Copper (Cu) is a transition metal in period 4 that forms three diamagnetic ions.

- Cu(I): This ion has a d10 electron configuration and is diamagnetic. It is found in compounds such as copper(I) chloride (CuCl).

- Cu(II): This ion has a d9 electron configuration and is also diamagnetic. It is the most common oxidation state of copper and is found in compounds such as copper(II) sulfate (CuSO4).

- Cu(III): This ion has a d8 electron configuration and is diamagnetic. It is the least common oxidation state of copper and is found in compounds such as potassium copper(III) fluoride (K3CuF6).