A transition element is best defined as one that:

forms an ion with a partially filled d subshell. The defining feature is a partially filled d subshell in at least one of its ions.

Transition metals show variable oxidation states because their:

4s and 3d electrons are close in energy. The small 4s/3d energy gap lets different numbers of electrons be lost, giving several stable ions.

Transition-metal compounds are usually coloured because the d orbitals:

absorb specific visible wavelengths. Partly filled d orbitals absorb particular wavelengths of visible light (d–d transitions), so the compound looks coloured.

A ligand in a complex ion is a species that:

donates a lone pair to the central metal ion. Ligands (e.g. water, ammonia, chloride) bond to the central metal ion by donating a lone pair.

Iron acts as a catalyst in the:

Haber process. Iron catalyses the Haber process; vanadium(V) oxide catalyses the Contact process.

Sodium forms only Na⁺ but iron forms Fe²⁺ and Fe³⁺ because iron:

has 4s and 3d electrons of similar energy. The close energies of iron's 4s and 3d electrons make more than one oxidation state accessible.

In a complex ion, the bond between a ligand and the central metal is a:

dative (coordinate) covalent bond. The ligand donates a lone pair to the metal, forming a dative (coordinate) covalent bond.

Transition-metal compounds are coloured because of:

d–d electron transitions. Splitting of the d-orbitals lets a d-electron absorb visible light (a d–d transition), producing colour.

Transition Elements

The d-block — variable oxidation states, colour, catalysis and complexes

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What makes a transition element

The transition elements are the d-block metals — iron, copper, chromium, manganese and the rest — defined more precisely as elements that form at least one ion with a partially filled d subshell. That partly filled d shell is the source of nearly all their special properties.

Physically they are typical metals but pushed to extremes: hard, dense, with high melting points, and good conductors.

Variable oxidation states

Unlike a Group 1 or 2 metal, a transition metal can lose different numbers of electrons, giving variable oxidation states. Iron forms Fe²⁺ and Fe³⁺; manganese ranges up to +7 (in MnO₄⁻); copper shows +1 and +2. This is because the 4s and 3d electrons are close in energy, so different numbers can be removed — which is also why transition metals make such versatile catalysts and take part in so many redox reactions.

Colour and complexes

Transition-metal compounds are typically coloured — copper(II) blue, iron(III) yellow-brown, manganese(VII) purple — because the partly filled d orbitals absorb specific wavelengths of visible light. The exact colour depends on the metal, its oxidation state, and what surrounds it.

They readily form complex ions: a central metal ion bonded to surrounding molecules or ions called ligands (water, ammonia, chloride), which donate a lone pair into the metal. Changing the ligands changes the colour — the basis of many colour-change tests.

Catalysis

Their variable oxidation states let transition metals (and their compounds) act as catalysts by temporarily accepting and giving back electrons, offering a lower-energy pathway. Iron catalyses the Haber process, vanadium(V) oxide the Contact process, nickel the hydrogenation of oils, and manganese(IV) oxide the decomposition of hydrogen peroxide. Industry relies on them heavily.

One cause, four properties

The partly filled d subshell explains it all: variable oxidation states (d electrons close in energy), colour (d–d light absorption), complex formation (space and orbitals for ligands), and catalysis (easy electron give-and-take).

A worked example

Worked example

Why can iron form both Fe²⁺ and Fe³⁺, while sodium only ever forms Na⁺?

Sodium has one outer electron; losing it gives the stable Na⁺ (a full shell), and losing more would break into a full inner shell — far too costly.Group 1 metals have only one easily-lost electron.
Iron's 4s and 3d electrons are close in energy, so it can lose two (→ Fe²⁺) or three (→ Fe³⁺).The small energy gap makes several oxidation states accessible.
Hence the variable oxidation states typical of transition metals.A partly filled d subshell allows more than one stable ion.

Iron's 4s and 3d electrons are similar in energy, so different numbers can be removed (Fe²⁺, Fe³⁺); sodium has just one outer electron, giving only Na⁺.

