The common oxidation states of copper are +1 and +2.

　　Copper(I)

　　Copper(I) is commonly referred to as cuprous copper. Cuprous chloride (CuCl), cuprous oxide (Cu₂O), and cuprous sulfide (Cu₂S) are all common monovalent copper compounds. [Cu(NH₃)₂]²⁻ is a complex ion formed by cuprous copper and ammonia. It is colorless, easily oxidized, and spontaneously undergoes disproportionation in acidic solutions to form Cu(II) and Cu.

　　Copper(II)

　　Copper(II) is the most common oxidation state of copper. It can form salts with most common anions, such as the well-known copper sulfate, which exists as a white anhydrous compound and a blue pentahydrate. Basic copper carbonate, also known as verdigris, exists in several different forms. Copper chloride and copper nitrate are also important copper salts.

　　Copper(II) can form a series of coordination compounds, such as Cu(H₂O)₄ (blue), CuCl₄ (yellow-green), and Cu(NH₃)₄ (dark blue), each with its own distinct color.

　　Common Copper Compounds

　　Copper(II) sulfate (CuSO₄), copper(II) acetate ((CH₃COO)₂Cu), copper(II) oxide (CuO) and cuprous oxide (Cu₂O), copper(II) chloride (CuCl₂) and cuprous chloride (CuCl), copper(II) nitrate (Cu(NO₃)₂), copper(II) cyanide (Cu(CN)₂), copper fatty acid salts, and copper naphthenate (C₂₂H₁₄CuO₄), among others.