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This article is about the electrically charged molecule. For other uses of this word, see ion (disambiguation).

An ion is an atom or group of atoms with a net electric charge. A negatively charged ion, which has gained one or more electrons, is known as an anion, and a positively charged ion, which has lost one or more electrons, is known as a cation. The process of producing ions is called ionization.

Ionized atoms or groups of atoms are denoted by superscripting with the number of electrons lost or gained (if more than one) and the sign (+ or −) of the electric charge, e.g. H+ or O2−.

Ionization energy

The energy required to produce a cation by stripping an electron from a more nearly neutral atom is the ionization energy. More generally, the n-th ionization energy of an atom is the energy required to strip it of its n-th electron after the first n − 1 have already been removed.

Each successive ionization energy is markedly greater than the last. Particularly dramatic increases occur after any given block of atomic orbitals is exhausted. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium is found as Na+, but not usually Na2+ due to the large amount of ionization energy required. Likewise, magnesium is found as Mg2+, but not Mg3+, and aluminium may exist as an Al3+ cation.


Ions were first theorized by Michael Faraday around 1830, to describe the portions of molecules that travel either to an anode or to a cathode. However, the mechanism by which this was achieved was not described until 1884 by Svante August Arrhenius in his doctoral dissertation to the University of Uppsala. His theory was initially not accepted (he got his degree with a minimum passing grade) but his dissertation won the Nobel Prize in Chemistry in 1903.


The word ion is a name given by Michael Faraday, from Greek ', neutral present participle of ἰέναι, "to go", thus "a goer". This nomenclature is based on the tendency of anions to travel toward anodes, and of cations to travel toward cathodes. So, anion (ἀνιόν) and cation (κατιόν) mean "(a thing) going up" and "(a thing) going down", respectively, and anode, ἄνοδος, and cathode, κάθοδος, mean "a going up" and "a going down", respectively, from ὁδός, "way".

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