The full, readable lecture — the d-block and its 3d configuration, variable oxidation states, the colours of their ions, paramagnetism, catalysis, complexes and alloys. As you scroll, the panel on the right shows each idea through a real object you already know: a gemstone, a wardrobe, a bicycle chain, a magnet, a bottle of blue solution. In the first scene, slide the d-orbital gap and switch a sapphire to a ruby yourself.
The most beautiful fact about transition metals is that their ions are coloured. A ruby, a sapphire and an emerald are all the same idea: a transition-metal ion sitting in a crystal, splitting white light. When ligands surround the ion they split its five d-orbitals into two levels separated by a small gap, Δ. The ion absorbs the colour of light whose energy equals Δ, promoting an electron across the gap — a d-d transition — and the light that passes through is the complementary colour you see.
Sc³⁺ (3d⁰, empty) and Zn²⁺ (3d¹⁰, full) cannot make a d-d transition — one has no electron to promote, the other has no empty level to promote into — so they are colourless. Move the slider on the right to change Δ and watch the same gemstone shift from ruby red to emerald green to sapphire blue.
A transition metal is like a person with a wardrobe of outfits: one element, many oxidation states, each its own colour. This happens because the 3d and 4s electrons sit very close in energy, so both sets are available for bonding — different numbers of electrons can be lost with little energy difference.
| Metal | Common oxidation states | Example species |
|---|---|---|
| Mn | +2, +3, +4, +6, +7 | Mn²⁺ (pink) … MnO₄⁻ (+7, purple) |
| Fe | +2, +3 | Fe²⁺ (green), Fe³⁺ (yellow-brown) |
| Cr | +2, +3, +6 | Cr³⁺ (green), Cr₂O₇²⁻ (+6, orange) |
Manganese shows the widest range, +2 pale pink up to +7 in the deep-purple permanganate ion (MnO₄⁻). Sc wears only +3 and Zn only +2 — they have the smallest wardrobes, so they are the least "transition-like" of the series.
The defining feature is a partly filled d-subshell. The 3d (first transition) series runs Sc → Zn (Z = 21–30), Groups 3–12, between the s-block and the p-block. The general valence configuration is (n−1)d¹⁻¹⁰ ns¹⁻²; electrons fill 4s first, then the five 3d orbitals one at a time. There are two famous anomalies:
A half-filled (d⁵) or completely-filled (d¹⁰) d-set is unusually stable (extra exchange energy and a symmetrical charge cloud), so one 4s electron slips across into 3d.
Most transition-metal ions have unpaired d-electrons, so a magnet picks them up. The more unpaired electrons, the stronger the pull. The magnetic moment is estimated by the spin-only formula:
Mn²⁺ / Fe³⁺ (3d⁵) → n = 5 → μ ≈ 5.92 BM (strongly attracted); Zn²⁺ (3d¹⁰, all paired) → n = 0 → diamagnetic, ignored by the magnet.
Transition metals and their compounds are superb catalysts, for two reasons: their variable oxidation states let them shuttle electrons easily, and their surface adsorbs reactant molecules, weakening bonds and offering a low-energy path. A car's catalytic converter is exactly this surface chemistry, cleaning the exhaust.
| Process | Catalyst | Role |
|---|---|---|
| Haber (NH₃) | Fe | adsorbs N₂ + H₂ |
| Contact (H₂SO₄) | V₂O₅ | SO₂ → SO₃ (V cycles +5⇌+4) |
| Hydrogenation | Ni | adds H₂ across C=C |
Add ammonia to pale-blue copper sulfate and the solution turns a stunning royal blue — a complex has formed. Small, highly charged transition-metal ions with empty d-orbitals readily accept lone pairs from surrounding species.
The spoon and fork in your kitchen drawer are stainless steel — iron with chromium swapped in. A door handle or trumpet may be brass — copper with zinc.
| Alloy | Main metals | Use |
|---|---|---|
| Stainless steel | Fe + Cr (+ Ni) | cutlery, sinks, surgical tools — resists rust |
| Brass | Cu + Zn | fittings, instruments |
| Bronze | Cu + Sn | bearings, statues, coins |
An old bicycle chain rusting in the rain is the everyday face of variable oxidation states: iron metal gives up electrons to become Fe²⁺ then Fe³⁺, the orange-brown hydrated oxide we call rust. The same iron, kept clean, runs your blood (haemoglobin), your buildings (steel) and a Haber plant (catalyst).