Final answer:
The colors of Cu²⁺ compounds such as CuCl₂ and CuO are influenced by ligand types, metal-ligand geometry, and electron transitions within different crystal lattices, which affect light absorption and reflection.
Step-by-step explanation:
The color of Cu²⁺ compounds depends on several factors, including the type of ligands attached to the metal ion and the metal-ligand geometry.
Although Cu²⁺ is indeed associated with a blue color, in CuCl₂ the presence of chlorine ligands and the geometry of the complex influences the d-electron configurations, leading to a green color due to the different wavelengths of light absorbed and reflected.
Similarly, in CuO, the copper ion is paired with oxygen in a 1:1 ratio resulting in a powdery black compound. This is because in solid CuO, the Cu²⁺ ions create a crystal lattice that affects the d-electron transitions differently from copper ions in a solution, thereby altering the color.
The change in color is not solely due to the presence of the copper ion but also to the surrounding atoms/molecules and their interactions, which can alter the electronic structure and light absorption characteristics of the compound.