Most compounds of metals and non metals do not exhibit color. For example, sodium chloride is a white crystalline solid, much like magnesium oxide. But complexes such as Copper oxide and Iron oxide are colored compounds. To understand this first we must understand how color is created from compounds.
In general chemicals are colored as electron shells have different energies. Electron shells can be treated as energy levels with those that are further from the nucleus being higher in energy. All atoms are capable of absorbing energy and releasing this energy again. This happens when energy is absorbed by an atom or compound and an electron in the shells is excited. When it is excited it moves from a lower energy level/shell to a higher energy level/shell. When it comes back down from this higher energy level/shell it has to release the energy that it absorbed. This happens when the electron comes back down to its original energy level releasing the extra energy as a photon.
A photon is a particle that can also be treated as a wave and carries energy usually as light. The wavelength and frequency of the photon that is emitted can provide information on the amount of energy that was absorbed by the electron to cause it to be excited. The equation E=hf relates the f - frequency of the colour to h - Planck’s constant and the E energy of the electron. We can measure the frequency or wavelength of light from an atom or compound and relate this to the energy of the compound or atoms electron that was excited.
In transition metals the D block is easily split, the influence of ligands, and the coordination number of the complex causes the D block of electrons to be split into two. This creates two small energy levels and as the D block elements are often unfilled this means there is space in the energy levels of the D block for electrons to be excited from the lower D block energy level to the higher energy level and when they come back down they emit photons of wavelengths in the visible region of the spectrum.
For example copper sulphate has a copper 2+ ion which has the electron configuration (Ar) 3d9 having 9 electrons in the d block. This means there is space in the split d orbitals for an electron to be excited into the upper d block energy levels when it is split and for it to then come back down to its original energy level emitting a photon. Where as zinc whilst it is in the d block of metals it has a full d block. this means even though the d block splits into two energy levels as it has a full number of electrons, 3d10, this means no electrons can be promoted to a higher energy level as there is no space.
The reason the compounds is colored is when the electron that was excited into the higher energy level comes back down emits a photon. The wavelength of the photon will depend on the light which is absorbed. As seen in the diagram a solution of copper sulphate absorbs light from the red end of the spectrum of visible light and the photon that is released as a result is given off in the wavelength of blue light. The wavelength and frequency of the light that is emitted is effected by how big the energy gap is in the D block that has been split. The magnitude of this splitting of the d block orbitals is determined by the ligands, metal ion charge and the coordination number of the complex. The bigger the gap the higher the frequency of energy released as a photon.
The shell is the path that electrons follow outside the nucleus. Shells can be considered as energy levels and the further away from the nucleus the higher in energy.
Energy levels is the term used when treating electrons as waves/particles we can treat them as energy levels. The sub orbitals s p d and f are in ascending energy levels with electrons further away from the nucleus being in higher energy levels
The nucleus is the term given to the centre of the atom comprising of the proton and neutron
Excited is a term used to describe when an electron absorbs energy to promote itself to a higher energy level
A photo is a particle/wave of energy that is emitted from complexes when an electron that was promoted in energy levels comes back down to the energy level it started in
Wavelength is a measure of the peak to peak of a wave. The wavelength of light differs in the visible section from red to purple measured in nanometers nm
Frequency is the number of waves per second. Measured in Hertz, the higher a frequency the more energy emitted
The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus
D block metals is a term given to metals in the transition metals section of the periodic table due to the d-orbital sub shells
Visible region is a term used to describe the section of the electromagnetic spectrum where colour can be seen
The electron configuration is the numerical layout of electrons in the orbitals around the atom. Can be done simply for example lithium has 3 electrons and has 2 in the first shell (as this is the maximum) and 1 in the second shell giving it the configuration 2,1.