Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

# Unit 5: The Structure of Molecules—Lewis Structures and Molecular Geometries

## Section 7: VSEPR Theory

When we look at Lewis structures, we are only seeing a two-dimensional picture. To get the full picture of molecules, we need to find ways to represent those atoms in three dimensions. Why? Because the shape, or geometry, of a molecule helps us understand how that molecule will behave. In fact, a molecule's geometry affects its physical interaction with other molecules, determines which region of the molecule is likely to undergo chemical changes, influences properties like melting point and density, and influences the products that will form when it reacts with different substances. So let's take a look at how we can determine the shapes of molecules.

Figure 5-12. Linear Geometry

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### Figure 5-12. Linear Geometry

In carbon dioxide, there is a central atom with two repelling regions of electron density (the electrons in the bonds). This forms a bond angle of 180° .

There is a simple model that allows us to predict, based on the Lewis structure, a molecule's three-dimensional geometry. It is called the "Valence Shell Electron Pair Repulsion theory," or VSEPR (pronounced "vesper"). In this model, we look at the arrangement of atoms and lone pairs around each atom individually. We call the atom under consideration the "central atom" and the other atoms or lone pairs attached to it the substituents of that atom. We consider each atom one at a time and then combine them to get the full picture.

Figure 5-13. Trigonal Planar Geometry

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### Figure 5-13. Trigonal Planar Geometry

In sulfur trioxide, SO3, three repelling substituents form an equilateral triangle in a plane around the central atom. Each bond angle is 120°.

The way VSEPR theory works is that we imagine that the electrons in the bonds or lone pairs desperately want to get as far away from each other as they can since all are negatively charged. But, since they are bound to the central atom, the best they can do is distribute themselves around the central atom with the angles between the atoms or lone pairs as large as possible. Imagine a central atom with bonds to two other things, for example the carbon in carbon dioxide, which is bound to two oxygens (Figure 5-12). The repelling electrons in the bonds will move to opposite sides of the central atom until they are 180° away from each other. We define the bond angle as the angle between the bonds to the central atom. When atoms arrange themselves in a straight line like this, we call it "linear geometry."

If there are three substituents around the central atom, as in sulfur trioxide (SO3), the farthest they can get from each other while still remaining bound to the central atom is to form a triangle in a plane around it, a geometry called "trigonal planar." The bond angles here are all 120°, as expected for an equilateral triangle. (Figure 5-13)

Figure 5-14. Tetrahedral Geometry

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### Figure 5-14. Tetrahedral Geometry

In methane, CH4, the four mutually repelling bonds adopt a symmetric distribution around the central carbon, a geometry known as tetrahedral. The angle that separates each of the bonds is 109.5°.

If there are four substituents repelling each other, the resulting geometry is called "tetrahedral," as shown below in Figure 5-14, and each of the bond angles is approximately 109.5°. Even though the Lewis structure makes any molecule seem flat, by combining the Lewis structure with VSEPR theory, we can understand and determine the 3-D structures of molecules.

Figure 5-15. Trigonal Pyramidal Geometry

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### Figure 5-15. Trigonal Pyramidal Geometry

In ammonia (NH3), the three hydrogens and one lone pair adopt a tetrahedral arrangement to avoid electron-electron repulsion. Since we cannot see the lone pair, the geometry gets called "trigonal pyramidal."

When describing molecular geometry, we deduce the shape by looking at the distribution of electrons around the central atom, including not just atoms but also lone electron pairs. But our description of the molecule depends only on the position of atomic nuclei. This means a molecule like ammonia, which has four repulsive regions of electron density (three bonds and the lone pair), has the geometry shown in Figure 5-15. The substituents arrange themselves tetrahedrally, just as we saw with methane. But when naming the molecular shape, we consider only the positions of the atoms, not the lone pairs; so rather than being tetrahedral, we would refer to this molecule as being trigonal pyramidal.

## Glossary

### Central atom

The atom for which we are determining the geometry when applying VSEPR theory.

### Substituents

The lone pairs and atoms of a molecule attached to the central atom when applying VSEPR theory.

### VSEPR Theory

A theory that describes the shapes of molecules based on the mutual repulsion of the electron clouds surrounding the nucleus.