Introduction to Transition Metals

The transition metals occupy the central block of the periodic table, spanning Groups 3 to 12 in Period 4 (and further periods below). For Edexcel A-Level, the focus is on the first-row transition metals from scandium (Sc) to zinc (Zn), with particular attention to the elements titanium through copper.

Defining a Transition Metal

The A-Level definition of a transition metal is precise and frequently tested:

A transition metal is a d-block element that forms at least one stable ion with an incomplete d sub-shell.

This definition is important because it excludes two d-block elements from the transition metal category:

Scandium (Sc): Its only stable ion is Sc³⁺, which has the electron configuration [Ar] — no d electrons at all. The d sub-shell is empty, not incomplete.
Zinc (Zn): Its only stable ion is Zn²⁺, which has the configuration [Ar] 3d¹⁰ — a completely full d sub-shell, not incomplete.

All other first-row d-block elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) form ions with partially filled d sub-shells and are therefore transition metals.

Common exam mistake: Writing "d-block element" when asked to define a transition metal. Not all d-block elements are transition metals. You must include the phrase "incomplete d sub-shell" in your definition.

Electron Configurations of the First-Row d-Block

The 3d and 4s sub-shells are very close in energy. In atoms, the 4s sub-shell fills before the 3d. The expected filling pattern is:

Element	Symbol	Z	Electron Configuration	Notes
Scandium	Sc	21	[Ar] 3d¹ 4s²
Titanium	Ti	22	[Ar] 3d² 4s²
Vanadium	V	23	[Ar] 3d³ 4s²
Chromium	Cr	24	[Ar] 3d⁵ 4s¹	Anomalous
Manganese	Mn	25	[Ar] 3d⁵ 4s²
Iron	Fe	26	[Ar] 3d⁶ 4s²
Cobalt	Co	27	[Ar] 3d⁷ 4s²
Nickel	Ni	28	[Ar] 3d⁸ 4s²
Copper	Cu	29	[Ar] 3d¹⁰ 4s¹	Anomalous
Zinc	Zn	30	[Ar] 3d¹⁰ 4s²

Anomalous Configurations: Chromium and Copper

Chromium has the configuration [Ar] 3d⁵ 4s¹ rather than the expected [Ar] 3d⁴ 4s². This is because a half-filled 3d sub-shell (3d⁵) is particularly stable due to the symmetrical distribution of electrons and exchange energy stabilisation.

Copper has the configuration [Ar] 3d¹⁰ 4s¹ rather than [Ar] 3d⁹ 4s². A completely filled 3d sub-shell (3d¹⁰) is also especially stable.

Ion Formation: 4s Electrons Are Lost First

When transition metal atoms form positive ions, the 4s electrons are removed before the 3d electrons. This is because in the presence of d electrons, the 4s sub-shell is at slightly higher energy due to inter-electron repulsion effects.

For example:

Fe: [Ar] 3d⁶ 4s² → Fe²⁺: [Ar] 3d⁶ (lost two 4s electrons)
Fe²⁺: [Ar] 3d⁶ → Fe³⁺: [Ar] 3d⁵ (lost one 3d electron)
Cu: [Ar] 3d¹⁰ 4s¹ → Cu⁺: [Ar] 3d¹⁰ (lost one 4s electron)
Cu: [Ar] 3d¹⁰ 4s¹ → Cu²⁺: [Ar] 3d⁹ (lost 4s then one 3d electron)

Common exam mistake: Writing Fe²⁺ as [Ar] 3d⁴ 4s² instead of [Ar] 3d⁶. Students forget that 4s electrons are lost first, not 3d. This is one of the most common errors in transition metal questions.

Variable Oxidation States

One of the defining properties of transition metals is their ability to form ions with multiple stable oxidation states. This occurs because the 3d and 4s sub-shells are close in energy, so varying numbers of electrons can be removed without a prohibitive increase in ionisation energy.

