homework help 2826.
There is no such rule. The best way to memorize the oxidation number of an ion or radical is to know which elements or compound they are usually in partner with. Knowing the partners you will know the oxidation number.
NaCl = Cl is -1 since it is partnered with Na which is in Group 1 which is +1
##H_2CO_3## = we know H is +1 , if there where 2 H, it will be 2+ therefore ##CO_3## should be 2-
Or you could just memorized them but will not be retained in your long term memory…. or follow my advice… knowing the truth about the molecules will last you forever.