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 steric
number (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.