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

BF3 has trigonal planar geometry.

Step 1: Understanding the Trigonal Planar Geometry

Trigonal planar geometry occurs in molecules where the central atom is bonded to three other atoms and has no lone pairs. This results in bond angles of 120° around the central atom.

Step 2: Evaluating BF3

The molecule \( \mathrm{BF}_3 \) contains a boron atom that forms three single bonds with three fluorine atoms. Boron has no lone pairs, meeting the conditions for trigonal planar geometry.

Step 3: Evaluating NH3

\( \mathrm{NH}_3 \) (ammonia) has a nitrogen atom bonded to three hydrogen atoms. Nitrogen has one lone pair, which results in a trigonal pyramidal geometry, not planar.

Step 4: Evaluating PCl3

In \( \mathrm{PCl}_3 \), the phosphorus atom is bonded to three chlorine atoms and has one lone pair, leading to a trigonal pyramidal geometry, not planar.

Step 5: Evaluating IF3

\( \mathrm{IF}_3 \) has iodine bonded to three fluorine atoms with two lone pairs on iodine. This leads to a T-shaped molecular geometry.

Step 6: Identifying the Correct Answer

Among all molecules, only \( \mathrm{BF}_3 \) meets the criteria for trigonal planar geometry, having the central atom bond to three others without lone pairs.

Key Concepts

Trigonal Planar Geometry

Trigonal planar geometry is a term used in molecular chemistry to describe a specific shape of a molecule. This geometry occurs when a central atom is bonded to three other atoms, and it should not have any lone pairs of electrons on it. This arrangement allows the bonded atoms to spread out evenly in a flat, triangular plane. The main characteristic of a trigonal planar geometry is that it provides a flat structure rather than a three-dimensional one, making it different from other molecular shapes which might have more of a pyramid or linear formation. This arrangement helps minimize repulsion between the electron pairs, leading to the most stable energy state for the molecule. In short, whenever you see a molecule with three bonded atoms and no lone pairs, think of it as potential trigonal planar.

Bond Angles

Bond angles are crucial in understanding molecular geometry as they influence the shape and properties of a molecule. In a trigonal planar molecule, such as \(\mathrm{BF}_{3}\), the bond angles are 120 degrees. This angle is the result of equal spacing between the atoms bonded to the central atom, allowing them to be as far apart as possible within the plane to minimize repulsive forces.
The bond angle can affect the physical and chemical properties of the molecule, including its polarity and reactivity.
Other geometries like trigonal pyramidal or tetrahedral would have different bond angles due to different atomic and lone pair arrangements.
Understanding bond angles helps predict the behavior and interaction of molecules, which is vital in fields like pharmacology, where molecular shape can determine how a drug interacts with the human body.

Lone Pairs

Lone pairs are non-bonding pairs of electrons that are located on an atom but not shared with other atoms in a molecule. These electrons can significantly influence the geometry of a molecule. In the case of trigonal planar geometry, the central atom must not harbor any lone pairs. The absence of lone pairs allows the molecule to adopt and maintain its planar shape, as seen in \(\mathrm{BF}_{3}\).

• Lone pairs occupy more space compared to bonding pairs due to their electron density. This can lead to distortion or alterations in bond angles and overall geometry if present.
• In \(\mathrm{NH}_{3}\) or \(\mathrm{PCl}_{3}\), the presence of one lone pair each on nitrogen and phosphorus respectively causes a shift from trigonal planar to trigonal pyramidal geometry, even if they have three bonded atoms.

Recognizing how lone pairs affect molecular shapes helps in predicting and explaining a molecule's steric properties and reactivity.

Molecular Shape

Understanding molecular shape is fundamental in chemistry as it influences how molecules interact with one another. The shape of a molecule is determined largely by the number of atoms bonded to the central atom and whether any lone pairs are present on the central atom. In the case with \(\mathrm{BF}_{3}\), which has a trigonal planar shape, the flat arrangement allows for uniform distribution of electron density, which can affect how this molecule reacts with other chemicals.
Different shapes can lead to different physical and chemical behaviors. For example, a flat trigonal planar shape can influence the dipole moment of a molecule and its interactions in a chemical reaction or bonding situation.
Understanding the concept of molecular shape helps chemists design new materials and compounds, predict reactions, and understand the functionalities of complex molecules in biological systems.