Transition metals · Year 13
The transition metals sit in the middle block of the periodic table, between groups 2 and 3. Compared with group 1 metals they are hard and dense, have high melting points and react slowly or not at all with water. They form ions with different charges, coloured compounds and complex ions, and they make good catalysts. At A-level a transition metal is an element that forms at least one stable ion with an incomplete d sub-shell.
Key facts
- Typical properties: high melting points and densities, hard and strong, much less reactive than group 1; ions with different charges (Fe²⁺ and Fe³⁺); coloured compounds; good catalysts.
- The 4s sub-shell fills before 3d, but ions lose their 4s electrons first: Fe is [Ar] 3d⁶ 4s² and Fe³⁺ is [Ar] 3d⁵. Chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹) are the exceptions.
- Scandium and zinc are in the d block but are not transition metals: Sc³⁺ has no d electrons and Zn²⁺ has a full 3d sub-shell.
- A complex is a central metal ion joined to ligands by co-ordinate bonds. A ligand donates a lone pair of electrons: water, ammonia, chloride and cyanide are common ligands. The co-ordination number is the number of co-ordinate bonds.
- Shapes: six small ligands give an octahedral complex (90°); four large ligands such as Cl⁻ give a tetrahedral one (109.5°); platinum(II) complexes such as cisplatin are square planar; silver(I) forms linear [Ag(NH₃)₂]⁺.
- Colour: the ligands split the d orbitals. An electron absorbs visible light of energy ΔE = hν to move up, and the light that is left is the colour you see. Changing the oxidation state, the ligands or the co-ordination number changes the colour.
- Catalysts: a heterogeneous catalyst is in a different phase from the reactants (iron in the Haber process, V₂O₅ in the Contact process); a homogeneous catalyst is in the same phase and works by changing its oxidation state (Fe²⁺ in the reaction of I⁻ with S₂O₈²⁻).