Hydrogen was identified as a distinct substance in the 18th century. As early as the 17th century, scientists such as Robert Boyle observed that an inflammable gas was released during the reaction of a metal with an acid. However, it was not until 1766 that Henry Cavendish isolated this gas and studied it systematically. He called it "inflammable air" and demonstrated that it produced water upon combustion.
In 1783, Antoine Lavoisier correctly interpreted Cavendish's results and demonstrated that water is a compound, not an element. He named this gas hydrogen, meaning "water-forming" (from the Greek hydro = water and genes = create). This discovery was crucial in the advent of modern chemistry.
Hydrogen (symbol H, atomic number 1) is the simplest chemical element, consisting of a single proton and an electron. Its most common isotope, protium (¹H), has no neutrons. Two other isotopes exist: deuterium (²H), which is stable and naturally present in small proportions, and tritium (³H), which is radioactive with a half-life of approximately 12.3 years.
At room temperature, hydrogen is found as a diatomic gas (H₂), extremely light (density ≈ 0.08988 g/L), colorless, odorless, and highly flammable. It melts at 13.99 K and boils at 20.27 K.
Hydrogen is a powerful reducing agent and forms chemical bonds with many elements: halogens, oxygen, sulfur, metals, etc. It forms hydrides and can behave as an acid (proton donor) or a base depending on the context. It is notably used for the reduction of metal oxides or the hydrogenation of organic compounds.
Hydrogen is used:
Hydrogen represents approximately 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⁺) form. Its 21 cm line is a major tool in radio astronomy for mapping 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 (Lyman, Balmer series...), allows constraints to be placed on fundamental constants and explores hypotheses about the variation of these constants over time or space.
