A negatively charged ion, which has more electrons than it has protons, is known as an anion (ἀνά ana: Greek 'up') (pronounced /ˈænaɪən/; an-eye-on). Conversely, a positively-charged ion, which has fewer electrons than protons, is known as a cation (κατά kata: Greek 'down') (pronounced /ˈkætaɪən/; cat-eye-on).

A neutral atom contains an equal number of Z protons in the nucleus and Z electrons in the electron shell. The electrons' negative charges thus exactly cancel the protons' positive charges. In the simple view of the Free electron model, a passing electron is therefore not attracted to a neutral atom and cannot bind to it. In reality, however, the atomic electrons form a cloud into which the additional electron penetrates, thus being exposed to a net positive charge part of the time. Furthermore, the additional charge displaces the original electrons and all of the Z + 1 electrons rearrange into a new configuration

A collection of non-aqueous gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma, often called the fourth state of matter because its properties are quite different from solids, liquids, and gases. Astrophysical plasmas containing predominantly a mixture of electrons and protons, may make up as much as 1% of visible matter in the univers