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Iron in steel, copper in wires, titanium in hip replacements, platinum in catalytic converters, chromium in shiny taps, gold in jewellery. The transition metals — the broad block of elements in the middle of the periodic table, groups 3 to 12 — are the working metals of modern civilization.
Where they are
The transition metals fill the d-block: four rows of ten elements, from scandium and titanium in period 4 to the superheavy elements in period 7. As you move across a row, electrons are being added to an inner d subshell (3d in period 4, 4d in period 5, 5d in period 6).
IUPAC defines a transition element as one whose atom has a partly filled d subshell, or which can form ions with a partly filled d subshell. By that strict definition, zinc, cadmium and mercury (group 12), whose d subshells are always full, aren’t true transition metals — though they’re usually shown in the block and often discussed alongside them.
The characteristic properties
Transition metals share a set of features that main-group metals mostly don’t.
1. Variable oxidation states
Because the 4s and 3d electrons are close in energy, transition metals can lose different numbers of electrons. Iron forms Fe²⁺ and Fe³⁺; copper forms Cu⁺ and Cu²⁺; manganese ranges from +2 to +7.
| Element | Common oxidation states |
|---|---|
| Titanium | +2, +3, +4 |
| Vanadium | +2, +3, +4, +5 |
| Chromium | +2, +3, +6 |
| Manganese | +2, +3, +4, +7 |
| Iron | +2, +3 |
| Cobalt | +2, +3 |
| Copper | +1, +2 |
That’s why transition metal compound names carry Roman numerals — iron(II) chloride, iron(III) chloride. See how to name ionic compounds.
2. Coloured compounds
Most transition metal compounds are coloured, while main-group compounds like sodium chloride are usually white. The colour comes from the d electrons: surrounded by other atoms or molecules, the d orbitals split into slightly different energy levels, and electrons jumping between them absorb particular colours of visible light. We see the colours that are left.
- Copper(II) sulfate solution: blue
- Nickel(II) salts: green
- Potassium permanganate: deep purple
- Potassium dichromate: orange
- Cobalt(II) chloride: pink when hydrated, blue when dry
- Iron(III) compounds: yellow-brown
Zinc compounds, with a full d subshell and no possible d–d jumps, are white — another reason zinc isn’t a “true” transition metal. Gemstones owe their colours to transition metal traces too: chromium makes rubies red and emeralds green.
3. Catalysis
Transition metals and their compounds are some of the most important catalysts, because they can switch between oxidation states and temporarily bond to reacting molecules. Iron in the Haber process, vanadium(V) oxide in sulfuric acid manufacture, nickel for hydrogenating vegetable oils, platinum, palladium and rhodium in catalytic converters. See reaction rates and catalysts.
4. Complex ions
Transition metal ions attract molecules or ions with lone pairs (water, ammonia, chloride), which bond to them to form complex ions like [Cu(H₂O)₆]²⁺ or [Cu(NH₃)₄]²⁺. Adding ammonia to blue copper sulfate solution turns it a deep royal blue as ammonia replaces water around the copper ion. Haemoglobin, which carries oxygen in your blood, is essentially a complex built around iron.
5. Typical metal properties, turned up
Transition metals are generally hard, dense, strong, and have high melting and boiling points — tungsten has the highest melting point of any metal. They conduct heat and electricity well; silver, copper and gold are the best conductors of all. See best conductor of electricity.
6. Magnetism
Iron, cobalt and nickel are ferromagnetic — they can be permanently magnetized — thanks to unpaired d electrons that line up together. See magnetic elements.
A few stars of the block
- Iron: the backbone of industry as steel; the most-used metal by far.
- Titanium: as strong as many steels but about 45% lighter, and highly corrosion-resistant; used in aircraft and medical implants.
- Copper: wiring, plumbing, and alloys like brass and bronze.
- Chromium: stainless steel (about 10.5% or more chromium) resists rust because chromium forms a thin, self-healing oxide layer.
- Zinc: galvanizing steel.
- Silver, gold and platinum: precious metals for jewellery, electronics and catalysis.
Quick answers
How many transition metals are there? Groups 3–12 contain 40 elements. Leaving out group 12 under IUPAC’s strict definition gives 36, and counts also vary with which elements a table places in group 3.
Why are transition metal compounds coloured? Electrons in partly filled d orbitals absorb specific wavelengths of visible light.
Is zinc a transition metal? By IUPAC’s strict definition, no — its d subshell is always full.
Related
The f-block elements below the table are sometimes called inner transition metals — see rare earth elements and the actinides.
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