Question

Why are the subscripts in a chemical formula whole numbers (for example, `N_2O_5` instead of `NO_2.5`)?

Answer 1

Because it doesn't make sense to have half an atom. `"NO"_2.5`, which we could call a "pseudo-formula", implies you have (two and a) half oxygen atoms for every molecule of "`"NO"_2.5`", which is physically impossible. So, we scale to whole numbers and rewrite as `"N"_2"O"_5` instead.

Same thing with something like `"C"_1.5"H"_4"O"_0.5`. We instead write `"C"_3"H"_8"O"`.