Finding Oxidation Number of Perchlorate Ion

[Spencer T]

How do we know the oxidation number of the four surrounding oxygen ions of a central perchlorate ion? Once we determine the oxidation number of the oxygen ions, how does the molecule's net charge of -1 assist in the calculation of the perchlorate's oxidation number?

Thanks for the help

[Lucas Vu 2G]

Oxygen tends to have an oxidation of -2 in most compounds. With this number, we know that the chlorine's oxidation number has to be +7 because the overall charge is -1.