8.4 Transition elements
- Syllabus
- 0620–2026–2027
- Topic
- 8.4
- Level
- —
Learning objectives
8.4.1Transition elements as metals that• Describe transition elements as metals that: (a) have high densities (b) have high melting points (c) form coloured compounds (d) often act as catalysts as elements and in compounds8.4.2Transition elements as having ions• Describe transition elements as having ions with variable oxidation numbers, including iron(II) and iron(III)
Recognise typical transition-element properties
Transition elements are metals in the central block of the Periodic Table. The required identifying pattern combines physical properties with characteristic compound and catalyst behaviour.
| Required property | What to recognise | Useful contrast with Group I metals |
|---|---|---|
| density | high | Group I metals have low densities |
| melting point | high | Group I metals have low melting points |
| compounds | often coloured | Group I compounds are usually white or colourless |
| catalytic behaviour | elements and their compounds often act as catalysts | Group I metals and compounds are not typically catalysts |
Iron, nickel, cobalt, copper, chromium and platinum are transition elements. Iron is used in the Haber process, nickel in hydrogenation, and transition-metal oxides can also catalyse reactions.
No single generic metal property is enough: conducting electricity or being malleable is common to many metals. Use the high-density/high-melting-point pattern together with coloured compounds or catalytic behaviour.
Use variable oxidation numbers
Transition elements can form ions with different oxidation numbers. The oxidation number is written as a Roman numeral in a compound's name.
| Ion | Oxidation number | Name in compounds | Example |
|---|---|---|---|
| Fe²⁺ | +2 | iron(II) | FeCl₂, iron(II) chloride |
| Fe³⁺ | +3 | iron(III) | FeCl₃, iron(III) chloride |
Find an unknown oxidation number by balancing charge. In Fe₂O₃, three O²⁻ ions total −6, so two iron ions total +6 and each iron is +3. In FeCl₂, two Cl⁻ ions require Fe²⁺.
The Roman numeral gives the oxidation number of the metal ion, not the number of metal atoms. Fe(III) means Fe³⁺; it does not mean that every formula contains three iron atoms.