Ion (physics)

An ion (/ˈaɪən, -ɒn/) is an atom, or a molecule, in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. An atom, or molecule, with a net positive charge is a cation. An atom, or molecule, with a net negative charge is an anion. 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 either 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 moved through the solution in a current, conveying matter from one place to the other.

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