Introduction:
The oxidation state of an element refers to the charge it would have if all the shared electrons in a compound were assigned to the more electronegative atom. Chromium can exhibit different oxidation states depending on the compound it is present in. In the case of chromate and dichromate salts, the difference in oxidation state of chromium is zero.

Chromate and Dichromate Salts:
Chromate and dichromate salts are compounds that contain the chromate ion (CrO4 2-) and dichromate ion (Cr2O7 2-), respectively. These ions are formed by the combination of chromium with oxygen and are negatively charged due to the presence of extra electrons.

Oxidation State of Chromium in Chromate and Dichromate Ions:
In the chromate ion (CrO4 2-), the oxidation state of chromium is +6. This is because oxygen is assigned an oxidation state of -2, and since there are four oxygen atoms in the chromate ion, the total oxidation state contributed by oxygen is -8. Therefore, for the overall charge of the ion to be -2, the oxidation state of chromium must be +6.

In the dichromate ion (Cr2O7 2-), the oxidation state of chromium is +6 as well. Similar to the chromate ion, each oxygen atom in the dichromate ion is assigned an oxidation state of -2, resulting in a total oxidation state of -14 contributed by oxygen. To balance the overall charge of -2, each chromium atom in the dichromate ion must have an oxidation state of +6.

Difference in Oxidation State:
The difference in oxidation state of chromium in chromate and dichromate salts is zero. Both ions have the same oxidation state of +6 for chromium. The only difference between the two ions is the number of chromium atoms present. The chromate ion contains one chromium atom, while the dichromate ion contains two chromium atoms.

Conclusion:
In chromate and dichromate salts, the oxidation state of chromium is +6. The difference in oxidation state between these two ions is zero. The oxidation state of an element depends on the compound it is present in and can vary based on its bonding and electron distribution.