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Group 13 (B, Al, Ga, In, Tl) has ns^2np^1 configuration. Boron is anomalous: a metalloid that never forms B^3+ (always covalent), creates electron-deficient compounds with unique 3c-2e banana bonds (B2H6), and shows diagonal relationship with silicon.

Boric acid B(OH)3 is a monobasic Lewis acid (accepts OH^- from water rather than donating H^+). Borax (Na2B4O7.10H2O) gives the borax bead test for metal identification.

Aluminium forms amphoteric Al2O3 and Al(OH)3. AlCl3 dimerises to Al2Cl6 (electron-deficient Al seeks to complete octet). AlCl3 is a key Lewis acid catalyst (Friedel-Crafts). The thermite reaction (2Al + Fe2O3) produces molten iron at ~2500C. Al is extracted by Hall-Heroult electrolysis of Al2O3 in cryolite.

Lewis acid strength of BX3: BI3 > BBr3 > BCl3 > BF3 (counter-intuitive due to ppi-ppi backbonding from F being strongest). The inert pair effect dominates in Tl: Tl^+ (6s^2 inert) is far more stable than Tl^3+. This effect increases down the group: Al (none) to Tl (strong).