Hydrogen (1$s^1$) does not fit neatly into any group. Similarities with Group 1: $ns^1$ configuration, forms $H^+$ cation, electropositive. Similarities with Group 17: one electron short of noble gas configuration (1$s^2$), forms $H^-$ hydride ion, high IE (1312 kJ/mol, much higher than any alkali metal), forms diatomic H2 molecule like halogens. Placed in Group 1 by IUPAC convention, but its chemistry is largely covalent. $H^+$ (bare proton) never exists free in solution — always hydrated as $H3O^+$ (hydronium). Hydrogen has three isotopes: protium (^1H, 99.98%), deuterium (^2H or D, 0.02%), tritium (^3H or T, radioactive, beta emitter, t1/2 = 12.3 years).