The Periodic Table: chemical periodicity

Period 3 trends and oxides

Across period 3 atomic radius decreases (more protons, same shielding). Melting points rise from Na to Si (metallic bonding strengthens, then giant covalent Si), then fall sharply to simple molecular P₄, S₈, Cl₂ and atomic Ar. Conductivity is high for metals, low for Si (semiconductor), negligible after. Oxides go from basic ionic ($\mathrm{Na_2O}$, MgO) through amphoteric ($\mathrm{Al_2O_3}$) and giant covalent ($\mathrm{SiO_2}$, acidic) to acidic simple molecular ($\mathrm{P_4O_{10}}$, $\mathrm{SO_2}$, $\mathrm{SO_3}$). Chlorides go from ionic (NaCl dissolves, pH 7) to covalent ones that hydrolyse to acidic solutions ($\mathrm{SiCl_4}$, $\mathrm{PCl_5}$).