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why does platinum has silvery shine whereas platinum black (finely powdered platinum) is black in colour although both have the same constitution of pure platinum?

Due to similar composition, both of them will have electrons in similar kind of energy states. So we can't assume this due to charge transfer. Neither is there any scope of charge transfer.

In bulk metals, electrons in the conduction band are shared, creating a "sea" of reflective electrons. A nanoparticle, though, is so small that there is no conduction "band", just discrete levels enforced by the Pauli exclusion principle. Other metals in finely divided form, such as gold or silver, are also are black or darkly colored. Traditional silver emulsion photography relies on white silver compounds turning to black nanoparticles of silver when reduced.

Nanoparticles exhibit interesting color effects due to plasmons. Finely-divided gold particles produce ruby glass, used in stained glass windows, though thin bulk gold films appear green or blue in transmitted light.