state of ionization

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. The term state of ionization (or, more briefly, just ionization) is used to characterize this surplus or deficit, such as +2 for "missing two electrons". In such a case, fully ionized means missing all the electrons, and ground state means missing none. In astrophysics, the following terminology is often used: the chemical symbol followed by I means neutral, II means singly ionized, III for doubly ionized, etc., e.g., CI, CII, CIII for carbon at these three states of ionization. The more common symbol in other sciences is (e.g., for hydrogen), H⁺ for singly ionized, H⁺⁺ for doubly ionized, etc.

In context, state of ionization (and the term fully ionized) may be used to mean the degree of ionization, referring to a number of atoms, characterizing how many are ions and how many are at each state. For a collection of atoms/molecules, e.g., a gas, the percentages of the gas at each state of ionization is of interest, because of its relation to temperature and to the production of the spectral lines, allowing the study of spectral lines to be used to determine temperature, through the Saha equation.

The term ionization has been used metaphorically in the study of multiple star systems (e.g., binary stars), to indicate the escape of one of the companions.