Ionization of a Gas in Thermal Equilibrium

The amount of ionization that could be expected by a gas existing in a thermal equilibrium is predicted by the Saha Equation

$$\frac{n\_i}{n\_n}\approx 2.4\times 10^{21}\frac{T^{3/2}}{n\_i}{e}^{-U\_i/KT}$$

With $n\_i\text{and}n\_n$ being the density (amt. per $m^3$) of ionized and of neutral atoms, respectively. $T$ is the gas temperature in °K, K is Boltzmann’s constant, and $U\_i$ is the ionization energy of the gas (The amount of joules required to remove the outermost electron from an atom).