Transition Metal Ion Colours — Chemistry Mnemonic
Target: mnemonic for transition metal ion colours chemistry
Why is this hard to memorize?
Coloured ions are a hallmark of d-block chemistry — and NEET/JEE test them as direct recall or through salt analysis problems. The colour arises from d-d electronic transitions when light is absorbed. Key associations: Cu²⁺ = blue, Fe²⁺ = green, Fe³⁺ = yellow/brown, Cr³⁺ = green, Mn²⁺ = pale pink, MnO₄⁻ = deep purple, CrO₄²⁻ = yellow, Cr₂O₇²⁻ = orange, Ni²⁺ = green, Co²⁺ = pink. Zinc and Scandium are colourless because they have d¹⁰ and d⁰ configurations respectively — no d-d transitions possible.
Classic mnemonics you should know
"Cu=Blue sky, Fe²⁺=Green grass, Fe³⁺=Yellow rust, Cr³⁺=Green leaf, Mn²⁺=Pink candy, MnO₄⁻=Purple grape"
Associate each ion with a familiar object of that colour: Cu²⁺ = blue sky, Fe²⁺ = green grass, Fe³⁺ = yellow/brown rust, Cr³⁺ = green leaf, Mn²⁺ = pink candy, MnO₄⁻ = purple grape juice.
"Cr³⁺=Green, CrO₄²⁻=Yellow, Cr₂O₇²⁻=Orange — "Green-Yellow-Orange = GYO traffic light""
Chromium species form a traffic light: Cr³⁺(green, bottom), CrO₄²⁻(yellow, middle), Cr₂O₇²⁻(orange, top). Adding acid converts yellow chromate to orange dichromate.
"Sc³⁺(d⁰), Ti⁴⁺(d⁰), Zn²⁺(d¹⁰), Cu⁺(d¹⁰) = ALL COLOURLESS because d-d transitions need partially filled d"
No d electrons (d⁰) or fully filled d (d¹⁰) = no d-d transition = colourless. This explains why ZnSO₄ solution is colourless but CuSO₄ is blue.
The complete list
- Cu²⁺ — Blue
- Fe²⁺ — Green
- Fe³⁺ — Yellow/Brown
- Cr³⁺ — Green
- Mn²⁺ — Pale Pink
- MnO₄⁻ — Deep Purple
- CrO₄²⁻ — Yellow
- Cr₂O₇²⁻ — Orange
- Ni²⁺ — Green
- Co²⁺ — Pink
- Zn²⁺ — Colourless (d¹⁰)
- Sc³⁺ — Colourless (d⁰)
Frequently asked questions
Why are transition metal compounds coloured?
Transition metal ions have partially filled d-orbitals. In a ligand field, these d-orbitals split into different energy levels. Electrons can be excited from lower to higher d-orbitals by absorbing visible light — the complementary colour of the absorbed light is what we see. This is called a d-d transition.
Why is CuSO₄ blue but CuCl colourless?
CuSO₄ contains Cu²⁺ (d⁹) — it has one unpaired d-electron that can undergo d-d transition, absorbing red/orange light and appearing blue. CuCl contains Cu⁺ (d¹⁰) — all d-orbitals are fully filled, so no d-d transition is possible, making it colourless.
How do I remember the difference between CrO₄²⁻ and Cr₂O₇²⁻ colours?
CrO₄²⁻ (chromate) is yellow — think "Chroma-yellow" like a yellow crayon. Cr₂O₇²⁻ (dichromate) is orange — "di" means two, and mixing two yellows makes it darker/orange. In acidic solution: yellow CrO₄²⁻ converts to orange Cr₂O₇²⁻. In basic solution: orange → yellow.
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