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

Because it doesn't make sense to have half an atom. ${\text{NO}}_{2.5}$, which we could call a "pseudo-formula", implies you have (two and a) half oxygen atoms for every molecule of "${\text{NO}}_{2.5}$", which is physically impossible. So, we scale to whole numbers and rewrite as ${\text{N"_2"O}}_{5}$ instead.
Same thing with something like ${\text{C"_1.5"H"_4"O}}_{0.5}$. We instead write $\text{C"_3"H"_8"O}$.