An ion (/ˈaɪən, -ɒn/) is 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.
Ions consisting of only a single atom are termed atomic or monatomic ions, while two or more atoms form molecular ions or polyatomic ions. In the case of physical ionization in a medium, such as a gas, “ion pairs” are created by ion collisions, where each generated pair consists of a free electron and a positive ion. Ions are also created by chemical interactions, such as the dissolution of a salts in liquids, or by other means, such as passing a direct current through a conducting solution, dissolving an anode via ionization.