Answer:
Döbereiner singled out triads of the elements lithium, sodium and potassium as well as chlorine, bromine and iodine. He noticed that if the three members of a triad were ordered according to their atomic weights, the properties of the middle element fell in between those of the first and third elements.
Explanation:
Mass plus atomic and number
CIO4-=-1
CI=4O=-1
O has a 2- oxidation change so
CI+4(-2)=-1
CI+(-8)=-1
CI=-1+8=7
So the oxidation number of chlorine is 7 in this case
