What you'll learn
This topic examines the transition elements found in the central block of the periodic table, focusing on their characteristic properties that distinguish them from other metals. Understanding these properties and their practical applications is essential for CIE IGCSE Chemistry examinations, where questions frequently test your ability to identify transition metals, explain their coloured compounds, and describe their catalytic behaviour.
Key terms and definitions
Transition elements — metals found in the central block of the periodic table between Groups 2 and 3, which form at least one ion with a partially filled d sub-shell of electrons.
Catalysis — the process by which a substance increases the rate of a chemical reaction without being permanently changed itself.
Oxidation state — a number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element in the compound.
Complex ion — an ion formed when a central metal ion is surrounded by molecules or ions that are bonded to it.
Ligand — a molecule or ion that forms a coordinate bond with a transition metal ion by donating a pair of electrons.
Variable oxidation state — the ability of transition metals to exist in different positive oxidation states in their compounds.
d-block elements — elements in which the d sub-shell is being filled with electrons, located in Groups 3-12 of the periodic table.
Catalytic converter — a device containing transition metal catalysts that converts harmful exhaust gases into less harmful substances.
Core concepts
Location and electronic structure of transition elements
The transition elements occupy the central block of the periodic table, specifically in the d-block. For CIE IGCSE Chemistry, you need to focus on the first row of transition metals from scandium (Sc) to zinc (Zn), though chromium (Cr), manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn) receive particular attention in examinations.
Key characteristics of their position:
- Located between Group 2 and Group 3 in the modern periodic table
- Found in Period 4 between calcium and gallium
- Include elements such as titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper
- Zinc is sometimes considered separately as it doesn't form ions with partially filled d orbitals
The electronic configuration of transition metals involves filling the 3d sub-shell. For example:
- Iron (Fe): 2,8,14,2
- Copper (Cu): 2,8,18,1
- Zinc (Zn): 2,8,18,2
General physical properties
Transition elements share several physical properties that distinguish them from Group 1 and Group 2 metals:
High melting and boiling points
- Most transition metals have melting points above 1000°C
- Iron melts at 1538°C compared to sodium at 98°C
- Strong metallic bonding due to availability of d electrons for bonding
- Used in high-temperature applications like furnace construction
High density
- Transition metals are dense compared to Groups 1 and 2
- Iron has a density of 7.9 g/cm³; copper 8.9 g/cm³
- Sodium by comparison has a density of only 0.97 g/cm³
- Makes them suitable for applications requiring strength in small volumes
Good conductors of heat and electricity
- Similar to other metals but often better conductors
- Copper is the second-best conductor of electricity after silver
- Used extensively in electrical wiring and heat exchangers
Hard and strong with high tensile strength
- Much harder than Group 1 metals
- Iron and its alloys (steel) used in construction and engineering
- Titanium used in aircraft construction due to strength-to-weight ratio
Formation of coloured compounds
One of the most distinctive properties of transition elements tested in CIE IGCSE Chemistry examinations is their ability to form coloured compounds. This occurs because partially filled d orbitals can absorb certain wavelengths of visible light.
Common examples you should know:
Copper(II) compounds — typically blue or green
- Copper(II) sulfate solution: bright blue
- Copper(II) carbonate: green
- Hydrated copper(II) sulfate crystals: blue
Iron(II) compounds — typically pale green
- Iron(II) sulfate solution: pale green
- Iron(II) hydroxide precipitate: green
Iron(III) compounds — typically orange/brown
- Iron(III) chloride solution: yellow-brown
- Iron(III) hydroxide precipitate: orange-brown
- Rust (hydrated iron(III) oxide): red-brown
Manganese compounds — various colours depending on oxidation state
- Potassium manganate(VII) solution: purple
- Manganese(IV) oxide: black
Chromium compounds — green or orange depending on oxidation state
- Chromium(III) compounds: green
- Potassium chromate(VI): yellow
Contrast with zinc: Zinc compounds are white or colourless because Zn²⁺ ions have a completely filled d sub-shell (d¹⁰), meaning no d-d electronic transitions can occur. This is why zinc is sometimes not classified as a true transition element.
Variable oxidation states
Transition metals exhibit variable oxidation states — they can form ions with different charges. This property arises because the 3d and 4s electrons are close in energy, making it possible to remove different numbers of electrons.
