VSEPR, or valence shell electron pair repulsion, is a model used to predict the geometry of a molecule. This arrangement is a result of the electron pairs that surround the central atom.
The VSEPR theory is also referred to as the Gillespie-Nyholm theory. The geometry of the model is a result of the repulsion between the valence electron pairs with the goal being to reduce this repulsion. As the number of valence electron pairs increases, the angle between the pairs is reduced. For example, a central atom with two electron pairs, neither with a lone pair of electrons, has an angle of 180 degrees between them. If a lone pair is introduced to this molecule, the angles are reduced to a range of 104 to 109.5 degrees.
Using the VSEPR theory, the different geometries are ordered based on the total pairs of electrons orbiting the central atom and the total number of lone electron pairs. The most basic of these geometries is the aforementioned linear, which consists of just three elements. The most complex geometry is the tricapped triganol prismatic, which has 10 total elements. Regardless of the geometry, the most lone electron pairs that can exist is three, in the trigonal bypramidal linear version.