Cation

A cation is a type of ion that has a positive electric charge.^[1] This means it has fewer electrons than protons. The opposite of a cation is an anion, which has a negative charge.

Cations can have only one atom (monatomic cations) or be made of multiple atoms together (polyatomic cations). Most metals form monatomic cations, while polyatomic cations are rarer.

Most metals make one or more monatomic cations. Alkali metals like sodium can lose one electron to make cations like Na⁺. Alkaline earth metals like calcium lose two electrons to make cations like Ca²⁺. These are the only ions these elements form, and so are just named after the element: the sodium cation Na⁺ is just called "sodium" in compounds like sodium chloride.

Transition metals and post-transition metals can make more than one type of cation: iron forms two cations, Fe²⁺ and Fe³⁺. The charge on transition metal cations is usually between +1 (such as silver in silver iodide) and +4 (such as titanium in titanium tetrachloride).

Because transition metal cations can have more than one charge, the charge (formally, the oxidation state), is included in the name of the cation in compounds using Roman numerals. Pyrite is made of Fe²⁺ and the sulfide anion S²⁻, so it is called iron(II) sulfide. Magnetite is made of Fe³⁺ and the oxide anion O²⁻, so it is called iron(III) oxide. Sometimes in older sources these cations have specific names: another name for iron(II) is the "ferrous" cation, while iron(III) is the "ferric" cation.

Ammonium is an example of a polyatomic cation. It is made of a nitrogen atom connected to four hydrogen atoms. The formula for ammonium is written NH+4. Ammonium is made when an acid gives a hydrogen ion to a molecule of ammonia.

1. ↑ International Union of Pure and Applied Chemistry. "Cation". Compendium of Chemical Terminology Internet edition.