Ion (chemistry)

An ion (/ˈaɪən, -ɒn/) is a an atom or molecule that has a non-zero net electrical charge (its total number of electrons is not equal to its total number of protons). A cation is a positively-charged ion, while an anion is negatively charged. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds, such as salts.

Ions can be created by chemical means, such as the dissolution of a salt into water, or by physical means, such as passing a direct current through a conducting solution, which will dissolve the anode via ionization .

Ions consisting of only a single atom are atomic or monatomic ions. If they consist of two or more atoms, then they are called molecular ions or polyatomic ions.

In the case of physical ionization of a medium such as a gas, what are known as "ion pairs" are created by ion impact, and each pair consists of a free electron and a positive ion.

The word ion comes from the Greek word ἰόν, ion, "going", the present participle of ἰέναι, ienai, "to go". This term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. Faraday did not know the nature of these species, but he knew that since metals dissolved into and entered a solution at one electrode and new metal came forth from a solution at the other electrode; that some kind of substance has moved through the solution in a current. This conveys matter from one place to the other.

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