Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

An element \(\mathrm{M}\) reacts with chlorine to form a compound \(\mathrm{X}\). The bond angle in \(\mathrm{X}\) is \(120^{\circ}\). What is \(\mathrm{M}\) ? (a) \(\mathrm{B}\) (b) \(\mathrm{N}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{Mg}\)

Step-by-Step Solution

Answer

The element \( \mathrm{M} \) is \( \mathrm{B} \).

Step 1: Identify the Bond Geometry

Since the bond angle in compound \( \mathrm{X} \) is \( 120^{\circ} \), this suggests that the molecule has a trigonal planar geometry, which is typical for molecules with \( sp^2 \) hybridization.

Step 2: Determine the Central Atom's Hybridization

To have a trigonal planar geometry, the central atom \( \mathrm{M} \) should have \( sp^2 \) hybridization. This hybridization occurs when an atom forms three equivalent bonds and has no lone pairs, resulting in a \( 120^{\circ} \) bond angle.

Step 3: Analyze the Given Options

Check each element in the options:- \( \mathrm{B} \): Forms \( \mathrm{BCl_3} \) with a trigonal planar structure; bond angle \( 120^{\circ} \).- \( \mathrm{N} \): Typically forms \( \mathrm{NCl_3} \), a pyramidal structure, not planar; bond angle less than \( 120^{\circ} \).- \( \mathrm{Be} \): Forms \( \mathrm{BeCl_2} \), a linear structure; bond angle \( 180^{\circ} \).- \( \mathrm{Mg} \): Forms \( \mathrm{MgCl_2} \), also a linear structure; bond angle \( 180^{\circ} \).Thus, only \( \mathrm{B} \) matches the description with a bond angle of \( 120^{\circ} \).

Step 4: Choose the Correct Answer

Based on the bond angle and the geometry, \( \mathrm{B} \) is the correct element \( \mathrm{M} \) that forms a compound with chlorine that has a \( 120^{\circ} \) bond angle. Therefore, \( \mathrm{M} \) is \( \mathrm{B} \).

Key Concepts

Trigonal Planar Geometry

The concept of trigonal planar geometry is vital when analyzing molecules with certain bond angles, like the one described in the exercise. If a molecule exhibits a bond angle of exactly 120 degrees, we associate it with a trigonal planar shape. This geometry is characterized by three equally spaced bonds surrounding a central atom.

A trigonal planar molecule essentially forms a flat, triangular structure. All atoms within it lie on the same plane, which makes it different from other geometries, such as tetrahedral or linear shapes. This specific configuration arises due to the repulsion between electron pairs surrounding the central atom.

When observing a molecule, check if the bond angles form this neat 120-degree relationship, like the example of boron trifluoride (BF₃), which displays perfect trigonal planar geometry. Understanding this geometric arrangement is crucial for recognizing molecules with similar spatial structures.

sp2 Hybridization

The phenomenon of sp2 hybridization often correlates with trigonal planar geometry. In chemistry, hybridization describes how atomic orbitals mix to create new, equivalent, hybrid orbitals. These orbitals then form the specific shapes that dictate molecular structure.

For sp2 hybridization, one s orbital and two p orbitals combine, yielding three identical orbitals. These are known as sp2 hybrid orbitals. Such hybridization results in a molecule with three bonds, no lone pairs on the central atom, and a bond angle of 120 degrees.

• Boron in BF₃ illustrates sp2 hybridization when it bonds with three chlorine atoms, forming a flat, trigonal planar shape.
• Ethylene (C₂H₄) is another example, where carbon atoms exhibit sp2 hybridization, leading to a planar and double-bonded configuration.

Recognizing sp2 hybridization helps predict and understand the geometry and reactivity patterns of molecules.

Molecular Structure Analysis

Analyzing a molecule's structure requires understanding both the geometry and the hybridization of the central atom. This comprehension allows us to accurately predict the molecule's properties, interactions, and behaviors.

When given the bond angle, like 120 degrees in the exercise, we can deduce that the molecular geometry is trigonal planar, driven by sp2 hybridization. By examining possible elements, we can determine which matches this structural requirement.

• Identify the central atom's possible bonding configuration.
• Evaluate the electron pair repulsions that influence the geometry.
• Predict molecular behavior based on observed or calculated geometry and hybridization.

In our case, upon recognizing that boron forms a compound with chlorine exhibiting a 120-degree bond angle, we deduced it had a trigonal planar shape. This understanding stems from correctly analyzing the molecular components and their spatial arrangements. Through such analysis, chemists can design and synthesize new molecules with desired properties.