I am reading wikipedia page on Molybdenum bronze and they mention this:

Blue bronzes, with limiting composition $\ce{A_{0.3}MoO3}$ that is, $\ce{A_3Mo10O30}$.

I used to see these fractional numbers (e.g. 0.3) and always wondered whether technically speaking they are correct or not. I understand that, as the wikipedia page says, they are equivalent to the 'full' numbers (e.g. 3) but still I am not sure whether in literature one of them is preferred or more common than the other?


These are called non-stoichiometric compounds or berthollides. Because the ratio of the fractional component is not fixed, but can vary over a small but continuous range, it would be pointless to try to express the formula as least common multiples (LCM).

For example, iron(II) sulfide can have a ratio from about $\ce{Fe_{$0.8$}S}$ to $\ce{FeS}$. So in LCM terms, the formula varies continually from $\ce{Fe4S5}$ through $\ce{FeS}$, including $\ce{Fe9S10}$, $\ce{Fe19S20}$ and so forth, all with similar physical and chemical properties.


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