What is the oxidation number of chromium in k2cr2o7? (1) +12 (3) +3(2) +2 (4) +6

(4) +6 is your answer.
Oxygen has an oxidation number of -2, and Potassium, as an alkali metal, has a oxidation number of +1. There are 7 Oxygen atoms and 2 Potassium atoms, therefore it has a change of -14 + 2 = -12. Since the oxidation number for this entire compound is 0, we know that the two Chromium atoms have oxidation number sums adding up to 12, since it has to balance out the -12. 12/2 = 6, so the Chromium atoms each have +6 as their oxidation number.