VSEPR Theory

We will explore the meaning and explanation of VSEPR theory and who proposed it, as well as delve into the applications of the theory with examples and their assumptions.

The main points that will be covered include Lewis structures and molecular geometries based on two, three, four, five, and six electron geometries, as well as some assumptions and inconsistencies with the theory, such as lone pair behaviour.

• First, we'll have a look at who proposed VSEPR theory.
• We'll then look at what VSEPR theory means.
• We'll follow this by going over Lewis structures.
• We'll then go over the assumptions of VSEPR theory.
• Finally, we'll be looking at the different molecular geometries based on two, three, four, five, and six electron geometries.

Who proposed VSEPR theory?

VSEPR theory was not proposed by a single researcher. Rather, it is an idea that many renowned scientists contributed to. It was firstly proposed by Sidgwick and Powell in 1940, and later developed into a full area of theoretical chemistry by Ronald Gillespie and Sir Ronald Nyholm in 1957.

The theory was then tested through many different methods and confirmed. It really showed how a mathematical theory can explain the shape of molecules, which can further explain the interaction and behaviours of molecules and compounds with each other.

VSEPR theory meaning

The VSEPR acronym stands for Valence Shell Electron Pair Repulsion (theory).

The VSEPR theory can explain why certain molecules are shaped the way they are in 3D. It can also help to deduce the shape of molecules from 2D representations to 3D structures. It is necessary to understand the basic principles the theory is based on, such as the principles of electron pair behaviour within a molecule, most commonly represented by Lewis structures.

VSEPR theory is based on the very straightforward idea that electron groups - which can either consist of lone pairs of electrons, single bonds, multiple bonds, or unpaired electrons - repel each other. This theory focuses on these electron repulsions since, according to VSEPR, the geometry that the molecule will adopt is the one in which the electron groups have the maximum separation possible from each other.

Assumptions of VSEPR theory

Key assumptions we have to consider are the differences in the bonds, and species of electron domains. Firstly, a double bond will behave differently to a single bond, yet the theory will regard it as a single electron domain. Secondly, we have to consider that a lone pair of electrons will have a greater repulsion than a bonded pair of electrons. Often in molecular geometries, the shape will be different, especially the values of the angles, as lone pairs will exert greater repulsion than bonded electrons. This can skew the shape to different conformations.

Lewis (electron dot) structures

Lewis structures highlight the presence of bonded electrons and electron lone pairs. Making the distinction between these species with regard to a specific atom allows for the creation of VSEPR-based shapes. In a Lewis structure, a pair of electrons can be represented either by two dots or a line.

Below is an example of how the structure shifts from the bonds created to their Lewis structures:

Fig. 1 - The distinction between the bonded pair and lone pair of electrons is shown in Lewis structures

Think about the 3D shape the molecule will adopt. Since this is a diatomic molecule, we can assume the most likely shape it will adopt will be a linear geometry. But what about more complicated molecules? In the next section, we will explore how VSEPR theory is based on the repulsion of electron pairs, as explained above.

VSEPR theory on two and three electron domains

In the above example of the diatomic F₂ molecule, we can see that there is one bond. The individual lone pairs want to spread out as far as possible to create a linear geometry. We can state the linear geometry is achieved (as we know the angle is not curved), thus the molecular shape has a 180° angle.

If a molecule has three atoms, what would be the shape adopted? It is assumed that with three atoms around a central atom, the bonds would spread out as far as possible, giving a trigonal planar (triangular planar) geometry. The angles between the atoms created would be 120°.

The diagram below shows the molecular geometries of molecules with two or three atoms bonded to a central atom.

Fig. 2 - A graphical representation can be seen here of the linear and trigonal planar molecular geometries, as based on VSEPR theory

VSEPR theory on four electron domains

How would a molecule with four atoms around a central one behave in 3D? In a 2D representation, a molecule with four electron domains would be represented with right angles (90°). But in 3D space, is that the most optimal spread of the electron pairs to avoid each other? Would a 90° angle allow for maximum repulsion between the bonded atoms?

Here is where you have to think outside the box. A molecule with four electron domains adopts a tetrahedral geometry. The bond angles are 109.5°, as it maximises a 3D space, rather than just a plane. Think of the four electron domains as creating a certain pyramidal structure. The figure below represents this molecular geometry and the repulsion of the electron pairs that creates a tetrahedral molecular geometry.

Fig. 3 - You can see how a molecule with four electron domains spreads those domains out maximally in a 3D space, creating a tetrahedral molecular geometry with bond angles of 109.5°

VSEPR theory on five and six electron domains

Now, consider what would happen if you have five bonded atoms to a central atom. What about six? What molecular geometry would these molecules adopt in 3D? In 2D representations, it is easy to draw on another bond and add it symmetrically to an atom. In reality, 3D structures often deviate from this notion.

Five electron domains create a trigonal bipyramidal (triangular bipyramidal) geometry. The 'bipyramidal' literally means two pyramids. Interpret this as two pyramids stacked on top of each other, where the pyramids have a triangular base.

A molecule with six electron domains creates an octahedral geometry. You can think of it as a shape with 8 sides, such as a diamond - hence octahedral, even though there are 6 atoms involved (and 6 vertices of the shape). You can even think of it as two pyramids. stacked on top of each other, where the base of the pyramid is a square. The angles created in this shape are 180° and 90°.

Using the above predictions, you can see how VSEPR theory allows us to make conclusions regarding the 3D shapes of molecules, depending on their electron domains (bonded pairs or lone pairs of electrons).

Examples of VSEPR theory

Let's explore some examples of each shape predicted by VSEPR theory, and how they can be applied to many contexts. Each of the theorised models will be accounted for, and some common examples shown and discussed.

Fig. 5 - Carbon dioxide linear geometry

Fig. 6 - Boron trichloride trigonal planar geometry

Above, you can see two structures. One represents the CO₂ molecule as an example of linear geometry, whilst the other shows a molecule of BCl₃ as an example of trigonal planar geometry, as predicted by VSEPR theory. The angles are displayed. These structures are experimentally confirmed, which suggests that VSEPR theory is relevant and holds up in real-life examples.

Below, you can see how the tetrahedral geometry as predicted by VSEPR theory is shown in a molecule of methane, CH₄. Here the bond angles form the predicted angle of 109.5°.

Fig. 7 - Methane tetrahedral geometry

What you can see below is an example of the trigonal bipyramidal geometry of PF₆. Here you can definitely see the difference between the 2D representation, a depiction of the Lewis structure of the molecule, compared to the 3D model. The five electron domains spread themselves as far as possible based on the repulsion of electron pairs to create the shape predicted - trigonal bipyramidal.

Fig. 8 - Phosphorus pentafluoride trigonal bipyramidal geometry

Below you will find an example of the octahedral VSEPR geometry, represented by a molecule of SF₆. You can see how the six fluorine atoms arrange themselves away from the central sulfur atom creating an octahedral 'diamond' shape in 3D.

Fig. 9 - Sulfur hexafluoride octahedral geometry