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.