Last update: April 11, 2025

Hydrogen: An Invisible Element with Very Real Effects

History of the Discovery of Hydrogen

As early as the 17th century, chemists observed that a flammable gas was released during the reaction of a metal with an acid. In 1766, Henry Cavendish (1731-1810) isolated this gas and called it "flammable air," demonstrating that it produced water when burned. In 1783, Antoine Lavoisier (1743-1794) correctly interpreted Cavendish's results and demonstrated that water is a compound, not an element. He named this gas hydrogen (from the Greek hydro = water and genes = create).

Structure and Fundamental Properties

Hydrogen (symbol H, atomic number 1) is the simplest chemical element, consisting of a single proton and electron, known as protium (¹H). Other isotopes exist: deuterium \(\,^{2}\mathrm{H}\), tritium \(\,^{3}\mathrm{H}\), \(\,^{4}\mathrm{H}\)...
At room temperature, hydrogen exists as a diatomic gas (H₂), extremely light (density ≈ 0.08988 g/L), colorless, odorless, and highly flammable. The temperature at which the liquid and solid states of hydrogen can coexist in equilibrium (melting point): 13.99 K (−259.16 °C). The temperature at which it transitions from liquid to gas (boiling point): 20.271 K (−252.879 °C).

Table of Hydrogen Isotopes

Hydrogen isotopes (key physical properties)
Isotope / Notation	Protons (Z)	Neutrons (N)	Atomic mass (u)	Natural abundance	Half-life / Stability	Decay / Remarks
Protium — \(\,^{1}\mathrm{H}\,\)	1	0	1.007825 u	≈ 99.985 %	Stable	Nucleus reduced to a proton; basis of atomic hydrogen.
Deuterium — \(\,^{2}\mathrm{H}\) (D)	1	1	2.014102 u	≈ 0.0156 %	Stable	One proton + one neutron; bound nucleus, used in NMR and fusion.
Tritium — \(\,^{3}\mathrm{H}\) (T)	1	2	3.016049 u	Trace	12.32 years	Radioactive β\(^-\) decay to \(\,^{3}\mathrm{He}\). Produced in reactors and used for D–T fusion.
Extreme neutron isotopes — \(\,^{4}\mathrm{H},\,^{5}\mathrm{H},\,^{6}\mathrm{H},\,^{7}\mathrm{H}\)	1	3 — 6	— (resonances)	Unnatural	\(10^{-22}\) — \(10^{-21}\) s	Very unstable states observed in laboratories; immediate decay by neutron emission.

Chemical Reactivity

Hydrogen is a powerful reducing agent and forms chemical bonds with many elements: halogens, oxygen, sulfur, metals, etc. It forms hydrides and can act as an acid (proton donor) or a base (proton acceptor) depending on the context. Hydrogen is involved in the reduction of metal oxides by releasing a proton when acting as an acid, and in the hydrogenation of organic compounds by capturing a proton when acting as a base.

Industrial and Technological Applications of Hydrogen

To produce ammonia, a basic product used in fertilizers;
To clean and improve the quality of fuels in refineries;
In fuel cells, which directly convert hydrogen into electricity;
As an ultra-cold fuel to propel rockets;
As a clean energy source in an economy aiming to reduce CO₂ emissions;
To hydrogenate oils and fats, for example in the production of margarine;
To produce methanol, used in plastics, solvents, and chemicals;
As a chemical reducing agent to transform certain metallic or non-metallic compounds;
As fuel for vehicles (cars, buses, trains, or boats) with near-zero emissions;
To store excess electricity produced by solar or wind energy;
In cryogenics and research on materials at very low temperatures;
As experimental fuel in nuclear fusion projects (deuterium and tritium);
In NMR imaging, with the deuterium isotope to trace certain molecules or in deuterated solvents;
As a protective gas to prevent the oxidation of sensitive chemical compounds.

Role in Astrophysics and Cosmology

Hydrogen accounts for about 75% of the baryonic mass of the universe. It was synthesized in large quantities during the Big Bang. In stars, it serves as fuel for thermonuclear fusion reactions via the proton-proton cycle or the CNO cycle. In the interstellar medium, it is found in atomic (H I), molecular (H₂), or ionized (H⁺) forms. Its 21 cm line is a major tool in radio astronomy for mapping the galactic structure.

The hydrogen atom is the simplest quantum system and serves as a model for testing the predictions of quantum mechanics and quantum electrodynamics (QED). Its electronic spectrum, which is very well measured, allows constraints to be placed on fundamental constants and explores hypotheses about the variation of these constants over time or space.

N.B.:
The 21 cm line is a radio signal emitted by neutral hydrogen in space. It occurs when a slight change in the orientation of the spins of the proton and electron in the hydrogen atom releases a photon. Although this transition is rare and very weak, it is very useful for astronomers to "see" the distribution of hydrogen in our galaxy and nearby galaxies, as it easily passes through dust clouds that block visible light.