In my chemistry course, the addition of the “per” prefix generally means that the oxoanion of relevance will have one more oxygen than the “base” ion.
e.g. chlorate = $\ce{ClO3-}$, perchlorate = $\ce{ClO4-}$
However, permanganate is $\ce{MnO4-}$ and manganate is $\ce{MnO4^2-}$. The charges are different but there are still four oxygen atoms in each, so it seems to violate the convention. Any thoughts?
They ran out of names, maybe?
Permanganate does have more oxygen in the sense that it could formally be considered a possible product of adding oxygen to manganate:
$\ce{4MnO4^{2-} + O2 + 2H2O -> 4 MnO4^- + 4 OH^-}$
This theoretical reaction goes along with manganese having oxidation state +7 in permanganate versus only +6 in manganate. We may say that "per-" simply means higher oxidation state.
The general convention does not follow the number of oxygen atoms, but the order of oxidation states of the central atom.
For the increasing order of oxidation states, there are these prefixes/suffixes:
The naming starts with -ate (-ic), Then -ite ( -ous ) is added if there are 2 oxidation states. Which of the other two is added as the third one depends on the oxidation state of the least common one.
