Hydrogen and electrolysis

Hydrogen sits uniquely in the periodic table: it has a single electron and can either lose it to form H⁺ (behaving somewhat like an alkali metal cation) or gain one to form H⁻ (behaving like a halide anion), and this dual character shows up directly in the compounds it forms. With highly electropositive metals such as the alkali and alkaline-earth metals, the large electronegativity gap lets hydrogen accept an electron, forming ionic hydrides (such as NaH) containing the discrete hydride anion, H⁻ — these compounds are salt-like, high-melting solids that react vigorously with water, releasing hydrogen gas. With nonmetals such as carbon, nitrogen, oxygen, and the halogens, hydrogen instead shares electrons, forming covalent hydrides (such as CH4, NH3, H2O, and HCl) whose properties range continuously across the periodic table, from flammable gases to strongly acidic solutions in water. Hydrogen's compounds with transition metals form a third, more variable category, often described as interstitial or metallic hydrides, where hydrogen atoms occupy gaps within the metal lattice.

Electrolysis is the process of using electrical energy to drive a non-spontaneous chemical reaction, most commonly the decomposition of water into hydrogen and oxygen gas, or the decomposition of a molten ionic compound into its elements. An external power source forces electrons onto one electrode (the cathode) and pulls them away from the other (the anode); at the cathode, the surplus of electrons drives reduction — in water electrolysis, hydrogen cations (effectively H3O⁺ or H2O) gain electrons and are reduced to hydrogen gas — while at the anode, the deficit of electrons drives oxidation — water molecules or hydroxide ions lose electrons and are oxidised to oxygen gas. This electrode assignment for electrolysis is worth fixing firmly: reduction always happens at the cathode and oxidation always happens at the anode, in both electrolytic and galvanic (spontaneous) cells, even though which electrode is positively or negatively charged differs between the two cell types.

Electrolysis and hydrogen chemistry meet directly in fuel cell technology, a device that runs the electrolysis of water in reverse: hydrogen gas is oxidised at one electrode and oxygen gas is reduced at the other, with the overall reaction releasing electrical energy directly rather than heat, making hydrogen fuel cells an actively researched route to clean energy conversion.