An ion is an atom or molecule with an overall electrical charge.

For example, a hydrogen atom, written H, has one (positive) proton and one (negative) electron, and an overall charge of zero. Should that hydrogen atom lose that electron in a reaction, it will become a hydrogen ion with a net positive charge, written H⁺.

An oxygen atom, written O, has eight protons and eight electrons and is also electrically neutral. Oxygen atoms gain two electrons in reactions, and become oxygen ions with a net negative charge of two. This is written O^2-.

An example of a positively-charged molecular ion is ammonium, NH⁴⁺, and an example of a negatively-charged molecular ion is carbonate, CO₃^2-.