Final answer:
The oxidation number of a chromium atom in K₂Cr₂O₇ is +6. This is calculated considering potassium's +1 and oxygen's -2 oxidation states, balancing to the neutral charge of the compound.
Step-by-step explanation:
The oxidation number of the chromium atom in K₂Cr₂O₇ is determined by accounting for the charges of all atoms in the compound. Oxygen generally has an oxidation number of -2. Therefore, for seven oxygen atoms, the total oxidation number is -2 × 7 = -14. Since the compound is neutral, the sum of the oxidation numbers of all the atoms must equal zero. The potassium atoms (K₂) each have an oxidation number of +1, giving a total of +2 for both. This means that the total oxidation number for the two chromium atoms must balance the remaining charge to make the compound neutral. Accordingly, the oxidation number for each chromium atom in K₂Cr₂O₇ is +6, because +6 × 2 (for two Cr atoms) + (-2 × 7) (for seven O atoms) + +2 (for two K atoms) = 0, which satisfies the condition that the sum of oxidation numbers in a neutral compound is zero. Therefore, the correct answer is D) +6.