Element	Common Oxidation States	Example Compounds
Ti	+2, +3, +4	TiO₂ (+4), TiCl₃ (+3)
V	+2, +3, +4, +5	V₂O₅ (+5), VO²⁺ (+4), V³⁺ (+3), V²⁺ (+2)
Cr	+2, +3, +6	Cr₂O₇²⁻ (+6), Cr³⁺ (+3)
Mn	+2, +4, +7	MnO₄⁻ (+7), MnO₂ (+4), Mn²⁺ (+2)
Fe	+2, +3	FeSO₄ (+2), FeCl₃ (+3)
Co	+2, +3	CoCl₂ (+2), [Co(NH₃)₆]³⁺ (+3)
Ni	+2	NiCl₂ (+2)
Cu	+1, +2	Cu₂O (+1), CuSO₄ (+2)

The maximum oxidation state for the early transition metals corresponds to the total number of 3d and 4s electrons (e.g., Mn can reach +7, corresponding to loss/sharing of all 3d⁵ 4s² electrons). Beyond manganese, the maximum oxidation state tends to decrease as the increasing nuclear charge makes it harder to remove additional electrons.

Vanadium: A Showcase of Variable Oxidation States

Vanadium is a favourite exam topic because of its four distinct coloured oxidation states in aqueous solution:

Oxidation State	Species	Colour
+5	VO₂⁺	Yellow
+4	VO²⁺	Blue
+3	V³⁺	Green
+2	V²⁺	Violet/lavender

These can be demonstrated by reducing an acidified solution of ammonium vanadate (NH₄VO₃) with zinc in acidic conditions: the solution progresses through yellow → blue → green → violet.

Coloured Compounds

Most transition metal compounds are coloured. This is because the partially filled d sub-shell allows d-d electronic transitions when the ion is surrounded by ligands. The energy gap between split d orbitals corresponds to wavelengths in the visible spectrum.

Ion	Colour in Aqueous Solution	d Electron Count
Ti³⁺	Purple	3d¹
V²⁺	Violet	3d³
Cr³⁺	Green/violet	3d³
Mn²⁺	Very pale pink	3d⁵
Fe²⁺	Pale green	3d⁶
Fe³⁺	Yellow/brown	3d⁵
Co²⁺	Pink	3d⁷
Ni²⁺	Green	3d⁸
Cu²⁺	Blue	3d⁹

Compounds of Sc³⁺ and Zn²⁺ are typically colourless because Sc³⁺ has no d electrons (3d⁰) and Zn²⁺ has a full d sub-shell (3d¹⁰) — in neither case is a d-d transition possible.

Catalytic Activity

Transition metals and their compounds are widely used as catalysts. Their effectiveness is linked to:

Variable oxidation states — they can donate and accept electrons, providing an alternative reaction pathway with a lower activation energy
Ability to form complexes — reactant molecules can bind to the transition metal, bringing them into close proximity and the correct orientation

Key examples:

Iron in the Haber process (N₂ + 3H₂ → 2NH₃)
V₂O₅ in the Contact process (2SO₂ + O₂ → 2SO₃)
MnO₂ in the decomposition of hydrogen peroxide
Nickel in the hydrogenation of alkenes
Platinum/Palladium/Rhodium in catalytic converters

Other Properties of Transition Metals

Compared to Group 1 and Group 2 metals, transition metals generally have:

Higher melting points and boiling points (due to stronger metallic bonding involving d electrons)
Higher densities
Harder, stronger metals
Smaller atomic radii across the period (the d electrons do not shield as effectively, so the effective nuclear charge pulls the outer electrons in)

Property	Na (Group 1)	Mg (Group 2)	Fe (Transition)
Melting point / °C	98	650	1538
Density / g cm⁻³	0.97	1.74	7.87
Atomic radius / pm	186	160	126

Summary

Transition metals are d-block elements that form at least one stable ion with an incomplete d sub-shell (excluding Sc and Zn). Their characteristic properties — variable oxidation states, coloured compounds, and catalytic activity — arise from their partially filled d orbitals. In ions, 4s electrons are always lost before 3d electrons. Learn the common ions and colours of each transition metal for the exam.

