Inorganic Chemistry
Inorganic chemistry in Cambridge IAL Chemistry examines periodic trends and the descriptive chemistry of Groups 2 and 17 and the transition elements. Explanations must link observed properties to structure, bonding, charge density and redox, not just state a trend.
Core rules
- Across Period 3, atomic radius decreases and first ionisation energy generally increases, with dips at Al (new 3p subshell) and S (electron pairing in 3p). Melting point follows structure: giant covalent Si > metallic Na/Mg/Al > simple molecular P4/S8/Cl2/Ar. Oxide character shifts from basic/ionic through amphoteric to acidic/covalent.
- Down Group 2, reactivity increases; oxides and hydroxides become more basic; thermal stability of carbonates and nitrates increases because the larger cation polarises the carbonate/nitrate anion less. Hydroxide solubility increases; sulfate solubility decreases. Group 2 metals reduce water to hydrogen and form the metal hydroxide.
- Down Group 17, oxidising power decreases and reducing power of halides increases. Displacement reactions show the stronger oxidising agent. Silver halides become less soluble in ammonia down the group.
- Transition metals show variable oxidation states, complex formation, catalysis and coloured ions. Colour arises from d–d transitions; ligands donate electron pairs. Coordination number is not the same as oxidation state.
Worked example
Find the oxidation state and d-electron count of Fe in [Fe(CN)6]3−. Let Fe = x. Six cyanide ligands each carry −1, so x + 6(−1) = −3, giving x = +3. Fe has atomic number 26, so Fe3+ is [Ar]3d5. CN− is a strong-field ligand, so the d5 configuration is low-spin: t2g5, with one unpaired electron. The complex is paramagnetic. A weak-field ligand would give high-spin d5 with five unpaired electrons.
Traps
Forgetting ligand charges; confusing coordination number with oxidation state; claiming all transition-metal colours come from d–d transitions without checking; reversing Group 2 sulfate solubility; ignoring the Al and S dips in ionisation energy; and not balancing redox half-equations by atoms, charge and electrons. Describe precipitates with colour and formula where asked.