Transition metals
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
The topic on one screen
- A transition metal is a d-block element that forms at least one stable ion with a partially filled (incomplete) d-subshell.
- A ligand donates a lone pair of electrons to the metal ion, forming a coordinate (dative covalent) bond; the metal + ligands = a complex.
- Ligands can be monodentate (e.g. H2O, NH3, Cl-), bidentate (e.g. ethanedioate, en) or multidentate (e.g. EDTA).
- Common shapes: octahedral (6 ligands), tetrahedral or square planar (4 ligands), linear (2 ligands, e.g. in Tollens').
- Colour comes from d-d transitions: d orbitals split, electrons absorb visible light to jump, and the complementary colour is seen. Change the ligand, coordination number or oxidation state and the colour changes.
- Catalysis: variable oxidation states let transition metals accept and donate electrons (homogeneous), or their surface adsorbs reactants (heterogeneous).
Where students actually lose marks
Defining a transition metal COMPLEX, several answers didn't refer to an atom or ion, or gave the definition of a transition metal instead. Be precise about what the term is asking for.
June 2023 Paper 1 examiner report (Q01.1)
The chelate effect was poorly recalled: it is driven by entropy (more particles released than taken in), not by 'stronger bonds forming'.
June 2024 Paper 1 examiner report (Q03.2)
That variable oxidation states matter for transition-metal catalysis is usually known, but few stressed it in their answers. Say it explicitly.
June 2024 Paper 1 examiner report (Q03.3)
Try it — exam-style
Define a transition metal.
Explain what is meant by a ligand and state the type of bond it forms with a transition metal ion.
When six water ligands in [Cu(H2O)6]2+ are replaced by three bidentate ethanedioate ligands, the complex becomes more stable. Explain this in terms of entropy.
Explain why transition metals and their compounds are often good catalysts.
Explain why aqueous transition metal ions are coloured, and state one change that would change the colour.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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