Ion, atom or group of atoms that has become electrically charged by gain or loss of negatively charged electrons. In general, ions formed from metals are positive (cations), those from nonmetals negative (anions). Crystals of ionic compounds consist of negative and positive ions arranged alternately in the lattice and held together by electrical attraction. Many compounds undergo ionic dissociation in solution. Ions may be formed in gases by radiation or electrical discharge, and occur in the ionosphere. At very high temperatures gases form plasma, consisting of ions and free electrons.