Copper II sulfate pentahydrate [Cu H2O 4]SO4H2O is a coordination compound with copper II ion Cu2+ surrounded by water molecules. The electronic configuration of Cu2+ is [Ar]3d9, where one electron is removed from the 4s orbital and the other from the 3d orbital.The color of a coordination compound is mainly determined by the d-d transitions of the central metal ion. In the case of copper II sulfate pentahydrate, the Cu2+ ion has an incomplete d-orbital, which allows for electronic transitions within the d-orbitals when it absorbs light. These electronic transitions are responsible for the color of the compound.Copper II sulfate pentahydrate is known for its characteristic blue color. This color arises from the d-d transitions of the Cu2+ ion, which absorbs light in the red region of the visible spectrum, and the transmitted and reflected light appears blue to our eyes.