Complex ions and ligands

A defining feature of transition metals is that their ions form complex ions: a central metal ion surrounded by ligands — molecules or ions such as water, ammonia or chloride that each donate a lone pair into a dative (coordinate) bond. The number of such bonds is the coordination number, commonly 6 (octahedral) or 4 (tetrahedral).

Swapping one ligand for another — ligand exchange — often changes the colour, which is why adding ammonia to copper(II) sulfate turns the pale blue solution deep blue.

Where the colour and catalysis come from

In a complex, the metal's five d-orbitals split into two slightly different energy levels. A d-electron can absorb a wavelength of visible light to jump the gap, and the colour you see is what is left of the light — a d–d transition. Change the metal, its oxidation state or its ligands and the gap changes, so the colour changes too.

The same partly-filled, closely-spaced d-orbitals let transition metals shift easily between oxidation states, which is why they and their compounds make such good catalysts — iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in hydrogenation.

Colour is subtractive

A complex absorbs the colours matching its d-orbital gap and transmits the rest; the colour you see is the leftover light. Copper(II) absorbs red/orange, so it looks blue.

Practice questions

6 questions · 21 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

4 · 9 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[2 marks]

State three characteristic properties of transition elements.

Model answer

They form ions with more than one oxidation state, form coloured compounds, and act as catalysts (any three; also high melting points/densities).

Examiner tip. Variable oxidation states, coloured ions, catalytic activity — the classic three.

SQ2[2 marks]

Give one example of a transition metal used as a catalyst and the process it catalyses.

Model answer

Iron in the Haber process (or vanadium(V) oxide in the Contact process, or nickel in hydrogenation).

Examiner tip. Transition metals and their compounds are common industrial catalysts.

SQ3[3 marks]

Explain why transition metal compounds are often coloured.

Model answer

The d-orbitals split into different energy levels; a d-electron absorbs certain wavelengths of visible light to move between them, and the transmitted/reflected light gives the colour.

Examiner tip. Colour arises from d–d electron transitions absorbing part of the visible spectrum.

SQ4[2 marks]

Explain what is observed when excess aqueous ammonia is added to copper(II) sulfate solution, in terms of ligand exchange.

Model answer

A pale blue precipitate of copper(II) hydroxide forms first, then dissolves in excess ammonia to give a deep blue solution as water ligands are replaced by ammonia ligands (ligand exchange), changing the colour.

Examiner tip. Changing the ligands changes the d-orbital gap and so the colour.

Long questions

2 · 12 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[6 marks]

Copper shows typical transition-metal behaviour.

State the two common oxidation states of copper and the colour of each aqueous ion. [2]
Explain what is meant by a complex ion. [2]
State what is observed when excess aqueous ammonia is added to copper(II) sulfate solution. [2]

Mark scheme

+1 (Cu⁺) and +2 (Cu²⁺)[1]
Cu²⁺(aq) is blue (Cu⁺ compounds often white/colourless)[1]
A complex ion is a central metal ion[1]
surrounded by ligands bonded by coordinate (dative) bonds[1]
A pale blue precipitate forms first[1]
which dissolves in excess ammonia to give a deep blue solution[1]

Cu⁺ and Cu²⁺ (Cu²⁺ blue); complex = metal ion + ligands (dative bonds); excess NH₃ gives pale blue ppt then deep blue solution.

Examiner tip. Ligands donate lone pairs into dative bonds with the central metal ion.

LQ2[6 marks]

Transition elements have several characteristic properties.

Explain why transition metals show variable oxidation states. [2]
Explain why their compounds are coloured. [2]
Explain why they act as good catalysts, with one named example. [2]

Mark scheme

Their 3d and 4s sub-shells are close in energy[1]
so different numbers of electrons can be lost, giving several stable oxidation states[1]
In a complex the d-orbitals split into two energy levels[1]
a d-electron absorbs visible light to jump the gap (d–d transition); the transmitted light is the colour[1]
They can change oxidation state easily / provide a surface for reaction[1]
e.g. iron in the Haber process (or V₂O₅ in the Contact process)[1]

Variable states: close 3d/4s energies; colour: d–d transitions absorbing visible light; catalysis: easy change of oxidation state, e.g. iron in the Haber process.

Examiner tip. Nearly all transition-metal properties trace back to their partly-filled d-orbitals.