How can you tell when Cu (copper) is Copper II or I?

Question

): My book doesn't explain it and it messes me up when I need the ionized state/oxidation state

is it Cu(II) when it's CuBr2?

Answer 1

you know that Br's charge is -1, so times that charge by 2 and you get -2. A charge of positive 2 is what balances it out, so its Cu (II).