
Order of chemical symbols in chemical formula
I've searched for resources, but everything I could find led to the hill system but I was marked wrong for putting phosphorus after oxygen. I saw that the number of atoms was supposed to be ascending, but what I understood was it was for a second step in ordering of the same element like C2 is before C3. Is it something about how oxygen is always at the end of a formula or it's some sort of oxide formula?
I've never heard of an order of chemical symbols before so please don't use overly technical terms because it will go over my head, thank you :) The picture shows the correct answer, but my wrong answer had phosphorus and oxygen switched