In chemistry, a molecule or atom that is electrically charged is called an ion. An ion has more or less electrons than there are protons in an atom. The process of giving or taking electrons away 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 metals 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 giving atoms high energy to . This is done using electrical voltage or by high-energy radiation.
A simple ion is formed from a single atom.
Polyatomic ions are formed from a lot of atoms. Polyatomic ions usually consist of all non-metal atoms, but sometimes the polyatomic ion can have a metallic atom too.
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+, which is a proton) are anions (except NH4+).
Transition metals can form more than one simple cation with different charges.
Most ions have a charge of less than 4, but some can have higher charges.
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".
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