Cation

A cation is a type of ion that has a positive electric charge. 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.

Examples

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.

Cation Media

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  Electron transfer from a neutral lithium (Li) atom on the left to a neutral fluorine (F) atom on the right would give a Li⁺ and F⁻ ions.

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  Schematic of an ion chamber, showing drift of ions. Electrons drift faster than positive ions due to their much smaller mass.

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  Avalanche effect between two electrodes. The original ionization event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionizing electron and the liberated electron.

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  Equivalent notations for an iron atom (Fe) that lost two electrons, referred to as ferrous.

• 

  Mixed Roman numerals and charge notations for the uranyl ion. The oxidation state of the metal is shown as superscripted Roman numerals, whereas the charge of the entire complex is shown by the angle symbol together with the magnitude and sign of the net charge.

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  An electrostatic potential map of the nitrate ion (2NO−
  3). The 3-dimensional shell represents a single arbitrary isopotential.