Hydrogen

Hydrogen is the simplest element there is and the most abundant in the universe, making up roughly three quarters of all baryonic matter. Its atom is as sparse as an atom can be — one proton, one electron — which is why it became the model against which the whole quantum theory of atomic structure was tested.

On Earth the picture is different. There is almost no free hydrogen in the atmosphere: it is too light for the planet's gravity to hold. Bound up in other substances, though, it is everywhere — in water above all, and in every organic compound.

The main terrestrial reservoir of hydrogen is water. Two atoms of it accompany every atom of oxygen in the ocean, and by number of atoms hydrogen is second only to oxygen in the Earth's crust and hydrosphere.

The second reservoir is organic matter: oil, natural gas, coal, and everything alive. Methane, CH₄, carries four hydrogen atoms for every carbon, which is why natural gas remains the main industrial feedstock for producing it.

Free hydrogen turns up in volcanic gases and in the upper atmosphere, from which it escapes steadily into space.

The gas given off when acids attack metals was noticed as early as the sixteenth century — Paracelsus described the "air" that bursts out when iron dissolves in oil of vitriol. But inflammable air was treated as a curiosity.

Henry Cavendish established it as a distinct substance in 1766. He collected the gas, measured its density, and showed that burning it produced water — which meant water was no longer an element. The name, coined by Lavoisier in 1783, says exactly that: hydrogen, the thing that gives birth to water.

Hydrogen sits awkwardly in the periodic table. It is placed in group 1, but it is not an alkali metal. It forms both the cation H⁺, giving up its only electron, and the anion H⁻, completing its shell to match helium. Nothing else does both.

The H₂ molecule is strong, and at room temperature hydrogen is fairly unreactive. Heat it, or add a catalyst, and it reacts with almost anything: explosively with oxygen, with nitrogen to give ammonia, with metals to form hydrides.

The hydrogen bond deserves separate mention. It is twenty times weaker than an ordinary covalent bond, and yet it is what keeps water liquid at room temperature and what holds the two strands of DNA together. Biochemistry as we know it is impossible without it.

More than half of all hydrogen produced goes into making ammonia — which is to say, in the end, into fertiliser. Much of the rest goes to hydrotreating petroleum products and to hydrogenating fats.

As a fuel, hydrogen is attractive because burning it produces water. The difficulty is not the burning but the making: most hydrogen today comes from natural gas, and the carbon dioxide is released anyway — at the plant rather than in the engine.