In this example question, it says give the formal or systematic name for the following polyatomic ion PL33-. All right, so it has phosphorus within it. Looking back up, you know that phosphorus was one of our Tetra oxides and it came in the form of PO43-. Its name was Foss fate.
We just learned that if I decrease the number of oxygens by just one oxygen, it'll give me a new polyatomic ion. So reducing it by one oxygen gives me this PO33-. And remember what that does is it changes the eight ending to 8. So this would represent our phosphite ion.
So it all hinges on the fact that you remember your trioxides and your tetraoxides, and then from there changing the number of oxygen changes the ending of your polyatomic ion.