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In chemistry, an electrically charged molecule or atom is known as an ion because it has gained or lost electrons from its normal complement, a process known as ionization. In physics, completely ionized atomic nuclei, such as those in alpha radiation, are more commonly referred to as charged particles. Ionization is usually performed by the application of high energy to atoms, in the form of electrical voltage or by high-energy radiation. An ionized gas is called plasma.

Negatively charged ions are known as anions (which are attracted to anodes) and positively charged ions are called cations (which are attracted to cathodes). (Cation is pronounced "cat eye on", not "kay shun".)

Ions were first theorized by Michael Faraday around 1830, to describe the portions of molecules that travel either to an anion or to a cation. 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 he won the Nobel Prize in Chemistry in 1903 for the same dissertation.

The word "ion" is from Greek ion, present participle of ienai "go", thus "a goer". "Anion" and "cation" mean "up-goer" and "down-goer", and "anode" and "cathode" are "way up" and "way down" (hodos = road, way).

For single atoms in a vacuum, there are physical constants associated with the process of ionization. The energy needed to remove electrons from an atom is called the ionization energy, or ionization potential. These terms are also used to describe ionization of molecules and solids, but the values are not constant because ionization can be affected by the local chemistry, geometry, and temperature.

Ionization energies decrease down a group of the Periodic Table, and increase left-to-right across a period. These trends are exact opposite of the atomic radius periodic trends. Electrons in smaller atoms are attracted more strongly to the nucleus, therefore the ionization energy is greater. In larger atoms, the electrons are not held as strongly so the required ionization energy is lesser.

Element First Second Third Fourth Fifth Sixth Seventh
Na 496 4560
Mg 738 1450 7730
Al 577 1816 2744 11,600
Si 786 1577 3228 4354 16,100
P 1060 1890 2905 4950 6270 21,200
S 999 2260 3375 4565 6950 8490 11,000
Cl 1256 2295 3850 5160 6560 9360 11,000
Ar 1520 2665 3945 5770 7230 8780 12,000
Successive Ionization Energies in kJ/mol

The first ionization energy is the energy required to remove one electron, the second to remove two electrons, and so on. The successive ionization energies are always greater than the previous, and a certain nth ionization energy will be significantly larger than the rest. For this reason, ions tend to form with in certain ways. 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 aluminum may exist as an Al3+ cation.

In Greek mythology, Ion was a son of Xuthus and Creusa. He founded the Ionian race and became a king of Athens.

The term is also used for an element of the Plato texts, and a Window manager.