Unit 17
The main-group elements
This is where the periodic table becomes chemistry you can recognise. Why does caesium explode in water while lithium merely fizzes? Why do lead compounds prefer +2? Why is dinitrogen so lazy when nitrogen compounds both feed the world and blast rock? Why is fluorine the strongest oxidant of all? Each question is answered with tools from earlier units — thermodynamic cycles, electrode potentials and acid–base equilibria — and every number is computed from data.
- 1 Hydrogen and the s-block Lithium has the most negative ΔG° with water yet fizzes most gently; caesium, with almost the same ΔG°, explodes. Take the energy cycle apart, then follow flame colours, solubility trends and the diagonal relationship. 30 min
- 2 Boron, aluminium, carbon, silicon and the inert pair Electron-poor boron, self-protecting aluminium, carbon with four faces, silicon married to oxygen — and why the state two below the group number grows more stable down a group. 30 min
- 3 Boranes and clusters: counting skeletal electrons Diborane holds 8 atoms together with only 12 valence electrons: three-centre two-electron bonds are the answer. Wade’s rules need only a count of electron pairs to tell whether a borane or carborane cage is closo, nido or arachno — then compare with measured structures to see what the rules can predict. 30 min
- 4 Nitrogen and phosphorus The N≡N triple bond makes dinitrogen nearly inert, so fixing nitrogen is a problem for life and industry alike. Walk the nitrogen cycle across its oxidation-state map, then see why phosphorus prefers P₄ tetrahedra. 30 min
- 5 Oxygen and sulfur Paramagnetic oxygen, protective ozone, period-3 oxides running from strong base to strong acid, and the contact process creeping up on equilibrium bed by bed — this is how sulfuric acid is made. 30 min
- 6 Halogens and noble gases Fluorine’s electron affinity is smaller than chlorine’s, so why is it the strongest oxidant? Meet the halogens through energy cycles, bond enthalpies and computed colours, run displacement tests, and see how the “inert” gases bond to fluorine. 30 min