A-Level Deep Dive: Introduction to Transition Metals

Spec mapping

Edexcel 9CH0 specification Topic 15 — Transition Metals, sub-topic 15.1 covers the definition of a transition metal as a d-block element forming at least one ion with a partially filled d subshell, the typical properties (variable oxidation states, coloured ions, catalytic activity, complex ion formation), and the explicit exclusion of Sc and Zn (refer to the official specification document for exact wording). Examined in Paper 1 (9CH0/01) and Paper 3 (9CH0/03). Builds on Topic 1 (Atomic Structure) for d-orbital electron configurations and on Topic 8 (Redox I) for the variable-oxidation-state behaviour. Foundational for 15.2 (complex ions), 15.3 (colour), and 15.4 (catalysis).

Worked example with full mark scheme

Question (8 marks):

(a) Define a transition metal. Use this definition to explain why Cu is a transition metal but Zn is not, even though both are d-block elements. (4)

(b) Write the full electron configuration of Mn and predict, with reasoning, the most stable ionic oxidation state. State another commonly observed oxidation state and identify a familiar compound containing Mn in that state. (4)

Solution with mark scheme:

(a) Step 1 — definition.

A transition metal is a d-block element that forms at least one stable ion with an incomplete (partially filled) d subshell (1 ≤ d-electron count ≤ 9).

B1 — definition.

Step 2 — Cu vs Zn analysis.

Cu: [Ar] 3d¹⁰ 4s¹ as the atom; Cu²⁺ is [Ar] 3d⁹ — incomplete d subshell — therefore Cu is a transition metal. (Cu⁺ is [Ar] 3d¹⁰ — full — but the existence of Cu²⁺ with d⁹ qualifies Cu.)

M1 — Cu²⁺ d⁹ identified.

Zn: [Ar] 3d¹⁰ 4s² as the atom; Zn²⁺ is [Ar] 3d¹⁰ — full d subshell — therefore Zn is not a transition metal. Zn forms only one ion (Zn²⁺), and that ion has a completed 3d¹⁰ shell.

M1 — Zn²⁺ d¹⁰ identified, full subshell.

A1 — explicit linkage to definition (Zn fails the "partially filled" criterion).

(b) Step 1 — electron configuration of Mn.

Atomic number 25. Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s² (or [Ar] 3d⁵ 4s²).

B1 — full configuration.

Step 2 — most stable oxidation state.

Removal of the 4s² gives Mn²⁺: [Ar] 3d⁵ — half-filled d subshell, exchange-energy stabilised. Mn²⁺ is the most stable ionic state, found in MnSO₄, MnCl₂, etc.

M1 — Mn²⁺ identified as most stable, with reasoning (half-filled d⁵).

Step 3 — another common oxidation state.

Mn(VII) in MnO₄⁻ (purple permanganate); Mn²⁺ + 4O is shifted to Mn(+7) by removing all five 3d + two 4s electrons. Other common states: Mn(IV) in MnO₂, Mn(III) in Mn₂O₃.

A1 — Mn(VII) in KMnO₄ (or any other valid example with correct OS).

Total: 8 marks (M2 A2 B4).

Specimen question modelled on the Edexcel 9CH0 Paper 1 format

Question (6 marks): The d-block of the periodic table contains the elements Sc to Zn in the first row.

(a) Explain why Sc is not classified as a transition metal even though it is a d-block element. (2)

(b) Predict and write the electron configurations of (i) Cr atom and (ii) Cr³⁺ ion. Explain any anomaly. (4)

Mark scheme decomposition by AO:

Mark	AO	Awarded for
(a) M1	AO1	Sc has [Ar] 3d¹ 4s²; the only stable ion is Sc³⁺
(a) A1	AO2	Sc³⁺ is [Ar] (3d⁰) — empty d subshell, fails "partially filled" criterion
(b)(i) B1	AO1	Cr atom: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²)
(b)(i) M1	AO2	Anomalous configuration explained by exchange-energy stabilisation of half-filled d⁵ subshell
(b)(ii) B1	AO1	Cr³⁺: [Ar] 3d³
(b)(ii) A1	AO2	When ionising, the 4s electron is removed first (4s is higher in energy than 3d once the d subshell starts filling)