Science, Tech, Math › Science Electron Domain Definition and VSEPR Theory Share Flipboard Email Print Ian Cuming/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 20, 2019 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. Key Takeaways: Electron Domain An atom's electron domain is the number of lone pairs or chemical bond locations that surround it. It represents the number of locations expected to contain electrons.By knowing the electron domain of each atom in a molecule, you can predict its geometry. This is because electrons distribute around an atom to minimize repulsion with one another.Electron repulsion is not the only factor that affects molecular geometry. Electrons are attracted to positively charged nuclei. The nuclei, in turn, repel each other. Valence Shell Electron Pair Repulsion Theory Imagine tying two balloons together at the ends. The balloons automatically repel 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. For example, CO2 has two 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: AX2 - The two-electron domain structure produces a linear molecule with electron groups 180 degrees 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 degrees apart.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.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: Sketch the Lewis structure of the ion or molecule.Arrange the electron domains around the central atom to minimize repulsion.Count the total number of electron domains.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. Sources Jolly, William L. "Modern Inorganic Chemistry." McGraw-Hill College, June 1, 1984. Petrucci, Ralph H. "General Chemistry: Principles and Modern Applications." F. Geoffrey Herring, Jeffry D. Madura, et al., 11th Edition, Pearson, February 29, 2016.