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The transition metals form colored ions, complexes, and compounds in aqueous solution. The characteristic colors are helpful when performing a qualitative analysis to identify the composition of a sample. The colors also reflect interesting chemistry that occurs in transition metals.
Transition Metals and Colored Complexes
A transition metal is one that forms stable ions that have incompletely filled d orbitals. By this definition, technically not all of the d block elements of the periodic table are transition metals. For example, zinc and scandium aren't transition metals by this definition because Zn2+ has a full d level, while Sc3+ has no d electrons.
A typical transition metal has more than one possible oxidation state because it has a partially filled d orbital. When transition metals bond to one more neutral or negatively charged nonmetal species (ligands), they form what are called transition metal complexes. Another way to look at a complex ion is as a chemical species with a metal ion at the center and other ions or molecules surrounding it. The ligand attaches to the central ion by dative covalent or coordinate bond. Examples of common ligands include water, chloride ions, and ammonia.
When a complex forms, the shape of the d orbital changes because some are nearer the ligand than others: Some d orbitals move into a higher energy state than before, while others move to a lower energy state. This forms an energy gap. Electrons can absorb a photon of light and move from a lower energy state into a higher state. The wavelength of the photon that is absorbed depends on the size of the energy gap. (This is why splitting of s and p orbitals, while it occurs, does not produce colored complexes. Those gaps would absorb ultraviolet light and not affect the color in the visible spectrum.)
Unabsorbed wavelengths of light pass through a complex. Some light is also reflected back from a molecule. The combination of absorption, reflection, and transmission results in the apparent colors of the complexes.
Transition Metals May have More Than One Color
Different elements may produce different colors from each other. Also, different charges of one transition metal can result in different colors. Another factor is the chemical composition of the ligand. The same charge on a metal ion may produce a different color depending on the ligand it binds.
Color of Transition Metal Ions in Aqueous Solution
The colors of a transition metal ion depend on its conditions in a chemical solution, but some colors are good to know (especially if you're taking AP Chemistry):
Transition Metal Ion
brown to yellow
A related phenomenon is the emission spectra of transition metal salts, used to identify them in the flame test.