Problem 22 - Chemistry: An Atoms-Focused Approach

Question

Rank the following molecular geometries in order of increasing bond angles: (a) trigonal pyramidal; (b) trigonal planar; (c) square planar.

Step-by-Step Solution

Answer

Question: Arrange the following molecular geometries in order of increasing bond angles: trigonal pyramidal, trigonal planar, and square planar. Answer: The order of increasing bond angles for these molecular geometries is: square planar (90°), trigonal pyramidal (107.3°), and trigonal planar (120°).

Step 1: Understanding the molecular geometries

To rank these molecular geometries in order of increasing bond angles, we first need to understand their shapes and bond angles. - Trigonal pyramidal molecules have a central atom bonded to three other atoms with a lone pair of electrons on the central atom. The ideal bond angle in a trigonal pyramidal molecule is 107.3°. - Trigonal planar molecules have a central atom bonded to three other atoms without any lone pairs on the central atom. The ideal bond angle in a trigonal planar molecule is 120°. - Square planar molecules have a central atom bonded to four other atoms, with two lone pairs of electrons on the central atom. The ideal bond angle in a square planar molecule is 90°. Now that we have the bond angles for each molecular geometry, we can rank them accordingly.

Step 2: Ranking the molecular geometries

Based on the ideal bond angles of these molecular geometries, we can now rank them in order of increasing bond angles: 1. Square planar: 90° 2. Trigonal pyramidal: 107.3° 3. Trigonal planar: 120° So, the rank order of the molecular geometries in terms of increasing bond angles is: square planar, trigonal pyramidal, and trigonal planar.

Key Concepts

Trigonal Pyramidal

The trigonal pyramidal shape is a common type of molecular geometry. In this shape, a central atom is bonded to three other atoms and has one lone pair of electrons. This lone pair occupies more space than a bonding pair, resulting in a geometry where the bond angles are approximately 107.3°. This angle is lower than that of the trigonal planar arrangement due to the lone pair's repulsion. Trigonal pyramidal molecules can typically be found in compounds such as ammonia (NH₃).

• The presence of a lone pair leads to a spatial arrangement that looks like a pyramid with a triangular base.
• The bond angles are slightly smaller than the idealized tetrahedral angle of 109.5°.

Understanding this geometry is crucial in predicting the behavior of molecules, especially in terms of polarity and reactivity. The lone pair affects these properties because its repulsion skews the molecule's symmetry. This makes the trigonal pyramidal shape distinct from other triatomic geometries.

Trigonal Planar

A trigonal planar molecular geometry occurs when a central atom is directly bonded to three surrounding atoms with no lone pairs on the central atom. The bond angles in this configuration are ideally 120°, which means the atoms are spread out in a flat plane. This geometry leads to a highly symmetrical molecule which can have implications for its physical properties and interactions. Substances like boron trifluoride (BF₃) display a trigonal planar shape. Some key aspects include:

• All peripheral atoms are positioned equidistant from each other, maintaining an even spread around the center.
• The symmetry of the trigonal planar shape often leads to non-polar molecules if all surrounding atoms are the same.

Initially, one might think that trigonal planar might be less common due to its apparent strict bonding requirements, but it appears frequently in coordination chemistry and small covalent molecules. It is a simple yet elegant arrangement in molecular geometry.

Square Planar

Square planar geometry is seen in molecules where the central atom is bonded to four peripheral atoms and features two lone pairs. This configuration results in an ideal bond angle of 90°, making it easy to distinguish from other geometries.Typically, this geometry is observed in certain complex metal ions like \[ PtCl_4^{2-} \]. Key characteristics include:

• The molecule forms a flat, square shape, with atoms at each of the corners.
• The lone pairs occupy positions adjacent to each other on the central atom, which helps maintain the geometry.

Square planar geometry is less common than others like tetrahedral but plays an important role in areas such as coordination chemistry. Understanding how lone pairs influence bond angles and spatial arrangements helps in predicting molecular behavior. Additionally, it is essential for understanding certain catalytic processes and material properties in chemistry.