Transition metals form coloured compounds due to the presence of d-electrons in their outermost energy level. When light falls on these compounds, the electrons get excited and move from lower energy levels to higher energy levels. This excitation causes the absorption of certain colours of light, and the reflected colours are seen by our eyes, resulting in the characteristic colours of these compounds.

Example: Copper compounds

Copper compounds are a classic example of coloured transition metal compounds. Copper(II) sulfate is a blue compound, while copper(II) chloride is green. Copper(II) oxide is black, while copper(I) oxide is red.

Explanation

Copper has an atomic number of 29, which means it has 29 electrons. The electron configuration of copper is 1s2 2s2 2p6 3s2 3p6 4s1 3d10. The outermost energy level of copper has only one electron, which is in the 4s orbital. However, copper has ten electrons in the 3d orbital, which is one less than the full shell of 18. This makes copper a transition metal.

When copper forms compounds, the 4s electron is lost first, followed by the 3d electrons. Copper(II) compounds such as copper(II) sulfate have lost both the 4s and 3d electrons, leaving behind an empty 3d orbital. When light falls on copper(II) sulfate, the electrons in the 3d orbital get excited and move to higher energy levels. This excitation causes the absorption of red and green light, leaving behind blue light that is seen by our eyes.

Similarly, copper(I) oxide has lost only the 4s electron, leaving behind a partially filled 3d orbital. When light falls on copper(I) oxide, the electrons in the 3d orbital get excited and move to higher energy levels. This excitation causes the absorption of blue and green light, leaving behind red light that is seen by our eyes.

Conclusion

In conclusion, transition metals form coloured compounds due to the presence of d-electrons in their outermost energy level. When light falls on these compounds, the electrons get excited and move to higher energy levels, causing the absorption of certain colours of light and resulting in the characteristic colours of these compounds. Copper compounds are a classic example of coloured transition metal compounds.

Due to d-d transition..
Reason = electron present in lower energy d orbital in these complexes can absorb visible light and get excited to higher energy d-orbital..