What is the oxidation number of Cr in Cr2O7 2-?

To find the oxidation number of Cr in Cr2O7 2-, follow these steps:

1. Identify the charge of the entire compound. In this case, it's 2-.

2. Identify the charge of the oxygen atoms. Oxygen usually has an oxidation number of -2.

3. Multiply the oxidation number of oxygen by the number of oxygen atoms in the compound. In this case, it's -2 * 7 = -14.

4. The sum of the oxidation numbers in a compound is equal to the charge of the entire compound. So, the sum of the oxidation numbers of the two Cr atoms and the seven O atoms is -2 (the charge of the compound).

5. Set up the equation: 2(Cr) + (-14) = -2.

6. Solve for Cr: 2(Cr) = -2 + 14 = 12.

7. Divide by 2 to find the oxidation number of one Cr atom: Cr = 12 / 2 = +6.

So, the oxidation number of Cr in Cr2O7 2- is +6.