Hydrogen

A hydrogen atom is one proton with one electron bound to it. That is the whole thing. In the commonest isotope, protium, there is no neutron at all, which makes it the only atom whose nucleus is a single particle. About one hydrogen atom in 6,400 is deuterium, carrying one neutron; tritium, with two, is radioactive and decays with a half-life of a little over twelve years.

Because it is the simplest, it is the one atom whose behaviour can be solved exactly. The spectral lines of hydrogen, the ladder of frequencies it absorbs and emits, were measured before anyone could explain them, and fitting them is what forced quantum mechanics into existence: Johann Balmer's formula of 1885 was numerology until Niels Bohr's model of 1913 gave it a reason, and Erwin Schrodinger's equation of 1926 gave it a proper one. Every account of atomic structure since is an approximation to a problem that hydrogen alone poses cleanly.

Hydrogen is roughly three quarters of the ordinary matter in the universe by mass, and effectively all of it was made in the first few minutes after the Big Bang. Stars consume it: the proton-proton chain that powers the Sun fuses hydrogen into helium, and that reaction is the source of very nearly all the light that has ever fallen on the Earth.

On Earth free hydrogen is almost absent. The molecule is light enough to escape the atmosphere entirely, so what remains is bound: into water, which is a ninth hydrogen by mass, into every hydrocarbon, and into every protein and nucleic acid in every living thing. Hydrogen bonds, the weak attractions between a hydrogen atom and a nearby oxygen or nitrogen, hold the double helix together and give water its unreasonably high boiling point. Without that one small anomaly the planet would have no liquid oceans.

Henry Cavendish reported in 1766 that dissolving metals in acid released a distinct gas, lighter than air, which burned to leave nothing behind but a dew. He called it inflammable air, and within the phlogiston chemistry of his day he took it to be phlogiston itself, or something close to it. He was working carefully and he was wrong, which is the ordinary condition of a good experiment inside a bad theory.

Antoine Lavoisier repeated the work, showed in 1783 that the dew was water and nothing else, and renamed the gas hydrogene, the water-former. That single result did more damage to the phlogiston doctrine than any argument, because water had been an element since Empedocles and was now demonstrably a compound. The four classical elements did not survive it.

It is worth being exact about what this means for correspondence. Hydrogen was not one of the seven metals. It was not one of the three principles. No alchemical symbol exists for it, because no alchemist ever isolated it or suspected it. Any chart assigning hydrogen a planet, a chakra or a sephira is doing so on modern authority, and the honest course is to say whose.

What can be said is that the substance alchemy called water, and treated as one of the four roots of everything, turns out to be two thirds hydrogen by count of atoms. That is not a vindication of the old scheme; the old scheme was wrong about water being simple. But it is a genuine point of contact between the two vocabularies, and it is more interesting than an invented correspondence would be.

Industrially, hydrogen now goes overwhelmingly into ammonia for fertiliser and into refining petroleum. Almost all of it is made from natural gas, a process that releases carbon dioxide, and the entire current argument about hydrogen as a clean fuel turns on that fact rather than on anything about the gas itself.