Some elements like plutonium have multiple oxidation states. We tell them apart by writing the oxidation number with a roman numeral.

Periodic table with oxidation states

The oxidation state (or oxidation number) of an element is used to predict what sort of chemical compounds form.

For example, if you put hydrogen and oxygen together, they form H₂O, and not H₃O because the oxidation number of oxygen is −2, which means that we need two +1 hydrogens for the numbers to balance.

You can look up the oxidation state(s) of an element on many periodic tables. They are usually between −2 and +3.