Answer:
Most of the compounds of transition
elements are coloured due to the presence of unpaired electrons in d-sub shell
of transition metal ions. It can be explained with the help of crystal field
theory. For example, in an octahedral complex \[{{[Ti{{({{H}_{2}}O)}_{6}}]}^{3+}}\]
under the influence of ligands (\[{{H}_{2}}O\] molecules), all the five
degenerate d-orbitals of penultimate shell of transition metal ion \[T{{i}^{3+}}\]
split up in to two degenerate sets, one containing \[{{d}_{{{x}^{2}}-{{y}^{2}}}}\]
and \[d_{z}^{2}\] (higher energy) and the other containing \[{{d}_{xy}},\,\,{{d}_{yz}},\,{{d}_{zx}}\]
(with lower energy). When white light falls, a part of it corresponding to a
certain wavelength (yellow in this case) is absorbed. Due to small energy difference,
electronic excitations take place from one set to another (d-d transition). The
remaining colours of white light (red and blue) are transmitted and the compound
appears purple coloured.
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