The formulas for compounds called copper (I) chloride and (II) chloride are as follows:

- Copper (I) chloride: CuCl

- Copper (II) chloride: CuCl2

In copper (I) chloride, copper has a +1 oxidation state, while in copper (II) chloride, copper has a +2 oxidation state. This difference in oxidation state is reflected in the formulas of the two compounds. The Roman numerals in the names indicate the oxidation state of copper in each compound.