For the elements after the 3rd period, the number of covalent bonds is sometimes five (e.g. PCl5) or six (e.g. SF6), and the central atom of these molecules shows hypervalency. In this case, because s and p electrons run short to form more than four 2-electron covalent bonds, it was once believed that d electrons were partly involved. The present view is, however, that these hypervalent bonds use only s and p orbitals but that the bond orders are lower than those of single bonds