Three classes: (1) Ionic (saline) hydrides — formed by s-block metals (except Be, Mg partially). NaH, CaH2 contain $H^-$ ion. High melting point, conduct electricity when molten (H2 liberated at anode). React with water: NaH + H2O → NaOH + H2. CaH2 used as desiccant (drying agent). (2) Covalent (molecular) hydrides — formed by p-block elements. Can be electron-deficient (B2H6), electron-precise (CH4), or electron-rich (NH3, H2O). Stability decreases down a group. (3) Metallic (interstitial) hydrides — formed by d- and f-block metals. H atoms occupy interstitial voids. Non-stoichiometric (e.g., LaH2.87, PdH0.6). Pd absorbs 900x its volume of H2 — used for H2 purification and storage.