Explain why Sc3+(aq) is colourless, while Ni2+(aq) is green.

Colour arises due to partially filled d-orbitals. D-orbitals are split into two sub-orbitals with slightly different energies. When an electron in the lower energy d sub-orbital absorbs light, it moves up to the higher energy d-orbital. The energy gap between these two d sub-orbitals falls within the visible spectrum, which leads to colour. The electron configuration of Ni2+ is 1s22s22p63s23p6 3d8. Given that the d-orbital is full when there are ten electrons in it, the fact that Ni2+ has only eight electrons in the 3d orbital means that it has an impartially filled d-orbital, which explains why it is colourful. In Ni2+, when an electron in the lower energy 3d sub-orbital moves up to the higher energy sub-orbital, it absorbs red light, leading thus to the appearance of a green colour.Sc3+, on the other hand, has an electron configuration of 1s22s22p63s23p6 and since it does not have an impartially filled d-orbital, it is colourless.

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