Question

Of the ions Co²⁺, Sc³⁺ and Cr³⁺, which ones will give coloured aqueous solutions and how will each of them respond to a magnetic field and why?

[Atomic number: Co = 27, Sc = 21, Cr = 24]

Answer 1

1. Co²⁺ ion: It has 3 unpaired electrons in the 3d orbitals, so it shows colour in aqueous solution due to d-d transitions. It is paramagnetic and attracted by a magnetic field because of unpaired electrons.
2. Sc³⁺ ion: Has no d-electrons (3d⁰), so it is colourless in aqueous solution. It is diamagnetic and repelled by a magnetic field because all electrons are paired.
3. Cr³⁺ ion: Has 3 unpaired electrons in the 3d orbitals, so it gives coloured aqueous solutions. It is paramagnetic and attracted by a magnetic field due to unpaired electrons.

Thus, Co²⁺ and Cr³⁺ are coloured and paramagnetic due to unpaired d-electrons, while Sc³⁺ is colourless and diamagnetic as it has no unpaired electrons