The acronym VSEPR is short for 'Valence Shell Electron Pair Repulsion'. Consequently, the VSEPR theory is about the repulsion effect of the most chemically important valence electrons and the influence of the repulsion on the geometry of simple molecules. By saying 'simple' we mean the molecules that are able to be regarded as having one single central atom and at lease one ligand; in other words, a molecule with the form of 'AYn', where Y could be either a ligand (denoted X) or a lone electron pair (denoted E). The theory states that the repulsion of valence electron pairs makes a given molecule adopt a geometry that minimises the repulsion effect; moreover, the number of valence electron pairs shall dictate the geometry. For instance, suppose that n=4, then VSEPR states that overall an AY4 molecule shall have a tetrahedral geometry, possible lone electron pair taken in consideration. This means the ligands as well as the lone pairs (if any) contribute to a tetrahedral structure. AY4 molecules could look either actually like a tetrahedron (e.g. methane, CH4, in the form AX4) or like an 'uncompleted' tetrahedron (e.g. water, H2O, in the form AX2E2). When n= 2, 3, 5 and 6 (which are commonly encountered), the overall geometry are dictated to be respectively linear, trigonal planar, trigonal bipyramidal and octahedral.Though the VSEPR theory has a limited range of application and is not perfect in prediction, it does offer a quick and convenient solution for simple molecules, which are the blocks that build up the sophisticated building of chemistry.