To determine the electron-pair geometry, you must start with the Lewis structure of the molecule. In order to determine the correct electron-pair geometry, it is important to have the correct Lewis structure drawn.

The electron-pair geometry is the general shape of the molecule. To determine the electron-pair geometry, you need to count the number of electron-dense areas around the central atom of the molecule. Electron-dense areas can be electrons from bonded atoms or lone electron pairs. Multiple bonds (double or triple bonds) are treated as if they were single bonds. The electron pairs in the multiple bonds are treated as a "super group" of electrons.

Let’s look at few examples. You will first learn how to count the electron-dense areas around the central atom. You will learn the names of the geometries later in this lesson.

Geometries

Watch the video below to learn more about geometries. (Note that the video covers two expanded octet shapes that are an extension of what you need to know). You will need to know the following three types of geometries (shapes): linear, trigonal planar, and tetrahedral.

Match the electron-pair geometry names with the diagrams below. The green balloon-shaped structures represent the electron-dense areas, and the red circles represent the central atoms.

What about molecules that have more than one central atom? Let’s look at an example. The chemical formula for butane is C₄H₁₀. The Lewis structure is shown below.

For this molecule the electron pair geometry for all four carbon atoms is tetrahedral. Note that it is not always the case that the electron-pair geometry will be the same for multiple central atoms.