Key examples for IGCSE:
Iron
- Forms Fe²⁺ ions (iron(II) compounds)
- Forms Fe³⁺ ions (iron(III) compounds)
- Fe²⁺ can be oxidised to Fe³⁺
Copper
- Forms Cu⁺ ions (copper(I) compounds) — less common
- Forms Cu²⁺ ions (copper(II) compounds) — most common
Manganese
- Forms Mn²⁺, Mn⁴⁺, Mn⁶⁺, and Mn⁷⁺ ions in different compounds
- Potassium manganate(VII) contains Mn in +7 oxidation state
Chromium
- Forms Cr³⁺ and Cr⁶⁺ ions in compounds
This property contrasts sharply with Group 1 metals (always +1) and Group 2 metals (always +2). The variable oxidation states contribute to the catalytic properties of transition metals and their coloured compounds.
Catalytic properties
Transition elements and their compounds act as catalysts in many important industrial and biological processes. Their effectiveness as catalysts relates to:
- Variable oxidation states allowing them to form temporary intermediate compounds
- Ability to provide a surface for reactant molecules to adsorb onto
- Capacity to weaken bonds in reactant molecules
Industrial catalysts you must know:
Iron in the Haber Process
- Catalyst for nitrogen + hydrogen → ammonia
- N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
- Temperature: approximately 450°C
- Pressure: approximately 200 atmospheres
- Finely divided iron provides large surface area
Vanadium(V) oxide in the Contact Process
- Catalyst for sulfur dioxide → sulfur trioxide
- 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
- Used in sulfuric acid manufacture
- V₂O₅ alternates between V⁵⁺ and V⁴⁺ oxidation states
Nickel in hydrogenation
- Converts unsaturated vegetable oils to saturated fats (margarine production)
- Adds hydrogen across C=C double bonds
- Temperature: approximately 150-200°C
Manganese(IV) oxide
- Catalyses decomposition of hydrogen peroxide
- 2H₂O₂(aq) → 2H₂O(l) + O₂(g)
- Commonly demonstrated in school laboratories
Platinum and palladium in catalytic converters
- Convert harmful exhaust gases to less harmful products
- 2CO + 2NO → 2CO₂ + N₂
- Convert unburnt hydrocarbons to CO₂ and H₂O
- Reduce nitrogen oxides to nitrogen gas
Practical uses of transition metals
Understanding the link between properties and uses is frequently examined:
Iron and steel
- Construction: bridges, buildings, reinforcement bars
- Manufacturing: car bodies, machinery, tools
- Properties: strong, hard, relatively cheap, can be alloyed
Copper
- Electrical wiring: excellent conductor, ductile
- Water pipes: unreactive with water, malleable
- Cooking utensils: good thermal conductor
- Alloys: brass (copper + zinc), bronze (copper + tin)
Titanium
- Aircraft construction: high strength, low density, resists corrosion
- Hip joint replacements: strong, unreactive with body fluids, biocompatible
- Expensive due to difficult extraction process
Chromium
- Stainless steel: prevents rusting of iron
- Chromium plating: decorative and protective layer on steel
- Resists corrosion effectively
Zinc
- Galvanising iron and steel: protective coating prevents rust
- Brass production: alloy with copper
- Sacrificial protection of iron
Nickel
- Stainless steel alloy component
- Rechargeable battery electrodes
- Electroplating for corrosion resistance
Worked examples
Example 1: Identifying transition metal properties
Question: A metal X has the following properties:
- Forms a blue solution when its sulfate dissolves in water
- Forms compounds in which X has two different oxidation states
- Acts as a catalyst for some reactions
(a) Suggest the identity of metal X. [1] (b) Explain why compounds of X are coloured. [2] (c) State one large-scale use of X in everyday life. [1]
Model answer:
(a) Copper [1 mark]
(b) Copper is a transition element / copper ions have partially filled d orbitals [1] These d electrons can absorb certain wavelengths of visible light / energy from visible light causes d electrons to move to higher energy levels [1]
(c) Electrical wiring / electrical cables / water pipes / cooking pans [1 mark for any valid use]
Example 2: Catalysis
Question: Hydrogen peroxide decomposes slowly at room temperature: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)
(a) Name a transition metal compound that catalyses this reaction. [1] (b) Explain what is meant by the term catalyst. [2] (c) State two ways you could show that this substance is acting as a catalyst. [2]
Model answer:
(a) Manganese(IV) oxide / MnO₂ [1 mark]
(b) A catalyst increases the rate of a reaction [1] without being permanently changed / used up / is chemically unchanged at the end [1]
(c) The reaction happens faster when the catalyst is added / more oxygen is produced per minute [1] The mass of catalyst is the same at the start and end of the reaction [1]
Example 3: Comparing transition metals with Group 1 metals
Question: The table shows some properties of sodium (Group 1) and iron (a transition element).
