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MHT-CET Chemistry · Transition and Inner Transition Elements

Colour and Magnetism of Transition Metal Ions

A transition metal ion is coloured when it has unpaired d electrons that can be promoted from one d level to another by visible light, so d⁰ and d¹⁰ ions are colourless; the same unpaired electrons make it paramagnetic, with a spin-only moment of √(n(n+2)) Bohr magnetons, and iron, cobalt and nickel are also ferromagnetic.

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Why this matters

18 PYQs, none HARD. Four ask which ion is coloured or colourless (Sc³⁺, Zn²⁺ and Cu⁺ are the colourless ones); fourteen are magnetism — the spin-only value for Cu²⁺ (1.73 BM, twice), Zn²⁺ (0) and Cr²⁺ (4.90 BM), the ion with the highest or lowest moment, the ion with none, the pair with equal unpaired electrons, the formula itself, and the metal that is not ferromagnetic. Two cards.

Concept 1 of 2

Coloured and Colourless Ions

Intuition

Colour comes from a d–d transition: the ion absorbs part of visible light to lift an electron to a higher d level, and we see the rest. That needs at least one electron in the d subshell AND a vacancy to move into. A d⁰ ion (Sc³⁺, Ti⁴⁺) has nothing to promote; a d¹⁰ ion (Zn²⁺, Cu⁺) has nowhere to put it. Both are colourless. Anything in between — Ti³⁺ d¹, V³⁺ d², Cr³⁺ d³, Fe²⁺ d⁶, Cu²⁺ d⁹ — is coloured.

Definition

  • Colourless: d⁰ — Sc3+\text{Sc}^{3+}, Ti4+\text{Ti}^{4+}; d¹⁰ — Zn2+\text{Zn}^{2+}, Cu+\text{Cu}^{+}, Cd2+\text{Cd}^{2+}.
  • Coloured (d¹–d⁹): Ti3+\text{Ti}^{3+} purple, V3+\text{V}^{3+} green, Cr3+\text{Cr}^{3+} violet, Mn2+\text{Mn}^{2+} pale pink, Fe2+\text{Fe}^{2+} pale green, Fe3+\text{Fe}^{3+} yellow, Co2+\text{Co}^{2+} pink, Ni2+\text{Ni}^{2+} green, Cu2+\text{Cu}^{2+} blue.

Colour condition

coloured  ⟺  1≤nd≤9;d0, d10⇒colourless\text{coloured} \iff 1 \le n_d \le 9;\quad d^0,\ d^{10} \Rightarrow \text{colourless}

Worked example

Which of Ti⁴⁺, Ni²⁺, Cu⁺ and Sc³⁺ gives a coloured solution?
Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 1Transition and Inner Transition ElementsEASY
Which among the following cations produces colourless aqueous solution in their respective oxidation state?

[Q66 · 14th May Shift 1 · 2024]

Treating copper as always coloured

Cu²⁺ (d⁹) is blue, but Cu⁺ (d¹⁰) is colourless. Count the d electrons of the ION, not the element.
Drill 3 more on coloured and colourless ions

Concept 2 of 2

Unpaired Electrons and the Spin-Only Magnetic Moment

Intuition

Each unpaired electron adds to the magnetic moment; paired electrons cancel. Count the unpaired electrons n from the ion's d configuration (n for d¹–d⁵, 10 − n for d⁶–d¹⁰), then μ = √(n(n+2)) BM. The largest moment among 3d ions belongs to d⁵ (Mn²⁺, Fe³⁺: five unpaired, 5.92 BM); d⁰ and d¹⁰ ions (Sc³⁺, Zn²⁺) have none. Ions with the same d count have the same moment, whatever the element. Iron, cobalt and nickel go further — ferromagnetic, keeping their magnetism without a field; chromium does not.

Definition

  • μ=n(n+2)\mu = \sqrt{n(n+2)} BM: n = 0 → 0; 1 → 1.73; 2 → 2.83; 3 → 3.87; 4 → 4.90; 5 → 5.92.
  • Unpaired: Ti3+\text{Ti}^{3+} 1, V3+\text{V}^{3+} 2, Cr3+\text{Cr}^{3+} 3, Cr2+\text{Cr}^{2+} 4, Mn2+\text{Mn}^{2+} 5, Fe3+\text{Fe}^{3+} 5, Fe2+\text{Fe}^{2+} 4, Co2+\text{Co}^{2+} 3, Ni2+\text{Ni}^{2+} 2, Cu2+\text{Cu}^{2+} 1, Sc3+\text{Sc}^{3+} and Zn2+\text{Zn}^{2+} 0.
  • Same number of unpaired electrons: Fe3+\text{Fe}^{3+} and Mn2+\text{Mn}^{2+} (both d⁵).
  • Ferromagnetic: Fe, Co, Ni. Cr is not.

Spin-only magnetic moment

μ=n(n+2) BM\mu = \sqrt{n(n+2)}\ \text{BM}

Worked example

Find the spin-only moment of Fe²⁺ (Z = 26).
Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 2Transition and Inner Transition ElementsMODERATE
Which element from following in +2 state exhibits highest magnetic moment?

[Q68 · 9th May Shift 2 · 2023]

Counting all the d electrons as unpaired

Beyond d⁵ the electrons pair: Cu²⁺ (d⁹) has one unpaired electron, not nine, and Zn²⁺ (d¹⁰) has none.

Calling every paramagnetic metal ferromagnetic

Chromium has unpaired electrons and is paramagnetic, but only iron, cobalt and nickel are ferromagnetic — they stay magnetised when the field is removed.
Drill 13 more on unpaired electrons and the spin-only magnetic moment

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Watch out for (3)

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