Electron Domains and VSEPR Theory

What an Electron Domain Means in Chemistry

Carbon dioxide is a linear molecule with two O-H bonds that are 180 degrees apart.
Carbon dioxide is a linear molecule with two O-H bonds that are 180 degrees apart. MOLEKUUL/SCIENCE PHOTO LIBRARY / Getty Images

In chemistry, the electron domain refers to the number of lone pairs or bond locations around a particular atom in a molecule. Electron domains may also be called electron groups. Bond location is independent of whether the bond is a single, double or triple bond.

VSEPR Valence Shell Electron Pair Repulsion Theory

Imagine tying two balloons together at the ends. The balloons automatically repel one another, or "get out of the way" of one another.

Add a third balloon, and the same thing happens so that the tied ends form an equilateral triangle. Add a fourth balloon, and the tied ends reorient themselves into a tetrahedral shape.

The same phenomenon occurs with electrons: electrons repel one another, so when they are placed near one another they automatically organize themselves into a shape that minimizes repulsions among them. This phenomenon is described as VSEPR or Valence Shell Electron Pair Repulsion.

Electron domain is used in VSEPR theory to determine the molecular geometry of a molecule. The convention is to indicate the number of bonding electron pairs by the capital letter X, the number of lone electron pairs by the capital letter E, and the capital letter A for the central atom of the molecule (AXnEm). When predicting molecular geometry, keep in mind the electrons generally try to maximize distance from each other, but they are influenced by other forces, such as the proximity and size of a positively-charged nucleus.

Examples: CO2 (see picture) has 2 electron domains around the central carbon atom. Each double bond counts as one electron domain.

Relating Electron Domains to Molecular Shape

The number of electron domains indicates the number of places you can expect to find electrons around a central atom. This, in turn, relates to the expected geometry of a molecule.

When the electron domain arrangement is used to describe around the central atom of a molecule, it may be called the molecule's electron domain geometry. The arrangement of atoms in space is the molecular geometry.

Examples of molecules, their electron domain geometry, and molecular geometry include:

2 Electron Domains (AX2) - The two electron domain structure produces a linear molecule with electron groups 180° apart. An example of a molecule with this geometry is CH2=C=CH2, which has two H2C-C bonds forming a 180-degree angle. Carbon dioxide (CO2) is another linear molecule, consisting of two O-C bonds that are 180° apart.

2 Electron Domains (AX2E and AX2E2) - If there are two electron domains and one or two lone electron pair, the molecule can have a bent geometry. Lone electron pairs make a major contribution to the shape of a molecule. If there is one lone pair, the result is a trigonal planar shape, while two lone pairs produce a tetrahedral shape.

3 Electron Domains (AX3) - The three electron domain system describes a trigonal planar geometry of a molecule where four atoms are arranged to form triangles with respect to each other. The angles add up to 360 degrees. An example of a molecule with this configuration is boron trifluoride (BF3), which has three F-B bonds, each forming 120-degree angles.

Using Electron Domains to Find Molecular Geometry

To predict the molecular geometry using the VSEPR model:

  1. Sketch the Lewis structure of the ion or molecule.
  2. Arrange the electron domains around the central atom to minimize repulsion.
  3. Count the total number of electron domains.
  4. Use the angular arrangement of the chemical bonds between the atoms to determine the molecular geometry. Keep in mind, multiple bonds (i.e. double bonds, triple bonds) count as one electron domain. In other words, a double bond is one domain, not two.
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Helmenstine, Anne Marie, Ph.D. "Electron Domains and VSEPR Theory." ThoughtCo, Dec. 17, 2017, thoughtco.com/definition-of-electron-domain-605073. Helmenstine, Anne Marie, Ph.D. (2017, December 17). Electron Domains and VSEPR Theory. Retrieved from https://www.thoughtco.com/definition-of-electron-domain-605073 Helmenstine, Anne Marie, Ph.D. "Electron Domains and VSEPR Theory." ThoughtCo. https://www.thoughtco.com/definition-of-electron-domain-605073 (accessed January 20, 2018).