Molecular model of iodine pentafluoride (IF5), a toxic gas. The atoms are color coded: iodine (orange) and fluorine (red). The blue area is a lone pair of electrons that are not involved in a chemical bond. This molecule has a square-based pyramid shape, and is an example of the Valence Shell Electron Pair Repulsion (VSEPR) theory. This states that bonded atoms around a central atom adopt a configuration in order to be as far away from each other as possible. Without a lone pair, atoms surrounded by five others have a trigonal bipyramidal shape.