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, for it is attracted to anodes, and a positively charged ion, which has lost one or more electrons, is known as a cation (pronounced cat eye on), for it is attracted to cathodes. The process of producing ions is called ionization.

History

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. Hower, 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.

Analysis

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	4,560
Mg	738	1,450	7,730
Al	577	1,816	2,744	11,600
Si	786	1,577	3,228	4,354	16,100
P	1,060	1,890	2,905	4,950	6,270	21,200
S	999	2,260	3,375	4,565	6,950	8,490	11,000
Cl	1,256	2,295	3,850	5,160	6,560	9,360	11,000
Ar	1,520	2,665	3,945	5,770	7,230	8,780	12,000
Successive Ionization Energies in kJ/mol

The first ionization energy is the energy required to remove an electron from an isolated uncharged atom, the second to remove an electron from an isolated +1 ion, 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 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 aluminium may exist as an Al³⁺ cation.

Etymology

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).