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Coloured compounds in transition-series metal compounds is generally due to electronic transition of two types: charge transfer transition, or metal-to ligand charge transfer. In charge transfer transitions, an electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge transfer. These occur when the metal is typically involved in a high oxidation state. A metal-to-ligand charge transfer occurs when the metal involved is in a low oxidation state, and the ligand is easily reduced. These two charge transition states are what are responsible for the production of colours among the transition metals on the periodic table.
Most of the compounds of transition elements are coloured. The colour of these compounds can often be related to incompletely filled d- orbitals on the transition metal ion.
When light hits a substance, part is absorbed, part is transmitted (if the substance is transparent) and part may be reflected.
If the incident radiation is absorbed then the substance looks black. If all the incident radiation is reflected then the substance appears white. if only a very small proportion of the incident white light is absorbed and if all the radiations in the visible region of the spectrum are transmitted equally, then the substance will appear colourless.
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