An atom or molecule that has lost (or, more rarely, gained) one or more electrons. An ion is positively charged if it has lost one or more electrons, and negatively charged if it has gained one or more electrons. Ions are designated by the chemical symbol for the parent atom followed by a superscript indicating the charge; thus doubly ionized iron is represented as Fe²⁺. Alternatively, the symbol is followed by a Roman numeral which is one more than the number of electrons lost (e.g. Fe III for doubly ionized iron, Fe I for neutral iron).