I am going through the theory of saha equation but I am not able to understand the concept of ionisation state? What are ionization states actually?

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    $\begingroup$ I guess this question is more appropriate at physics.SE, but briefly: It's the degree to which an atom — or an ensemble of atoms — is ionized. That is, is it missing one, two, three, etc. electrons. But beware: Whereas chemists, and normal physicists I think, use the notation (with iron as an example) Fe, Fe$^{+1}$, Fe$^{+2}$, Fe$^{+3}$, etc. for neutral, singly-ionized, and doubly-ionized, triply-ionized iron, astronomers use Fe I, Fe II, Fe III, Fe IV, etc. That is, [astronomers' Roman number] = [chemists Arabic number] + 1. $\endgroup$ – pela May 18 '16 at 13:18
  • $\begingroup$ The comment by Pela is the correct answer. Astronomers needed a way to designate the neutral or un-ionized state. $\endgroup$ – TazAstroSpacial May 30 '16 at 0:04
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    $\begingroup$ @pela In fact, this goes to Chemist SE (ionizing atoms). $\endgroup$ – User123 Apr 6 '19 at 18:45

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