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

Question

Which of the following molecules has trigonal planar geometry? (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{PCl}_{3}\) (d) \(\mathrm{IF}_{3}\)

Step-by-Step Solution

Answer

The molecule with trigonal planar geometry is \( \mathrm{BF}_{3} \).

Step 1: Understanding Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. A trigonal planar geometry involves one central atom with three atoms bonded to it at 120-degree angles in a plane.

Step 2: Analyzing \( \mathrm{BF}_{3} \)

In \( \mathrm{BF}_{3} \), the central atom boron forms three single bonds with three fluorine atoms. With three bonding pairs and no lone pairs, the structure is trigonal planar with 120-degree bond angles.

Step 3: Analyzing \( \mathrm{NH}_{3} \)

The molecule \( \mathrm{NH}_{3} \) consists of one nitrogen atom bonded to three hydrogen atoms and has one lone pair on nitrogen. This results in a trigonal pyramidal geometry, not trigonal planar.

Step 4: Analyzing \( \mathrm{PCl}_{3} \)

In \( \mathrm{PCl}_{3} \), phosphorus is bonded to three chlorine atoms and also has one lone pair of electrons, making its geometry trigonal pyramidal rather than trigonal planar.

Step 5: Analyzing \( \mathrm{IF}_{3} \)

The molecule \( \mathrm{IF}_{3} \) consists of iodine with three bonded fluorine atoms and two lone pairs, leading to a T-shaped geometry, which differs from trigonal planar.

Step 6: Conclusion of Analysis

Among the options, only \( \mathrm{BF}_{3} \) has the trigonal planar geometry, characterized by its three bonded pairs and no lone pairs.

Key Concepts

Trigonal Planar

When it comes to understanding molecular geometry, the term "trigonal planar" specifically refers to a molecule where there is one central atom surrounded by three atoms spaced evenly around it. This configuration is not just flat but forms a triangle on a single plane. It's quite a streamlined and balanced shape.

This geometry occurs because the central atom forms chemical bonds with three other atoms, and these bonds naturally aim to get as far apart from each other as possible, minimizing repulsion and stabilizing the structure. This means each of the angles formed between the bonds is equal, specifically measuring 120 degrees.

• Three bonding atoms
• Flat triangular shape
• 120-degree angles

One classic example of a trigonal planar molecule is Boron trifluoride (\( \mathrm{BF}_3\)). In \( \mathrm{BF}_3\), boron is the central atom bonded symmetrically to three fluorine atoms with no lone pairs interfering.

Bond Angles

Bond angles are crucial in defining the shape of a molecule. In a trigonal planar molecular structure, the bond angles are vital and precisely measure 120 degrees. This degree measurement comes from the way the electron pairs in the bonds arrange themselves to minimize repulsion, effectively forming a wide triangle.

Having equal bond angles ensures the molecule remains flat and symmetrical, contributing to its unique geometry. Why is this 120-degree angle so special? It is because

• The atoms repel each other equally.
• It maintains a planar configuration.
• It ensures ideal distance among bonded pairs.

This angle is characteristic of a system where there is no strain enforced by lone pairs and all atoms are bonded equidistantly from each other.

Lone Pairs

Lone pairs are pairs of valence electrons that are not shared between atoms, playing a significant role in molecular geometry. In trigonal planar molecules like \( \mathrm{BF}_3\), there are no lone pairs on the central atom, which is why the shape remains symmetrical and planar.

Lone pairs tend to occupy more space than bonding pairs, as they are solely under the influence of one nucleus, and can push bonding pairs closer together, altering angles in a molecule. For instance, if a lone pair were present, it would not only affect the molecule's shape but also the bond angles, usually reducing them and potentially transforming the molecular geometry entirely. In molecules like \( \mathrm{NH}_3\) or \( \mathrm{PCl}_3\) with lone pairs, the resulting geometry is trigonal pyramidal, not planar, due to this lone pair's influence.

Molecular Structure

Molecular structure refers to the arrangement of atoms within a molecule, determining not only how a molecule behaves but also its physical appearance. This structure includes information on bond lengths, angles, and the spatial arrangement of all atoms.

In a trigonal planar structure, this arrangement is strikingly flat and organized, with all atoms in the same plane. It's not just about chemistry but also about spatial reasoning.

• Structure defines physical properties.
• Affects reactivity and interaction with other molecules.
• Determined by VSEPR theory and electron pair repulsion.

Understanding molecular structure helps in predicting how molecules interact and react, providing insights into their stability and behavior in different scenarios. So, for molecules like \( \mathrm{BF}_3\), knowing the trigonal planar structure can reveal much about its chemical properties.