ion

An ion is an atom or molecule that has more or less electrons than protons, as opposed to a neutral atom in which the numbers match. Because of this, the ion has an electric charge, either positive or negative, and is subject Coulomb force, i.e., in the presence of an electron, positron, or another ion, is subject to a force drawing them together (if oppositely charged) or pushing them apart (if both are positive or both are negative). An anion is a negatively charged ion, i.e., with one or more extra electrons, and a cation is a positively charged ion, with fewer electrons than protons. Some molecules are essentially ions bound together by the Coulomb force, i.e., an ionic bond (whereas others are bound through more complicated details of small-scale electromagnetism, i.e., its quantum-mechanical nature).

Sufficiently high temperatures dissociate molecules, and at a higher temperature, when electrons are being prodded away from nuclei as quickly as they can recombine, the material consists of ions and is termed plasma. By far, the majority of the baryonic (i.e., non-dark) matter in the universe is in this state, given that the interior of stars as well as much of the interstellar medium and intracluster medium is sufficiently hot.

In science, an ion is often indicated by a superscript of "+" or "-", e.g., for hydrogen (H), H⁺ is hydrogen missing its electron, and H^- has an extra electron. To specify more than one electron, the superscript is given multiple pluses or minuses, or the sign is prefixed by a number, e.g., He⁺⁺ or He²⁺. Specifically in astrophysics, Roman numeral suffixes are often used instead, an "I" suffix mean neutral, "II" meaning singly ionized (missing one electron), "III" meaning doubly ionized, i.e., missing two, the Roman numeral being one more than the missing number of electrons. Examples: HI and ionized hydrogen for hydrogen with one or zero electrons, and iron (Fe), which has 26 protons, is Fe XXVII (or Fe²⁶⁺) if it has no electrons.