Question

The salt of Sc³⁺ ion is colourless and the salt of Mn³⁺ ion is coloured. Explain.

[Z of Sc = 21and Z of Mn = 25]

Answer 1

Sc (Z = 21) → Electronic configuration of Sc³⁺:

Sc = [Ar] 3d¹ 4s²

Sc³⁺ = [Ar] 3d⁰

Since Sc³⁺ has no electrons in the 3d orbitals (3d⁰ configuration), no d-d electronic transition is possible. Therefore, its salts are colourless.

Mn (Z = 25) → Electronic configuration of Mn³⁺:

Mn = [Ar] 3d⁵ 4s²

Mn³⁺ = [Ar] 3d⁴

Mn³⁺ has four electrons in 3d orbitals (3d⁴ configuration). d–d transitions are possible, so it absorbs visible light and hence its salts are coloured.

Therefore, Sc³⁺ salts are colourless due to the absence of d-electrons, whereas Mn³⁺ salts are coloured due to the presence of partially filled d-orbitals.