Chemical formula
Non-stoichiometric chemical formulae

Chemical formulae most often use integers for each element. However, there is a class of compounds, called non-stoichiometric compounds, that cannot be represented by small integers. Such a formula might be written using decimal fractions, as in Fe O, or it might include a variable part represented by a letter, as in Fe O, where x is normally much less than 1.
punctuation:no_comma