Ion

An ion (/ˈaɪ.ɒn, -ən/)^[1] is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convention. The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons.

A cation is a positively charged ion with fewer electrons than protons^[2] (e.g. K⁺ (potassium ion)) while an anion is a negatively charged ion with more electrons than protons^[3] (e.g. Cl⁻ (chloride ion) and OH⁻ (hydroxide ion)). Opposite electric charges are pulled towards one another by electrostatic force, so cations and anions attract each other and readily form ionic compounds. Ions consisting of only a single atom are termed monatomic ions, atomic ions or simple ions, while ions consisting of two or more atoms are termed polyatomic ions or molecular ions.

If only a + or − is present, it indicates a +1 or −1 charge, as seen in Na⁺ (sodium ion) and F⁻
(fluoride ion). To indicate a more severe charge, the number of additional or missing electrons is supplied, as seen in O²⁻
₂ (peroxide, negatively charged, polyatomic) and He²⁺ (alpha particle, positively charged, monatomic).^[4]

In the case of physical ionization in a fluid (gas or liquid), "ion pairs" are created by spontaneous molecule collisions, where each generated pair consists of a free electron and a positive ion.^[5] Ions are also created by chemical interactions, such as the dissolution of a salt in liquids, or by other means, such as passing a direct current through a conducting solution, dissolving an anode via ionization.