Boron is a metalloid unlike other Group 13 members (all metals). Key anomalies: (1) Never forms $B^{3+}$ (too small, IE too high — always covalent). (2) Electron-deficient compounds — BH3 has only 6 electrons around B. (3) Forms 3-centre 2-electron (3c-2e) "banana bonds" in B2H6 (diborane): two B-H-B bridge bonds where 2 electrons are shared among 3 atoms. (4) Maximum covalence = 4 (no d-orbitals; achieves octet by accepting lone pair, hence Lewis acid). (5) Diagonal relationship with Si: both form covalent compounds, acidic oxides, similar chlorides. Boric acid B(OH)3 is monobasic and a Lewis acid: B(OH)3 + H2O → [B(OH)4]^- + $H^+$. Borax Na2B4O7.10H2O on heating gives sodium metaborate + B2O3 (glassy bead test).