Electrons repel each other because they are negatively charged. The quantum mechanical rules force some of them to be fairly close to each other in bonding pairs or lone pairs, but each pair repels all other pairs. According to the VSEPR model, therefore, molecules adopt geometries in which their valence electron pairs position themselves as far from each other as possible. A molecule can be described by the generic formula AXmE,, where A is the central atom, X stands for any atom or group of atoms surrounding the central atom, and E represents a lone pair of electrons. The stericnumber (SN = m + n) is the number of positions occupied by atoms or lone pairs around a central atom; lone pairs and bonds are nearly equal in their influcncc on molecular shape.