In Domains Valence electrons are grouped in pairs

In this lesson, you’ll learn about the VSEPR theory and how it can be used to explain molecule shapes. Then, learn how to predict the shape of a molecule by applying the VSEPR theory to the Lewis dot structure.

Introduction to VSEPR Theory

You might recall that Lewis dot structures are used to show the organization of electrons and atoms within a molecule. The nucleus and core electrons of atoms are represented by their element symbol, electrons are represented by dots, and bonds are represented by lines.

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These element structures are great for showing how atoms bond, but sadly, they do not give us an accurate picture of molecular geometry.For example, if water’s Lewis dot structure looks linear, then why is the molecule’s actual structure bent?Why does the Lewis dot structure for methane look like a flat diamond, while the real structure for methane is a three-dimensional pyramid?And just to make things more confusing, why is carbon dioxide linear, just like its Lewis dot structure?This leaves us wondering — is there a way to predict the shape of a molecule just by looking at its Lewis dot structure?The answer is yes! We can apply the Valence Shell Electron Pair Repulsion theory, usually referred to as the VSEPR theory, to predict the shape of any molecule given its Lewis dot structure.Before we jump into the VSEPR theory, let’s review a few things about electrons.

  • Electrons are negatively charged particles.
  • Electrons swarm around the nucleus of an atom.

    The inner electrons are called core electrons, the outermost electrons are called valence electrons, and the outermost layer of electrons is called the valence shell.

  • Atoms strive to obtain a full valence shell of electrons. (A full valence shell contains eight electrons.)
  • Valence electrons may be involved in bonding with other atoms (and they’re called bonding electrons), or they may exist as lone pairs (these are called nonbonding electrons).
  • Electrons in a full valence shell are paired.

According to VSEPR theory, the shape of a molecule is related to the organization of the central atom’s valence shell electrons.

The valence shell electrons are all negatively charged and therefore are constantly repelling each other. This repulsion is what gives a molecule its three-dimensional shape. Both bonding and nonbonding electrons are involved in repulsion.

While the Lewis dot structure for water makes water look linear, the molecule itself is bent. This is because water has four different pairs of electrons repelling each other. The bent structure is a result of these negatively charged areas trying to be as far away from each other as possible.

Electron Domains

Valence electrons are grouped in pairs around an atom. Any area of valence electron density around an atom is called an electron domain.

Areas of electron density include both bonding and nonbonding electrons.In some molecules, the central atom may have up to six electron domains! In this lesson, we will only look at central atoms with two, three or four electron domains. The shape created by the electron domains surrounding the central atom is called the electron domain geometry. Understanding both electron domains and electron domain geometry is very helpful in predicting molecular geometry. But we’ll go into more detail on that later.If an atom has two electron domains, those electrons are going to try to get as far away from each other as possible.

What angle will allow these two domains to be as far away from each other as possible?If you said 180 degrees, nice work. When an atom has two electron domains, they will always be 180 degrees apart, and the corresponding electron domain geometry is known as linear.If an atom has three electron domains, a similar phenomenon happens when their electrons try to get as far away from each other as possible. The electron domains are 120 degrees from each other in the horizontal plane and 180 degrees from each other in the vertical plane.

This results in an electron domain geometry of trigonal planar.If an atom has four electron domains, the electron domain geometry is tetrahedral, and the domains are 109.5 degrees from each other.

Molecular Geometry

Molecular geometry is based entirely on electron domain geometry.

Electron domain geometries show all of the electron domains, including bonding and nonbonding. Molecular geometries only show bonding domains, even though nonbonding domains are present.For molecules with two, three or four electron domains, there are five different possible molecular geometries: linear, trigonal planar, tetrahedral, bent, and trigonal pyramidal.Some of the molecular geometries are exactly the same as the electron domain geometries.

When all of the electron domains are bonding, then the electron domain geometry is the same as molecular geometry.We will learn how to determine a molecule’s geometry based on the number of and type of electron domains present. That involves following these simple steps:1) Find the Lewis dot structure of the molecule. If it is not given to you, you might have to draw it. If you need a refresher drawing Lewis dot structures, it’s never a bad idea to review.2) Locate the central atom, then count the number of electron domains around the central atom.

This gives you an idea of the electron domain geometry. Remember, electron domains are any area of electron density, bonds or lone pairs. Single, double and triple bonds each count as one area of electron density. I like to circle electron domains when doing this step.3) Identify the number of nonbonding domains and the number of bonding domains.4) Compare results to a chart that relates electron domains to molecule shape.

Electron domains Number of bonding domains Number of nonbonding domains Molecule shape
2 2 0 Linear
3 3 0 Trigonal planar
3 2 1 Bent
4 4 0 Tetrahedral
4 3 1 Trigonal pyramidal
4 2 2 Bent

Let’s do an example.

Here is the Lewis dot structure for water. Since we have the structure, we can skip step one.

Lewis dot structure for water
Lewis dot structure for water

Step 2) Locate the central atom and circle all the electron domains. Two sets of lone pairs and two bonds make four electron domains.Step 3) Identify the number of bonding and nonbonding domains.

Two of these domains are nonbonding lone pairs and two are bonding.Step 4) Compare results to a chart. I go to my chart and I look for molecule shapes that have four electron domains, two bonding, and two nonbonding.

The shape of this molecule will be bent.Let’s try another.Here is the Lewis dot structure for the carbonate ion. Step one is completed, so it’s on to the next step.

Lewis dot structure for carbonate ion
Lewis dot structure for ammonia
Electron domains Number of bonding domains Number of nonbonding domains Molecule shape
2 2 0 Linear
3 3 0 Trigonal planar
3 2 1 Bent
4 4 0 Tetrahedral
4 3 1 Trigonal pyramidal
4 2 2 Bent

The central atom in ammonia is nitrogen, and there are four areas of electron density around it; three are bonding and one is nonbonding. According to the chart, the shape of this molecule is trigonal pyramidal.

Lesson Summary

Molecule shapes can be predicted based on Lewis dot structure using the VSEPR theory. VSEPR stands for Valence Shell Electron Pair Repulsion.

It states that electron pairs in the valence shell of an atom repel each other; their molecular geometry is the result of this repulsion.Areas of valence electron density are known as electron domains. Electron domains can be nonbonding (lone pairs) or bonding.Molecular shape can be determined by doing these four steps:1) Find the Lewis dot structure of the molecule.2) Locate the central atom, then count the number of electron domains around the central atom.3) Identify the number of nonbonding domains and the number of bonding domains.4) Compare the results to a chart that relates electron domains to molecule shape.A chart like this will help you complete step number four.

Electron domains Number of bonding domains Number of nonbonding domains Molecule shape
2 2 0 Linear
3 3 0 Trigonal planar
3 2 1 Bent
4 4 0 Tetrahedral
4 3 1 Trigonal pyramidal
4 2 2 Bent

Learning Outcomes

Once you have finished this lesson you should be able to:

  • Recall some characteristics of electrons
  • Summarize VSEPR theory
  • Explain electron domain geometry and molecular geometry
  • Identify the shape of a molecule using VSEPR theory
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