Question

If you have three electron regions around a central atom, how can you have a triangular planar molecule? An angular molecule? What bond angles are predicted in each case?

Short answer

Triangular planar: 120°; Angular: less than 120°.

Step-by-step solution

Understand Electron Regions

An electron region is an area where electrons are likely to be found. These can be bonding pairs (shared electrons in a bond) or lone pairs (non-bonding electrons). When we talk about three electron regions, we refer to a trigonal planar arrangement, according to VSEPR theory.

Determine Triangular Planar Geometry

For a molecule to be triangular planar (also known as trigonal planar), all three regions must be bonding pairs with no lone pairs around the central atom. This creates a flat, triangular arrangement of atoms.

Bond Angles for Triangular Planar Molecule

In a trigonal planar geometry, the bond angles between the bonding pairs are 120 degrees, as the electrons are evenly distributed around the central atom to minimize repulsion.

Determine Angular Geometry

An angular, or bent shape, can occur when there are two bonding regions and one lone pair. The lone pair exerts a greater repulsive force, pushing the bonded atoms down to create an angle.

Bond Angles for Angular Molecule

The typical bond angle in an angular molecule with three electron regions is slightly less than 120 degrees due to the presence of the lone pair, which decreases the angle between the bonding pairs.

Key concepts

Electron Regions

In the context of VSEPR theory, electron regions are essential for determining the shape of a molecule. These regions represent areas where electrons are likely to be found around a central atom. This can either be through bonding pairs, which span shared electrons between atoms, or lone pairs, which are non-bonding electrons that occupy space around the central atom. By looking at the total number of electron regions, we can infer the molecular shape.
For instance, if a central atom has three electron regions, this generally suggests a particular geometric arrangement that minimizes electron repulsion, according to the VSEPR model. Understanding electron regions helps in visualizing how atoms orient themselves to maximize space efficiency and minimize repulsive interactions.

Trigonal Planar

A trigonal planar arrangement occurs when a central atom is surrounded by three bonding electron pairs with no lone pairs present. This configuration results in a flat, triangular layout, where all peripheral atoms are positioned at equal angles around the central atom.
In simpler terms, if you have three identical bonds stemming from a central point, each bond will spread out evenly, creating 120-degree angles with one another. This geometry is termed as 'trigonal planar' due to its shape resembling a flat triangle. The absence of lone pairs ensures that the shape remains even and symmetrical.
Trigonal planar structures are found in many organic and inorganic molecules. It's a classic depiction of equilibrium within molecular structures, providing an ideal space for bonding interactions.

Bond Angles

Bond angles are the geometric angles between two adjacent bonds in a molecule. In the VSEPR framework, bonding pairs, as well as lone pairs, influence these angles as they repel each other to maximize distance apart.
In a trigonal planar molecule, like in those determined above with three bonding regions, the ideal bond angle is 120 degrees. This angle arises from symmetrical electron pair distribution which minimizes repulsion between electrons. However, when lone pairs are present, as in angular molecules, the bond angle is less than 120 degrees. The lone pair exerts a greater repulsive force compared to a bonding pair. This additional repulsion compresses the angle slightly.
Understanding bond angles is crucial in predicting the behavior of molecules, including reactivity and physical properties.

Molecular Geometry

Molecular geometry refers to the three-dimensional shape that a molecule takes, which is primarily determined by the number and arrangement of electron regions around the central atom. VSEPR theory suggests that electron pairs, whether bonding or non-bonding, will arrange themselves to minimize repulsion.
Two common geometries with three electron regions are trigonal planar and angular. Trigonal planar forms when all three regions are bonds, leading to a flat triangular shape with 120-degree angles. In contrast, if one of these regions is a lone pair, the shape becomes angular or bent, with reduced bond angles.
This geometry impacts how molecules interact, their phase of matter, boiling and melting points, and reactivity with other substances. Identifying the molecular geometry is key to understanding and predicting these molecular properties.