Chromate

Chromate is an ion that contains one chromium atom (in its +6 oxidation state) and four oxide atoms. Its formula is CrO4. Its overall charge is -2. Dichromate is similar, but it contains two chromium atoms and seven oxide atoms (therefore, Cr2O7).
Chromate and dichromate are strongly oxidizing in acidic conditions, but weakly oxidizing in basic conditions. Chromates are generally bright yellow, while dichromates are generally orange or red. They can be made by reaction of chromium(III) oxide and a metal oxide in a basic environment. They can be destroyed by reaction with reducing agents. The oxygen in the air boosts the chromium atom's oxidation state from +3 to +6. 
Chromates and dichromates are harmful when powdered because the dust is carcinogenic (it can cause cancer). This can be avoided by reducing chromate with something like iron(II) sulfate.