An ion is an electrically charged atom or group of atoms. It is a part of an atom, or part of a group of atoms (molecule). It is "charged" so it will move near electricity. This is because atoms are made of three smaller parts: neutrons (with no charge), and equal numbers of (2) charged protons and (3) oppositely-charged electrons. An ion has unequal numbers of protons and electrons. Making an ion from an atom or molecule is called ionization.
The charge on a proton is measured as +1 (positively charged). The charge on an electron is measured as -1 (negatively charged). An atom that is ionized makes two ions, one positive, and one negatively charged. For example, a neutral hydrogen atom has one proton and one electron. Heating the atom breaks it into two parts: (1) a positively charged hydrogen ion, H+ (2) a negatively charged electron.
A liquid with ions is called an electrolyte. A gas with lots of ions is called a plasma. When ions move, it is called electricity. For example, in a wire, the metal ions do not move, but the electrons move as electricity. A positive ion and a negative ion will move together. Two ions of the same charge will move apart. When ions move they also make magnetic fields.
A simple ion is formed from a single atom.
Polyatomic ions are formed from a number of atoms. Polyatomic ions usually consist of all non-metal atoms. But sometimes the polyatomic ion can have a metallic atom too.
Transition metals can form more than one simple cation with different charges.
Most ions have a charge of less than 4, but some can have higher charges.
Michael Faraday was the first person to write a theory about ions, in 1830. In his theory, he said what the portions of molecules were like that moved to anions or cations. Svante August Arrhenius showed how this happened. He wrote this in his doctoral dissertation in 1884 (University of Uppsala). The university did not accept his theory at first (he only just passed his degree). But in 1903, he won the Nobel Prize in Chemistry for the same idea.
In Greek ion is like the word "go". "Anion" and "cation" mean "up-goer" and "down-goer". "Anode" and "cathode" are "way up" and "way down".
- Brescia, Frank; Arents, John; Meislich, Herbert; Turk, Amos (1966). Fundamentals of Chemistry: A Modern Introduction (First ed.). Academic Press. p. 5.