Where are the transition metals?
The periodic table has a wide middle section between Group 2 and Group 13. This is the d-block. The elements here are called transition metals (or B-group metals in some tables). The first row runs from scandium (Sc) to zinc (Zn): Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn. Below them are silver, gold, platinum and others.
In these atoms the new electrons go into an inner sub-shell called 3d (or 4d, 5d), not the outer shell. The outer shell (4s) usually keeps 2 electrons. That is why the transition metals are so alike across a row.
Strictly, a transition element has an incomplete d sub-shell in the atom or in one of its common ions. Zinc (3d¹⁰ in both the atom and Zn²⁺) does not fit this rule exactly, which is why its compounds are white; it is still placed in the d-block.
Transition metals compared with Group 1
| Property | Group 1 (Li, Na, K) | Transition metals (Fe, Cu, Ni, Cr) |
|---|---|---|
| Melting point | Low (Na 98 °C, K 63 °C) | High (Fe 1538 °C, Cr 1907 °C) – except mercury, a liquid |
| Density | Low (Na, K float on water) | High (Fe 7.9, Cu 9.0 g/cm³) |
| Hardness and strength | Soft: cut with a knife | Hard and strong |
| Reaction with water and oxygen | Very fast; stored under oil | Slow or none (iron rusts slowly; copper, gold barely react) |
| Ions | Only +1 | Often more than one charge (Fe²⁺/Fe³⁺) |
| Colour of compounds | White; solutions colourless | Usually coloured |
| Catalysts | No | Often yes |
Why the difference? Transition metal atoms are smaller and use more electrons (including d electrons) in metallic bonding. The bonds are stronger, so more energy is needed to melt them, and the atoms pack closely, so they are dense.
Coloured ions and more than one charge
Most transition metal compounds are coloured, in the solid and in solution. Partly filled d sub-shells absorb some colours of visible light; we see the colours that are left.
- Cu²⁺: blue (copper(II) sulfate)
- Fe²⁺: pale green; Fe³⁺: orange-brown (rust colour)
- Ni²⁺: green; Co²⁺: pink; Cr³⁺: green; MnO₄⁻: purple
Transition metals can form ions with different charges. Iron forms Fe²⁺ (iron(II) oxide, FeO) and Fe³⁺ (iron(III) oxide, Fe₂O₃). Copper forms Cu⁺ and Cu²⁺. The Roman numeral in a name tells you the charge. Group 1 metals only ever form +1 ions.
Test with sodium hydroxide
Adding sodium hydroxide solution makes coloured hydroxide precipitates: Cu²⁺ → blue, Fe²⁺ → green, Fe³⁺ → orange-brown. Chemists use this to identify the ion.
Transition metals as catalysts
A catalyst speeds up a reaction and is not used up. Transition metals and their compounds are excellent catalysts because their ions can easily change charge and their surfaces hold other molecules.
- Iron: Haber process, nitrogen + hydrogen → ammonia (for fertilisers).
- Nickel: adds hydrogen to vegetable oils to make margarine and vanaspati.
- Manganese(IV) oxide: breaks down hydrogen peroxide into water and oxygen.
- Vanadium(V) oxide: making sulfuric acid (contact process).
- Platinum, palladium, rhodium: car catalytic converters.
Electron 'slip' in chromium and copper; complex ions
Normally 4s fills before 3d. Two first-row metals break the pattern, because a half-full (d⁵) or full (d¹⁰) d sub-shell is extra stable:
- Chromium: expected 3d⁴4s², actual 3d⁵4s¹.
- Copper: expected 3d⁹4s², actual 3d¹⁰4s¹. Silver, below copper, does the same: 4d¹⁰5s¹.
One 4s electron 'slips' into 3d. When these atoms form ions, the 4s electrons are lost first: Fe (3d⁶4s²) → Fe²⁺ (3d⁶) → Fe³⁺ (3d⁵).
Complex ions (introduction)
A transition metal ion can hold several small molecules or ions around it, joined by bonds from their lone pairs. The result is a complex ion. Copper(II) in water is really [Cu(H₂O)₆]²⁺ (pale blue). Add ammonia and it becomes [Cu(NH₃)₄(H₂O)₂]²⁺, which is deep blue. Silver forms [Ag(NH₃)₂]⁺. The molecules around the ion are called ligands.
Four everyday transition metals: copper, silver, zinc, iron
- Iron (Fe): grey, strong, magnetic. Made in the blast furnace; most is turned into steel. Rusts in water and air (forms hydrated iron(III) oxide). Ions Fe²⁺ and Fe³⁺.
- Copper (Cu): reddish, an excellent conductor of electricity and heat, easy to draw into wires. Does not react with water or dilute acids; slowly turns green outdoors. Ions Cu⁺ and Cu²⁺.
- Silver (Ag): the best conductor of electricity; shiny; used in jewellery, coins and mirrors. Tarnishes black with sulfur compounds in air. Ion Ag⁺.
- Zinc (Zn): bluish-grey; reacts with dilute acids to give hydrogen. Used to coat iron (galvanising) and in brass (copper + zinc) and batteries. Only Zn²⁺; its compounds are white.
Try it: predict, then check
Before using the 3D picker, predict: which melts higher, titanium or potassium? Which is denser, silver or zinc? Then tap and check. At home: a new iron nail put into blue copper sulfate solution turns brown-coated as copper forms, and the blue fades as pale-green Fe²⁺ forms. Look at the colours of a copper vessel, steel spoon and galvanised bucket in your kitchen.
Key formulas and definitions
- Transition metals: d-block elements between Group 2 and Group 13 (Sc → Zn in row 4)
- Typical properties: high melting point, high density, hard, strong, coloured compounds, ions with different charges, catalysts
- Fe → Fe²⁺ + 2e⁻; Fe → Fe³⁺ + 3e⁻ (iron(II) and iron(III))
- Cr = [Ar] 3d⁵4s¹; Cu = [Ar] 3d¹⁰4s¹ (half-full / full d is extra stable)
- Ions lose 4s electrons first: Fe²⁺ = [Ar] 3d⁶
- Complex ion: metal ion + ligands, e.g. [Cu(H₂O)₆]²⁺, [Cu(NH₃)₄(H₂O)₂]²⁺, [Ag(NH₃)₂]⁺
Worked examples
1. A metal melts at 1455 °C, has density 8.9 g/cm³ and forms a green chloride. Is it a Group 1 metal or a transition metal?
Transition metal (it is nickel): high melting point, high density and a coloured compound.
2. Write the formulas of iron(II) chloride and iron(III) chloride.
Fe²⁺ with two Cl⁻: FeCl₂. Fe³⁺ with three Cl⁻: FeCl₃.
3. Write the electron configurations of Cu and Cu²⁺.
Cu = [Ar] 3d¹⁰4s¹. Cu²⁺ loses the 4s electron and one 3d electron: [Ar] 3d⁹.
4. Sodium hydroxide is added to a pale green solution and a green precipitate forms, which slowly turns orange-brown in air. Which ion was present?
Fe²⁺. Green Fe(OH)₂ forms; air oxidises it to orange-brown Fe(OH)₃ (iron(III)).
Common mistakes
- Thinking all transition metals are coloured as metals. It is their compounds and ions that are usually coloured; most of the metals are grey or silvery.
- Writing Fe₂O₃ as iron(II) oxide. In Fe₂O₃ iron is +3, so it is iron(III) oxide.
- Removing 3d electrons first when forming ions. The 4s electrons go first.
- Saying a catalyst is used up. It speeds up the reaction and is left unchanged at the end.