Question

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

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

Answer 1

A substance appears coloured when it absorbs light in the visible region due to electronic transitions.

Sc (Z = 21) has the electronic configuration:

Sc = [Ar] 3d¹ 4s²

∴ For Sc³⁺, three electrons are removed:

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) has the electronic configuration:

Mn = [Ar] 3d⁵ 4s²

∴ For Mn³⁺, three electrons are removed:

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.