question: What must be combined with a halogen to give it a positive oxidation number?
distractor3: carbon
distractor1: nitrogen
distractor2: calcium
correct_answer: oxygen
support: Halogens (Cl, Br, I) have negative oxidation numbers when they form halide compounds. When combined with oxygen, they have positive numbers. In the chlorate ion (ClO 3 - ), the oxidation number of Cl +5, and the oxidation number of O is -2.