| Property | Sodium | Iron |
|---|---|---|
| Melting point | 98°C | 1538°C |
| Density | 0.97 g/cm³ | 7.9 g/cm³ |
| Hardness | Soft | Hard |
(a) Suggest why iron has a much higher melting point than sodium. [2] (b) Iron forms compounds with iron in two different oxidation states. State what these oxidation states are and give the colour of an aqueous solution containing each one. [4]
Model answer:
(a) Iron has stronger metallic bonding (than sodium) [1] Because more electrons are available for bonding / d electrons contribute to bonding [1]
(b) Iron(II) or Fe²⁺ or oxidation state +2 [1] Pale green / green [1] Iron(III) or Fe³⁺ or oxidation state +3 [1] Yellow-brown / orange-brown / brown [1]
Common mistakes and how to avoid them
Mistake: Stating that all d-block elements are transition elements, including zinc. Correction: Zinc forms only Zn²⁺ ions with a full d¹⁰ configuration, so it doesn't meet the strict definition of a transition element. Zinc compounds are colourless/white, unlike typical transition metal compounds.
Mistake: Claiming that transition metals form coloured compounds because they are metals or because of their metallic bonding. Correction: The colour arises specifically because partially filled d orbitals allow d-d electron transitions when visible light is absorbed. This is unrelated to metallic bonding.
Mistake: Writing that catalysts speed up reactions by "taking part" in them without qualification. Correction: Catalysts do participate in reactions by forming temporary intermediates, but they are chemically unchanged at the end and can be recovered. The key phrase is "not permanently changed."
Mistake: Confusing the properties of different iron compounds — saying Fe²⁺ solutions are brown or Fe³⁺ solutions are green. Correction: Fe²⁺ solutions are pale green; Fe³⁺ solutions are yellow-brown/orange-brown. Learn these colour associations as they appear frequently in identification questions.
Mistake: Stating that transition metals have high melting points without explaining why. Correction: Always link the property to its cause: high melting points result from strong metallic bonding due to the involvement of d electrons in bonding, requiring more energy to overcome these forces.
Mistake: Naming transition metals as catalysts without being specific about the process or compound form. Correction: Be precise: "iron acts as a catalyst in the Haber process" or "vanadium(V) oxide catalyses the Contact process." The compound form matters.
Exam technique for "Transition elements: properties and uses"
Command word "Explain": When asked to explain why transition metals form coloured compounds or act as catalysts, provide both the observation and the reason. For colours, mention partially filled d orbitals and absorption of visible light. For catalysis, state that they increase reaction rate and provide a mechanism (surface adsorption or variable oxidation states). Expect 2-3 marks for explanations.
Comparison questions: CIE frequently asks you to compare transition metals with Group 1 or Group 2 metals. Structure answers in parallel: state the property for transition metals, then contrast with the other group. For example: "Transition metals have high melting points (>1000°C) due to strong metallic bonding involving d electrons, whereas Group 1 metals have low melting points (<200°C) due to weaker metallic bonding with only one electron per atom available."
Use and property links: Questions often ask "Suggest why metal X is suitable for use Y." Always link a specific property to the requirement: "Copper is suitable for electrical wiring because it is an excellent conductor of electricity and is ductile so can be drawn into wires." Two clear points score two marks.
Catalyst identification: Know the standard industrial catalysts by name. "A transition metal" scores zero marks where "iron" or "vanadium(V) oxide" would score. Similarly, know the associated processes: Haber (iron), Contact (V₂O₅), hydrogenation (nickel), decomposition of H₂O₂ (MnO₂), catalytic converters (platinum/palladium).
Quick revision summary
Transition elements are d-block metals with characteristic properties: high melting points, high density, hardness, and strength due to strong metallic bonding. They form coloured compounds because partially filled d orbitals absorb visible light. Transition metals exhibit variable oxidation states (e.g., Fe²⁺ and Fe³⁺) and act as catalysts in major industrial processes — iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in hydrogenation, and platinum/palladium in catalytic converters. Key uses include iron/steel in construction, copper in electrical wiring, titanium in aircraft, and chromium in stainless steel. Zinc is atypical as Zn²⁺ has a full d sub-shell, forming only colourless/white compounds.