To determine the oxidation number of oxygen in BaO2, we can use the fact that the overall charge of a compound must be zero.
Barium (Ba) is a Group 2 element and has an oxidation state of +2. The compound BaO2 has no net charge, so the sum of the oxidation states of all the atoms must be zero. Let x be the oxidation state of oxygen in BaO2.
Therefore, we have:
(+2) + 2(x) = 0
Solving for x, we get:
x = -1
Therefore, the oxidation number of oxygen in BaO2 is -1.