One proton and several isotopes
One proton determines hydrogen’s elemental identity. Ordinary hydrogen has no neutron, deuterium has one and tritium two. Their masses and nuclear properties differ. At ordinary conditions hydrogen gas consists mainly of diatomic molecules, distinguishing atomic hydrogen, molecular hydrogen and electrically charged hydrogen ions.
From a flammable gas to an element
Earlier metal-and-acid experiments produced a flammable gas, and Cavendish systematically investigated it in the eighteenth century. Lavoisier named hydrogen for its relationship with water. Its recognition grew through repeated experiments and changing accounts of water and combustion, rather than from observing bubbles alone.
References: [1]
Compounds on Earth and matter in stars
Hydrogen is the most abundant element in the Universe and participates in stellar nuclear processes. On Earth much of it is chemically combined in water and organic substances, with little free hydrogen in air. Elemental abundance therefore differs from the availability of directly usable pure hydrogen gas.
References: [1]
Industrial feedstock and energy carrier
Hydrogen supports ammonia production, refining and other manufacturing, and fuel cells can recover useful energy from it. Production through reforming or electrolysis requires inputs and creates system-dependent emissions. Water at the point of use does not by itself describe the environmental performance of production, storage and transport.