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It might be expected that the nomenclature used to describe ions in chemistry would be the same in astronomy, and it can be if modern notation is used. However, when Roman Numerals are used the situation can become confusing unless you remember which camp you are in.

In chemistry, if you talk about Iron (III) chloride you are talking about FeCl3. In this molecule the iron (Fe III ) is in the 3+ state, that is, it has lost 3 electrons and the clear notation for this is Fe3+.

In astronomy, often the Roman Numeral notation, for example Fe III is used to mean the Fe2+ state where two electrons have been lost.

Where a chemist would use Fe or Fe0 to describe neutral iron which has not lost an electron, an astronomer might use FeI ( usually when describing spectroscopic data), to mean the same thing.

So, if you see Roman Numerals to describe atoms / ions, consider the background of the author and the subject that you are reading about to determine which state is being discussed!