Copper sulfate CuSO4 is an inorganic compound that forms a blue crystalline solid. The color of this compound can be explained by its electronic configuration and crystal field theory.Copper has an electronic configuration of [Ar] 3d10 4s1. In CuSO4, copper forms a Cu2+ ion, which has an electronic configuration of [Ar] 3d9. The sulfate ion SO4 2- acts as a ligand, and the Cu2+ ion is surrounded by these ligands in an octahedral geometry.According to crystal field theory, the d orbitals of the central metal ion Cu2+ are split into two energy levels when surrounded by ligands in an octahedral geometry. The lower energy level consists of three orbitals dxy, dyz, and dxz known as t2g orbitals, and the higher energy level consists of two orbitals dx2-y2 and dz2 known as eg orbitals.In the case of Cu2+, there are nine d electrons. The first five electrons fill the t2g orbitals, and the next four electrons fill the eg orbitals. This leaves one unpaired electron in the eg orbital.The color of the compound is due to the absorption of light, which promotes the unpaired electron from the lower energy t2g orbital to the higher energy eg orbital. The energy difference between these orbitals corresponds to the energy of the absorbed light. In the case of copper sulfate, the absorbed light has a wavelength in the yellow region of the visible spectrum. Since the complementary color of yellow is blue, the transmitted and reflected light appears blue, giving copper sulfate its characteristic blue color.