- 2071 Magh · 2.5 marks
Cupric salts blue, cuprous colourless
Short answer
Cu²⁺ is 3d⁹, having a d-orbital vacancy that allows d–d transitions; absorption of red-orange light transmits blue, so cupric salts are blue. Cu⁺ is 3d¹⁰ with no vacant d orbital, so no d–d transition occurs and cuprous salts are colourless.
Explain the following property: Cupric salts are blue in color while cuprous salts are colorless.
- Cu²⁺ is 3d⁹: one d orbital has a vacancy (a "hole"). In [Cu(H₂O)₄]²⁺ an electron can jump from a lower d level to the half-filled upper level by absorbing red-orange light (d–d transition). The transmitted light is the complementary colour, blue. So cupric salts (CuSO₄·5H₂O) are blue.
- Cu⁺ is 3d¹⁰: all d orbitals are completely filled, so there is no vacant d orbital to accept an excited electron. No d–d transition occurs, no visible light is absorbed, and cuprous salts (CuCl) are colourless/white.
Thus the presence of an incomplete d subshell (d⁹) gives colour; the d¹⁰ configuration gives none.
✓ Answer checked by Claude Opus 5.5 (high effort)
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