An ion is an atom or group of atoms that has an electric charge. Ions with a positive charge are called cations. Ions with a negative charge are called anions.
Many normal substances exist in the body as ions. Common examples include sodium, potassium, calcium, chloride, and bicarbonate. (See: Electrolytes)
Ions can be created using radiation such as x-rays. Ionizing radiation is often used for diagnostic or therapeutic procedures, since the ions that are produced can be used to scan for or treat some diseases.