Ion

     From Wikipedia, the free encyclopedia.

An ion is, generally, an electrically charged atom or molecule. Negatively charged ions are known as anions (which are attracted to anodes) while positively charged ions are known as cations (which are attracted to cathodes). (Cation is pronounced "cat eye on", not "kay shun".)

Scientific fields

In chemistry, an ion is an electrically charged molecule or atom that has gained or lost electrons from its normal complement in a process known as ionization.

In physics, completely ionized atomic nuclei, such as those in alpha radiation, are more commonly called 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.

History

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.

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	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.

Greeks

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).

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

Other

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

External links

• Ion Window manager