Ion

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In chemistry, a molecule or atom that is electrically charged is called an ion. It is called this because it has got more electrons or lost some electrons (compared to how many it normally has). The process of getting them and losing them is called ionization.

Many ions are colourless. Elements in the main groups in the Periodic Table form colourless ions. Some ions are coloured. The transition metal usually form coloured ions.

In physics, atomic nuclei that have been completely ionized are called charged particles. These are ones in alpha radiation.

Ionization happens by applying high energy to atoms. This is done using electrical voltage or by high-energy radiation.

An ionized gas is called plasma.

A simple ion is formed from a single atom.

Polyatomic ions are formed from a group of atoms. Polyatomic ions usually consist of non-metals only, but some consist of a metal and a non-metal.

Positive ions are called cations. They are attracted to cathodes (negatively charged electrodes). (Cation is pronounced "cat eye on", not "kay shun".) All simple metal ions are cations.

Negative ions are called anions. They are attracted to anodes (positively charged electrodes). All simple non-metal ions (except H⁺) are anions (except NH4⁺).

Transition metals are rather unusual - the same metal can form more than one simple cation with different charges.

Ions with 4+ or 4- are uncommon.

Michael Faraday was the first person to write a theory about ions. He did this in 1830. In his theory, he said what the portions of molecules were like that moved to anions or cations. Svante August Arrhenius said how this happened. He wrote this in his doctoral dissertation in 1884. Arrhenius was studying at the University of Uppsala. The university did not accept his theory at first (he only just passed his degree). But in 1903, he won the Nobel Prize in Chemistry in 1903 for the same idea.

In Greek ion is like the word "go". "Anion" and "cation" mean "up-goer" and "down-goer". "Anode" and "cathode" are "way up" and "way down".

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