The degree of ionization refers to the proportion of neutral particles, such as those in a gas or aqueous solution, that are ionized to charged particles. For electrolytes, it could be understood as a capacity of acid/base to ionize itself. A low degree of ionization is sometimes called partially ionized, and a high degree of ionization as fully ionized. Ionization refers to the process whereby an atom or molecule loses one or several electrons from its atomic orbital, or conversely gains an additional one, from an incoming free electron. In both cases, the atom or molecule is no more a neutral particle and becomes a charge carrier. If the species has lost one or several electrons, it becomes positively charged and is called a positive ion, or cation. On the contrary, if the species has gained one or several additional electrons, it becomes negatively charged and is called a negative ion, or anion. Individual free electrons and ions in a plasma have very short lives typically inferior to the microsecond, as ionization and recombination, excitation and relaxation are collective continuous processes.
Chemistry usage
The degree of dissociationα, is a way of representing the strength of an acid. It is defined as the ratio of the number of ionized molecules and the number of molecules dissolved in water. It can be represented as a decimal number or as a percentage. One can classify strong acids as those having ionization degrees above 30%, weak acids as those with α below 30%, and the rest as moderate acids, at a specified molar concentration.
Physics usage
In gases, the degree of ionization refers to the proportion of neutral particles that are ionized into charged particles: where is the ion density and the neutral density. It is a dimensionless number, most of the time expressed as a percentage. In a plasma, the electron-ion collision frequency is much greater than the electron-neutral collision frequency. Therefore, with a weak degree of ionization, the electron-ion collision frequency can equal the electron-neutral collision frequency: is the limit separating a plasma from being partially or fully ionized. The term fully ionized gas introduced by Lyman Spitzer does not mean the degree of ionization is unity, but only that the plasma is in a Coulomb-collision dominated regime, i.e. when, which can correspond to a degree of ionization as low as 0.01%. A partially or weakly ionized gas means the plasma is not dominated by Coulomb collisions, i.e. when. A particular case of fully ionizes gases are very hot thermonuclear plasmas, such as plasmas artificially produced in nuclear explosions or naturally formed in our Sun and all stars in the universe. Stars contain largely hydrogen and helium gases, that are fully ionized into electrons, protons and helium ions. Equations to characterize such very hot, fully ionized thermonuclear plasmas in the presence of stellar magnetic fields can be approximated to ideal magnetohydrodynamics laws with high magnetic Reynolds number.
History
Ionized matter was first identified in a discharge tube, and so described by Sir William Crookes in 1879. The nature of the Crookes tube "cathode ray" matter was subsequently identified by English physicist Sir J.J. Thomson in 1897, and dubbed "plasma" by Irving Langmuir in 1928, perhaps because it reminded him of a blood plasma.
