The electronic configuration of copper is [Ar]3d 104s 1. If an electron is lost, an electron is lost in the S orbit, and a monovalent copper ion is formed. Two electrons are lost, one electron is lost in the S orbit, and one electron is lost in the D orbit, forming divalent copper ions. This is mainly because electrons occupy the lowest energy orbit possible.

Copper ion is obtained by losing two electrons in the outermost layer of copper atom, showing a positive valence of 2, which is recorded as Cu2+ and blue.

Schematic diagram of copper ion structure

Usually, copper ion Cu2+ actually exists in the form of hydrated ion [Cu(H2O)4]2+ in aqueous solution, and hydrated copper ion is blue, so most of our common copper salt solutions are blue.