Copper, with its atomic number 29, typically has a valency of either +1 or +2, corresponding to the loss of one or two electrons, respectively. However, there are compounds of copper that can exhibit a dark blue color due to the presence of five valence electrons. One such compound is copper(II) sulfate (CuSO4), which contains copper in its +2 oxidation state.
When copper(II) sulfate is hydrated (CuSO4·xH2O), it forms blue crystals or a blue aqueous solution. This compound is commonly known as blue vitriol or blue copper sulfate. The blue color arises from the absorption of certain wavelengths of light by the copper ions in the compound.
To produce dark blue copper with five valence electrons, you would typically start with a compound containing copper in a lower oxidation state and then oxidize it. One possible reaction is as follows:
2CuCl (copper(I) chloride) + Cl2 (chlorine gas) → 2CuCl2 (copper(II) chloride)
In this reaction, copper(I) chloride reacts with chlorine gas to form copper(II) chloride. The copper(I) chloride compound has copper in the +1 oxidation state, while copper(II) chloride has copper in the +2 oxidation state. Copper(II) chloride is a dark blue compound.
It's worth noting that in such reactions, the oxidation state and valence electron count of copper change, but the number of valence electrons remains the same. Copper always has 29 electrons, with two electrons in the inner shell and the remaining 27 in the outer shells.