An incomplete atom or a group of incomplete atoms, which thus has a charge. These can be simple, as in negative ions, which often consist of free electrons, or they can be fairly complex, as the nuclei of metal atoms with some or all electrons stripped off. Ions form because of dissociation (e.g. salt in water), strong radiation (UV, x-rays, radioactivity etc.), strong electrical forces (e.g. high voltage on a pointed electrode), radioactive decay, extreme heat, cold fusion and other processes. In general, negative ions have an excess of electrons, and positive ions lack electrons.