To find the correct oxidation state of Cu in CuCl2 (Copper (II) chloride), and each element in the compound, we use a few rules and some simple math.

First, since the CuCl2 doesn’t have an overall charge (like NO3- or H3O+) we could say that the total of the oxidation numbers for CuCl2 will be zero since it is a neutral compound.

We write the oxidation number (O.N.) for elements that we know and use these to figure out oxidation number for Cu.

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GENERAL RULES
Free elements have an oxidation state of zero (e.g. Na, Fe, H2, O2, S8).
In an ion the all Oxidation numbers must add up to the charge on the ion.
In a neutral compound all Oxidation Numbers must add up to zero.
Group 1 = +1
Group 2 = +2
Hydrogen with Non-Metals = +1
Hydrogen with Metals (or Boron) = -1
Fluorine = -1
Oxygen = -2 (except in H2O2 or with Fluorine)
Group 17(7A) = -1 except with Oxygen and other halogens lower in the group
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