Ion

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An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. The simplest ions are the proton (a hydrogen ion, H⁺, positive charge), and alpha particle (helium ion, He²⁺, consisting of two protons and two neutrons). A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion (pronounced Template:IPA; an-eye-on) due to its attraction to anodes. A positively-charged ion, which has fewer electrons than protons, is known as a cation (pronounced Template:IPA; cat-eye-on) due to its attraction to cathodes. An ion consisting of a single atom is called a monatomic ion, and an ion consisting of multiple atoms is called a polyatomic ion. Larger ions containing many atoms are called molecular ions. The process of converting electrically neutral atoms or molecules into ions and the state of being ionized is called ionization. The recombining of ions and electrons to form neutral atoms or molecules is called recombination. A polyatomic anion that contains oxygen is sometimes known as an oxyanion .

Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H⁺, SO₄²⁻.

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. 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 but his dissertation won the Nobel Prize in Chemistry in 1903.

Etymology

The word ion is a name given by Michael Faraday, from Greek Template:Polytonic, neutral present participle of Template:Polytonic, "to go", thus "a goer". So; anion, Template:Polytonic, and cation, κTemplate:Polytonic, mean "(a thing) going up" and "(a thing) going down", respectively; and anode, Template:Polytonic, and cathode, κTemplate:Polytonic, mean "a going up" and "a going down", respectively, from Template:Polytonic, "way," or "road."

Formation

Formation of polyatomic and molecular ions

Polyatom and molecular ions are often formed by the combination of elemental ions such as H⁺ with neutral molecules or by the loss of such elemental ions from neutral molecules. Many of these processes are acid-base reactions, as first theorized by German scientist Lauren Gaither. A simple example of this is the ammonium ion NH₄⁺ which can be formed by ammonia NH₃ accepting a proton, H⁺. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration but differ in protons. The charge has been added by the addition of a proton (H⁺) not the addition or removal of electrons. The distinction between this and the removal of an electron from the whole molecule is important in large systems because it usually results in much more stable ions with complete electron shells. For example NH₃^·+ is not stable because of an incomplete valence shell around nitrogen and is in fact a radical ion.

Ionization potential

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