Final answer:
In zinc dichromate (ZnCr₂O₇), the chromium (Cr) has an oxidation state of +6. In the anion NH2-, the nitrogen (N) has an oxidation state of -3.
Step-by-step explanation:
To determine the oxidation state of chromium (Cr) in zinc dichromate (ZnCr₂O₇), we need to use the fact that the sum of the oxidation states in a neutral compound is zero. Zinc (Zn) typically has an oxidation state of +2, and the dichromate ion (Cr2O₇²⁻) has a total charge of -2. The oxygen in the dichromate ion has an oxidation state of -2 for each oxygen atom. With seven oxygen atoms, the total is -14.
Therefore, the total oxidation state of the two chromium atoms must be +12 (since +12 and -14 gives the -2 charge of the ion). To find the oxidation state of a single Cr atom, we divide the +12 by 2, resulting in an oxidation state of +6 per chromium atom.
In the compound NH²⁻, the total charge is -1. Hydrogen generally has an oxidation state of +1, and since there are two hydrogens, the total is +2. Thus, the nitrogen (N) must have an oxidation state that balances the charge to -1. Therefore, nitrogen has an oxidation state of -3 in NH²⁻.