Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is a planar molecule? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{BCl}_{3}\) (d) \(\mathrm{PCl}_{3}\)

Step-by-Step Solution

Answer

The planar molecule is \( \mathrm{BCl}_3 \).

Step 1: Understand the term 'Planar Molecule'

A planar molecule is one in which all atoms lie in the same geometric plane. Typically, this corresponds to a molecular geometry that includes a trigonal planar shape.

Step 2: Examine each molecule's geometry

Let's review the molecular geometry of each option:(a) \( \mathrm{NH}_3\): This molecule has a tetrahedral electron geometry but a trigonal pyramidal shape. It is not planar.(b) \( \mathrm{H}_3 \mathrm{O}^{+}\): This molecule is also based on tetrahedral geometry but with a trigonal pyramidal shape, thus it is not planar.(c) \( \mathrm{BCl}_3\): This is a molecule with a central atom forming three bonds and no lone pairs, leading to a trigonal planar shape. It is planar.(d) \( \mathrm{PCl}_3\): Similar to \( \mathrm{NH}_3\), it has a tetrahedral electron geometry with a trigonal pyramidal shape. It is not planar.

Step 3: Identify the planar molecule

From the analysis, \( \mathrm{BCl}_3\) is the only option with a trigonal planar molecule arrangement, making it a planar molecule.

Key Concepts

Molecular Geometry

Molecular geometry refers to the spatial arrangement of atoms within a molecule. This arrangement affects the molecule's properties and reactivity. It is determined by the number of electron pairs around a central atom, including both bonding pairs (which form chemical bonds) and lone pairs (which are non-bonding electrons).
Understanding molecular geometry is crucial because it can help predict:

• The shape of the molecule
• The angles between bonds
• The overall polarity and behavior of the molecule in chemical reactions

In the context of planar molecules, the geometry is particularly important. Planar molecules, where atoms lie in a single plane, result from specific electron pair geometries such as trigonal planar formations. This geometry is prevalent among molecules with a central atom bonded to three other atoms with no lone pairs, creating a flat, triangle-like shape.

Trigonal Planar Shape

The trigonal planar shape is a type of molecular geometry where three atoms are symmetrically arranged around a central atom, forming a flat triangular shape. This occurs when the central atom has three bonding pairs and no lone pairs of electrons.
Characteristics of trigonal planar shapes include:

• A bond angle of exactly 120° between each pair of bonded atoms
• A completely flat molecular structure
• No lone pairs on the central atom

Molecules like boron trichloride (\(\mathrm{BCl}_3\)) are excellent examples of the trigonal planar shape. Since the central boron atom is bonded to three chlorine atoms with no lone pairs, all four atoms lie in a single plane. This configuration creates the planar structure making \(\mathrm{BCl}_3\) the only planar molecule among our choices.

Tetrahedral Geometry

Tetrahedral geometry is a condition where a central atom is bonded to four other atoms positioned at the corners of an imaginary tetrahedron. This type of geometry arises when there are four bonding pairs around the central atom, typically with no lone pairs or one lone pair, which deforms the geometry slightly.
Some characteristics of a tetrahedral structure include:

• A bond angle of approximately 109.5° when it is a perfect tetrahedron
• Three-dimensional structure, often resulting in non-planar molecules
• Presence of lone pairs slightly altering angles - for example, in \(\mathrm{NH}_3\), which has a lone pair, the shape becomes trigonal pyramidal

Notably, molecules with a tetrahedral electron geometry frequently appear in nature. However, their arrangements often lead to non-planar configurations, like those observed in \(\mathrm{NH}_3\) and \(\mathrm{PCl}_3\), where electron lone pairs significantly influence the shape.