The fact that different chemical species in solution appear different colors is related to the modern theory of the atom. Energy, including electromagnetic radiation from the sun, is quantized, meaning it exists in discrete units or "packets". Similarly, atoms have different electrons in various atomic orbitals around the nucleus. The difference between the energy levels of any two atomic orbitals is a discrete number. If light of a particular wavelength corresponding to that energy difference between orbitals comes in contact with an atom, the atom will absorb that energy and the electron will move to a higher energy level. The light that is not absorbed and passes through the solution will appear as a different color than white light since some of the wavelengths of light are not being transmitted. This occurs more commonly with transition metal elements like copper (blue in solution) since they have d shell electrons available for energy absorption.