+ **Hydrogen** is element 1: one [[Proton]], one [[Electron]], and in its commonest form no [[Neutron]] at all. It is the simplest thing that can be called an [[atom]], the most abundant [[chemical element]] in the universe, and — because it is so light — almost absent from Earth's atmosphere, since anything that light escapes.+ ++ ## Properties++ | property | value |+ |---|---|+ | atomic number (Z) | 1 |+ | standard atomic weight | [1.007 84, 1.008 11] |+ | electron configuration | 1s¹ |+ | group / period / block | 1 / 1 / s |+ | melting point (H₂) | 13.99 K (−259.16 °C) |+ | boiling point (H₂) | 20.271 K (−252.879 °C) |+ | density (gas, STP) | 0.000082 g cm⁻³ — lowest of all gases |+ | first ionisation energy | 1312.0 kJ/mol |+ | H–H bond dissociation energy | 435.7 kJ/mol |+ | electronegativity (Pauling) | 2.20 |++ The atomic weight is written as an interval, not a single number, and that is deliberate. Hydrogen's isotope mix varies by where you scoop it up: seawater sits near 1.007 98, while H₂ gas from a Kansas natural-gas well came in at 1.007 8507. CIAAW publishes the range rather than pretend to a precision the element does not have.++ ## Three isotopes++ | isotope | name | neutrons | natural amount fraction | stability |+ |---|---|---|---|---|+ | ¹H | protium | 0 | [0.999 72, 0.999 99] | stable |+ | ²H | deuterium | 1 | [0.000 01, 0.000 28] | stable |+ | ³H | tritium | 2 | trace | β⁻, [[half-life]] 12.32 y |++ Deuterium is the only isotope of any element with its own name and symbol in routine use (D, and T for tritium). It earned that because doubling the mass of the lightest atom actually changes how fast it reacts — the kinetic isotope effect — which almost no other [[Isotope]] pair does. Heavy water is D₂O.++ Deuterium was found in 1931 by Harold Urey's group at Columbia, decades after the element itself, precisely because there is so little of it.++ ## Why the universe is mostly hydrogen++ Hydrogen-1 needs no assembly. A proton and an electron *is* a hydrogen atom, so when the early universe cooled enough for nuclei to hold electrons, hydrogen was what there was. Big Bang nucleosynthesis left roughly 75% ¹H and 25% ⁴He by mass, with a smear of deuterium and traces of lithium. Nothing heavier than boron came out of it.++ Since then, stars have been burning it back down. In the Sun's core, at about 15 million °C, hydrogen fuses into helium; the outward pressure from that reaction is the only thing holding the Sun up against its own gravity. Fourteen billion years of this has not made much of a dent: hydrogen is still about 74% of ordinary matter by mass, against 23–25% for helium.++ | environment | hydrogen share |+ |---|---|+ | ordinary matter, by mass | ~74% |+ | ordinary matter, by atom count | ~92% |+ | Solar System, hydrogen-1 by mass | 705,700 ppm |++ ## On Earth, it is always bonded to something++ Free H₂ floats away, so terrestrial hydrogen is locked in compounds — water above all, then every hydrocarbon and nearly every molecule in your body. Getting it back out costs energy, and that cost is the entire economics of hydrogen fuel.++ Global hydrogen demand reached **97 Mt in 2023**, up 2.5% on 2022. Nearly all of it goes to oil refining and chemicals, and nearly all of it is made from unabated fossil fuels. Low-emissions hydrogen — electrolysis, or fossil production with carbon capture — came to **less than 1 Mt** of that.++ - **Ammonia** via the Haber process, which is how roughly half the world's food gets its nitrogen.+ - **Refining** — removing sulfur from fuels.+ - **Hydrogenation** — turning oils into fats, e.g. margarine.+ - **Rocket fuel** — liquid H₂ with liquid oxygen powered the Saturn V upper stages and the Space Shuttle main engines.+ - **Fuel cells**, where H₂ and O₂ react to make electricity and water, with no carbon in the exhaust.++ The catch is upstream. Hydrogen is only as clean as whatever made it, and today that is mostly methane.++ ## Odd states++ - **Ortho and para.** The two protons in H₂ can have aligned or opposed spins. At room temperature the equilibrium gas is about 75% ortho, 25% para, and the two forms have measurably different thermal properties. Ortho sits 1.455 kJ/mol higher in energy.+ - **Metallic hydrogen.** Above roughly 400 GPa, hydrogen is predicted to conduct electricity like a metal. It is thought to exist deep inside Jupiter, and remains extremely hard to make in a laboratory.++ Related: [[Isotope]] · [[chemical element]] · [[periodic table]] · [[uranium]] · [[Atom]] · [[Energy]]
History of hydrogen
back to the page · 1 revision · nothing here is ever deleted