Electronegativity and Atomic Size

Since fluorine is more electronegative, it is more effective in pulling electron density towards itself, which results in a smaller bond angle in
OF}_{2}. Conversely, bromine's large atomic size and moderate electronegativity result in a larger bond angle in
Br}_{2}
O as repulsion is decreased.

Step 4: Determine the Correct Sequence

Considering both atomic size and electronegativity, we predict the bond angle order based on decreasing angle due to increased lone pair-bond pair repulsion:
Br}_{2}
O >
Cl}_{2}
O >
OF}_{2}.

Key Concepts

Electronegativity

Electronegativity is a measure of how strongly atoms attract shared electrons in a chemical bond. In the context of bond angles, the electronegativity of atoms affects electron sharing and distribution around the central atom.

• Highly electronegative atoms, like fluorine, tend to pull electron density towards themselves.
• This pull can decrease the bond angles adjacent to these electronegative atoms due to increased lone pair repulsions.

For example, in oxygen difluoride ( OF_{2} ), the fluorine's intense electronegativity draws electrons closer, which strengthens lone pair repulsion on the oxygen. This ultimately reduces the bond angle compared to less electronegative atoms like chlorine or bromine, where the electron pull is weaker.

Lone Pair Repulsion

Lone pairs of electrons are pairs of valence electrons that are not shared with another atom in a bond. These pairs tend to occupy more space around the central atom and create repulsion forces that are crucial in determining bond angles.

• Lone pair-lone pair repulsion is greater than lone pair-bond pair, which is again greater than bond pair-bond pair repulsion.
• This hierarchy of repulsion affects the overall shape and angles in molecules.

Consider \(\mathrm{OF}_{2}\), which has lone pairs that repel each other strongly, leading to smaller bond angles. In comparison, \(\mathrm{Cl}_{2}\mathrm{O}\) and \(\mathrm{Br}_{2}\mathrm{O}\) have larger possibilities of lessening repulsion, thus potentially leading to wider bond angles because additional electron density is not as tightly pulled by these atoms.

Atomic Size

Atomic size refers to the distance between an atom's nucleus and its outermost electrons. This affects bond angles indirectly by influencing how tightly electron clouds are held.

• Bigger atoms often have more space to allow other atoms to be spaced apart.
• Larger atoms can lead to larger bond angles as they provide more room for repulsion minimization.

In the exercise, bromine in \(\mathrm{Br}_{2}\mathrm{O}\) is larger than both fluorine and chlorine. This increased size allows more dispersion of electrons and reduces effective repulsion, potentially resulting in more significant bond angles compared to smaller atoms such as fluorine, where electron cloud crowding reduces bond angles. This is why \(\mathrm{Br}_{2}\mathrm{O}\) has the largest bond angles in the comparison.