The following transition metal ions would be coloured in aqueous solution except________?

Analyze the question: We need the ion that is colourless in aqueous solution. Colour in transition metal ions comes from d-d electronic transitions, so the ion must have partially filled d-orbitals.

Concept: A d-d transition needs at least one d electron and at least one empty or half-empty d-orbital slot to move into. Ions with an empty d subshell (d⁰) or a full one (d¹⁰) are usually colourless.

Solution: Write the electron configuration of each ion.

• Cr³⁺: Cr is [Ar] 3d⁵ 4s¹. Losing 3 electrons gives 3d³. This is partially filled, so the ion is coloured (violet/green).
• Fe³⁺: Fe is [Ar] 3d⁶ 4s². Losing 3 electrons gives 3d⁵. This is partially filled, so the ion is coloured (yellow/brown).
• Mn³⁺: Mn is [Ar] 3d⁵ 4s². Losing 3 electrons gives 3d⁴. This is partially filled, so the ion is coloured (violet/red-brown).
• Sc³⁺: Sc is [Ar] 3d¹ 4s². Losing 3 electrons gives [Ar], which is 3d⁰. There are no d electrons, so no d-d transitions are possible and the ion is colourless.

Answer: The ion that is not coloured is Sc³⁺, which is option D.

Tip: Remember the colourless ions: Sc³⁺ (d⁰), Ti⁴⁺ (d⁰), Zn²⁺ (d¹⁰) and Cu⁺ (d¹⁰). If you see one of these in an exam, it is colourless. Quick check: count the d electrons. If the count is 0 or 10, there is no colour.