Hydrogen (H)

Hydrogen is the only atom that physics can solve exactly. One proton, one electron, no electron-electron repulsion to spoil the algebra — the Schrödinger equation for hydrogen has a closed-form analytic solution, and for helium and everything beyond it does not. Every energy level diagram in every chemistry textbook is either a hydrogen solution or an approximation scaffolded on one. That is a strange thing for the simplest element to be responsible for, and it is the reason hydrogen matters far beyond its own chemistry.

Balmer's guess and the model that worked once

In 1885 Johann Balmer, a Swiss schoolteacher with no particular standing in physics, noticed that the four visible lines in hydrogen's spectrum fitted a small whole-number formula. He had no idea why. Johannes Rydberg generalised it three years later, still without a mechanism. It sat there as numerology for twenty-eight years.

Then Niels Bohr, in 1913, assumed the electron could only occupy orbits with quantised angular momentum, and out fell the Rydberg constant — not fitted, but derived from the electron mass, the elementary charge and Planck's constant. It was one of the most convincing calculations of the century.

It was also very nearly the model's only success. Applied to helium, with two electrons, the Bohr model gives the wrong answer, and no patching fixed it. The gap between "works perfectly for hydrogen" and "fails immediately for the next element up" is what forced the full quantum mechanics of Heisenberg and Schrödinger a decade later. Hydrogen was simultaneously the evidence for the old quantum theory and the proof that it was insufficient.

The 21-centimetre line

There is a transition in hydrogen so faint that any individual atom will wait around eleven million years to make it, and it is the single most useful signal in radio astronomy.

In a hydrogen atom, the proton's spin and the electron's spin can be aligned or opposed. Flipping between the two states releases a photon with a wavelength of 21 centimetres. The transition is forbidden by the usual selection rules, hence the absurd lifetime — but interstellar space contains such colossal numbers of hydrogen atoms that the sky glows steadily at that wavelength. It was predicted by Hendrik van de Hulst in 1944, under German occupation in the Netherlands, and detected in 1951. Because 21 cm radiation passes straight through the dust that blocks visible light, it is how the spiral structure of our own galaxy was first mapped, and how the rotation curves that gave away dark matter were measured.

Inflammable air, and the man who named it properly

Henry Cavendish is the conventional credit, and the 1766 date is his paper On Factitious Airs. He was not the first person to make the gas — Paracelsus in the sixteenth century and Robert Boyle in 1671 both saw metals and acid produce something flammable — but Cavendish was the first to treat it as a distinct substance with a measurable density, and the first to show, around 1781, that burning it produced water and nothing else.

Cavendish, a committed phlogistonist, did not think he had made water from two elements. Antoine Lavoisier did, and in 1783 he coined hydro-genes, the water-former. The name is unusually honest: hydrogen really does have exactly that job in Lavoisier's system, and the symbol H agrees with it in every major language.

The only element whose isotopes have their own names

Deuterium and tritium are the only isotopes anywhere on the periodic table with individual names and individual chemical symbols. Nobody speaks of "carbon-13" as anything but carbon-13, but D and T are legitimate symbols that appear in structural formulae.

The reason is mass. Going from hydrogen-1 to deuterium doubles the mass of the nucleus, and doubles it again by half for tritium. No other element can change its mass by 100% by gaining a neutron, and the consequence is that hydrogen isotopes have genuinely different chemistry — bonds to deuterium break measurably more slowly, an effect large enough that heavy water is toxic to mammals at high replacement fractions and that pharmaceutical chemists now deliberately deuterate drug molecules to slow their metabolism. Harold Urey found deuterium in 1931 by distilling liquid hydrogen and looking for shifted spectral lines; he had the Nobel three years later.

The standard atomic weight is quoted as an interval rather than a single figure precisely because the deuterium fraction is not constant. Ocean water, Antarctic ice and commercial tank hydrogen genuinely differ, and the differences are large enough to be used as a tracer of where a water sample has been.

The colour chart is about the feedstock, not the gas

Almost all of the hydrogen the world uses is not burned for energy. The two dominant consumers are ammonia synthesis, which feeds the nitrogen fertiliser industry, and oil refining, where hydrogen is used to crack heavy fractions and to strip sulfur out of fuels. Methanol production takes most of the rest.

Nor is it mostly made by splitting water. The overwhelming majority comes from steam methane reforming — natural gas plus steam over a catalyst — with coal gasification supplying much of the remainder in China. This is what the colour vocabulary is tracking: grey hydrogen is reformed from methane with the carbon dioxide vented, blue is the same process with capture bolted on, turquoise is methane pyrolysis to solid carbon, and green is electrolysis run on renewable electricity. The molecules are identical in every case. The colour describes the accounting.

Hydrogen's awkwardness as a fuel is a volume problem, not an energy problem. Per kilogram it carries roughly three times the energy of petrol. Per litre, at any pressure a vehicle can reasonably carry, it carries far less — which is why the argument keeps returning to storage tanks rather than to combustion.

Metallic, in principle

Eugene Wigner and Hillard Bell Huntington predicted in 1935 that hydrogen squeezed hard enough would become a metal. Jupiter and Saturn almost certainly contain oceans of the stuff, and the circulating metallic hydrogen in Jupiter's interior is the best explanation for a magnetic field some twenty thousand times stronger than Earth's.

Making it in a laboratory is another matter. A Harvard group led by Ranga Dias and Isaac Silvera announced solid metallic hydrogen in a diamond anvil cell in 2017; the claim was contested immediately on the grounds that the reflectivity could have come from the alumina coating on the diamonds, and the sample was destroyed before anyone independent could examine it. The result has never been cleanly replicated, and the question is still open.

Why there is none of it in the air

Hydrogen is somewhere around 90% of the atoms in the universe by number and roughly three quarters of its ordinary matter by mass. Earth's atmosphere contains it at well under one part per million.

The reason is escape velocity. At the temperatures of the upper atmosphere, a molecule of hydrogen sits at the fast end of the speed distribution, and Earth's gravity cannot retain it over geological time. Any free hydrogen released at the surface eventually leaves the planet. What remains is chemically bound — in water above all, and in every hydrocarbon and every protein — and that is the only form in which Earth has ever had a meaningful supply.