You go by the oxidation numbers of potassium and oxygen.
You're actually looking for oxidation number, not charge.
The oxidation state of chromium in potassium dichromate,
- the oxidation states of potassium and oxygen;
- thefact that the compound is neutral.
Potassium is a group 1 metal, which means that its oxidation state is always
So, start by writing down what you know
The compound is neutral, which means that the oxidation states of all atoms that make up the compound must add up to give zero.
Since you have 2 potassium, 2 chromine, and 7 oxygen atoms, you'll get
The oxidation state of chromium in potassium dichromate is indeed