What is the proper name of the compound Cu2SO3?

The proper name of the compound Cu₂SO₃ is Copper(I) Sulfite.

To break this down, the “Cu” refers to copper, which can exhibit multiple oxidation states. In this compound, there are two copper ions for every sulfite ion (SO₃^2-). The Roman numeral (I) indicates that each copper ion has a +1 charge. Therefore, the overall charge balance is maintained, with the two Cu⁺ ions collectively contributing a +2 charge, which balances the -2 charge of the sulfite ion.

Using proper nomenclature, we display the oxidation state of the metal ion in the name of the compound. Hence, Cu₂SO₃ is correctly named Copper(I) Sulfite.