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Hydrogen (1$s^1$) is uniquely positioned between Group 1 and Group 17. Like alkali metals, it has $ns^1$ configuration and forms $H^+$ cations. Like halogens, it needs one electron for noble gas configuration (forms $H^-$ hydride) and exists as diatomic H2. Its IE $\frac{1312 kJ}{mol}$ is far higher than any alkali metal.

Three isotopes exist: protium (^1H, 99.98%), deuterium (^2H/D, 0.02%), and tritium (^3H/T, radioactive, t1/2 = 12.3 years). Heavy water D2O has higher m.p., b.p., and density than H2O; it serves as a nuclear reactor moderator.

Hydrides are classified as ionic (NaH, CaH2 — s-block), covalent (H2O, NH3, B2H6 — p-block), and metallic/interstitial (TiH, PdH0.6 — d/f-block). Covalent hydrides subdivide into electron-deficient (B2H6), electron-precise (CH4), and electron-rich (NH3). Pd absorbs 900x its volume of H2.

Water gas (CO + H2) is produced from C + H2O at ~1000 C. The hydrogen economy proposes H2 as a clean fuel: 2H2 + O2 → 2H2O (only product is water). Key challenges include energy-intensive production, storage, and safety.