- VSEPR theory predicts molecular geometry by minimizing electron repulsions between bonds and/or lone pair electrons
Valence Shell Electron Pair Repulsion (VSEPR) theory is based around the principle that molecular geometry can be predicted through the minimization of electron repulsion between bonds and/or lone pairs. While the basis behind this theory is fundamentally incorrect because it does not consider quantum mechanics, it is still taught as it provides a reasonably predictive approximation of molecular geometry.
To determine molecular geometry using VSEPR theory, first count all lone pairs of electrons and atoms around the central atom (A). This number can provide the parent shape of the molecule.
Once the parent shape is established, the next step is to determine the molecular shape, which is simply the shape of the atomic framework, excluding lone pair electrons. By definition, when there are no lone pairs present, the molecular shape is simply the parent shape. The table below does not comprehensively cover the molecular shapes according to VSEPR theory, but rather focuses on the geometries that you will frequently encounter in organic chemistry and biology.
Lets start our examples with the simplest hydrocarbon methane. As you previously learned, methane is comprised of a central carbon bound to 4 hydrogens. The total number of atoms bound to the central carbon is 4; therefore, the parent shape of methane is tetrahedral. Because there are no lone pairs, the molecular shape is also tetrahedral.
Now lets have a look at the molecular shape of water. The central oxygen atom is bound to 2 hydrogens with 2 lone pair electrons; therefore, the parent shape of water is tetrahedral. As there are 2 lone pair electrons present, the molecular shape is bent.
Interactive: