4.7: Molecular Geometry

Skills to Develop

• Predict the shape of simple molecules and their polarity from Lewis dot structures.
• Explain the meaning of the acronym VSEPR and state the concept on which it is based.

Although a convenient way for chemists to look at covalent compounds is to draw Lewis structures, which shows the location of all of the valence in a compound. Although these are very useful for understanding how atoms are arranged and bonded, they are limited in their ability to accurately represent what shape molecules are. Lewis structures are drawn on flat paper as two dimensional drawings. However, molecules are really three dimensional. In this section you will learn to predict the 3D shape of many molecules given their Lewis structure.

Many accurate methods now exist for determining molecular structure, the three-dimensional arrangement of the atoms in a molecule. These methods must be used if precise information about structure is needed. However, it is often useful to be able to predict the approximate molecular structure of a molecule. A simple model that allows us to do this is called the valence shell electron pair repulsion (VSEPR) theory. This model is useful in predicting the geometries of molecules formed in the covalent bonding of nonmetals. The main postulate of this theory is that in order to minimize electron-pair repulsion, the electron pairs around the central atom in a molecule will get as far away from each other as possible.

Predicting the Shape of Molecules

Consider, methane, \(\ce{CH_4}\), commonly known as natural gas. In this molecule, carbon has four valence electrons and each hydrogen adds one more so the central atom in methane has four pairs of electrons in its valence shell. The 3D shape of this molecule is dictated by the repulsion of the electrons. Those four pairs of electrons get as far away from each other as possible which forms a shape called tetrahedral. In the tetrahedral shape, the bond angle between any two hydrogen atoms is \(109.5^\text{o}\).

What if we look at ammonia instead, \(\ce{NH_3}\)? A molecule of ammonia has a nitrogen atom in the middle with three bonds to the hydrogen atoms plus one lone pair of electrons. That means there are four total pairs of electrons around the central atom, and the electrons will still be close to \(109.5^\text{o}\) apart from each other. However, when discussing the overall shape of the molecule, we only take into account the location of the atoms. When a central atom is bonded to three atoms and has one lone pair of electrons, the overall shape is trigonal pyramidal.

We have a similar problem in the case of a molecule such as water, \(\ce{H_2O}\). In water, the oxygen atom in the middle is bonded to the two hydrogen atoms with two lone pairs. Once again, we only consider the location of atoms when we discuss shape. When a molecule has a central atom bonded to two other atoms with two lone pairs of electrons, the overall shape is bent.

As you can probably imagine, there are different combinations of bonds making different shapes of molecules. Some of the possible shapes are listed in the table. However, it is important to note that some molecules obtain geometries that are not included here.

Example \(\PageIndex{1}\)

Determine the shape of ammonium, \(\ce{NH_4^+}\), given the following Lewis structure:

Solution

To answer this question, you need to count the number of atoms around the central atom and the number of unshared pairs. In this example, there are four atoms bonded to the \(\ce{N}\) with zero unshared pairs of electrons. The shape must be tetrahedral.

Example \(\PageIndex{2}\)

Determine the shape of carbon dioxide, \(\ce{CO_2}\), given the following Lewis structure:

Solution

To answer this question, you need to count the number of atoms around the central atom and the number of unshared pairs. In this example, there are two atoms bonded to the \(\ce{C}\) with zero unshared pairs of electrons. The shape must be linear, according to the table.

Example \(\PageIndex{3}\)

Determine the shape of sulfur dioxide, \(\ce{SO_2}\), given the following Lewis structure:

Solution

To answer this question, you need to count the number of atoms around the central atom and the number of unshared pairs. In this example, there are two atoms bonded to the \(\ce{S}\) with one unshared pair of electrons. The shape must be bent, according to the table.

Vocabulary

• VSEPR model: A model whose main postulate is that the structure around a given atom in a molecule is determined by minimizing electron-pair repulsion.
• Molecular geometry: The specific three-dimensional arrangement of atoms in molecules.

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