Degree of ionization

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The degree of ionization, also called ionization yield, is the fraction of particles that are ionized (charged). In a gas or solution, it tells you how much of the material has become charged.

What ionization means: when a neutral atom or molecule loses electrons, it becomes a positively charged ion (cation). If it gains electrons, it becomes a negatively charged ion (anion). Ionization creates charge carriers and can happen in many environments. In plasmas, ions and electrons exist and recombine very quickly, so the ionization state is a dynamic balance.

Two main uses of the term α (the degree of ionization):

- In chemistry, it describes acid strength: α = number of ionized molecules divided by the total number of dissolved molecules. At a given concentration, acids are often labeled as:
- Strong acids: α above about 30%
- Weak acids: α below about 30%
- Moderate acids: in between

- In plasma physics, α is the fraction of particles that are ionized: α = ni / (ni + nn), where ni is the ion density and nn is the neutral density. It’s a dimensionless number (often shown as a percent). The terms fractional ionization and ionization fraction are also used.

Fully ionized means there are no bound electrons left; you’re left with bare nuclei. Very hot plasmas, like thermonuclear ones, and many stars have fully ionized gas, with hydrogen and helium mostly in the forms of protons (H+) and He2+.

History note: Ionized matter was first observed in Crookes tubes (1879). J. J. Thomson studied cathode rays in 1897, and Irving Langmuir named the state “plasma” in 1928.