Two chemists, Ronald James Gillespie and Ronald Sydney Nyholm, developed the Valence Shell Electron Pair Repulsion (VSEPR) theory, which determines the geometries, or 3-D shape, of a molecule. The key to understanding this theory is to remember that like charges repel each other. In the animation below, the like charges of the negatively charged electrons move away, or repel each other. This is called electrostatic repulsion.

The VSEPR theory states that electron pairs, found in the valence shells around the central atom in a molecule, try to move as far away from each other as possible. The electrons spread out as much as possible to minimize the repulsion between their like, negative charges. The positions of both electrons that are bonded and lone pair electrons (non-bonded electron pairs) are explained by this theory. When the electron pairs move away from each other, they give the molecule its three-dimensional shape.

The VSEPR method has two different labels for the 3-D shapes, or geometries, of a molecule. The two labels are as follows:

• Electron-pair geometry
• Molecular geometry

You will learn more about these geometries in the following sections.