Question

What is the oxidation number of copper?

Answer 1

Hint : We know that the oxidation number of an atom in a species is the total number of electrons it gains or loses while forming a bond with another atom present in the species. We can find out the oxidation number of an atom in a species using two methods.

Complete Step By Step Answer:
There are two ways of solving this question since they have not mentioned what type of oxidation number. There are two types of oxidation numbers.
Calculated or average oxidation number and
Observed oxidation number.
Sometimes the values for both the calculated oxidation number and the observed oxidation number are the same. But they are different for some species. For such species we have to find the actual oxidation number by using the chemical structure of the species. For finding the calculated oxidation number of a particular atom in a species, we have to assume the oxidation number of all other atoms present in the species. The valence number of the atoms with their sign is taken as their assumed oxidation number.
For example, if we have to find the oxidation number of copper. The oxidation number of copper depends on its state. The oxidation number of metallic copper is zero. In these compounds, the most common oxidation number of $Cu$ is $+2.$ Less common is $+1.$ Copper can also have oxidation numbers of $+3\text{ }and\text{ }+4.$

Oxidation Number	Example
$+1$	$CuCl,C{{u}_{2}}O$ and $C{{u}_{2}}S.$
$+2$	$CuC{{l}_{2}},CuO$ and $CuS{{O}_{4}}.$
$+3$	$KCu{{O}_{2}}$ and ${{K}_{2}}Cu{{F}_{6}}.$
$+4$	$C{{s}_{2}}Cu{{F}_{6}}.$

Note :
Note that in this case we cannot calculate the oxidation state of copper using the other method since for calculating the observed oxidation number we need to know the structure of the compound and how the bonded electrons are shared among the individual atoms in the compound.