An ion 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. Ions can be created, by either chemical or physical means, via ionization.
In chemical terms, if a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation.
If an atom gains electrons, it has a net negative charge and is known as an anion.
Ions consisting of only a single atom are atomic or monatomic ions; if they consist of two or more atoms, they are molecular or polyatomic ions. Because of their electric charges, cations and anions attract each other and readily form ionic compounds, such as salts.
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.

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