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).
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).
| 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. |
N.B. :
Electron shells: How electrons organize around the nucleus.
Hydrogen has only 1 electron distributed in a single electron shell. Its complete electronic configuration is: 1s¹, in the K shell. Hydrogen is the simplest element in the periodic table.
K Shell (n=1): Contains only 1 electron in the 1s sub-shell. This single shell is incomplete, as it can hold up to 2 electrons. Therefore, 1 electron is missing to saturate this first shell and reach the stable configuration of helium.
The single electron (1s¹) is the valence electron of hydrogen. This configuration explains its unique chemical properties:
By losing its 1s electron, hydrogen forms the H⁺ ion (oxidation state +1), which is actually just an isolated proton. This state is common in acids and aqueous solutions.
By gaining 1 electron, hydrogen forms the hydride ion H⁻ (oxidation state -1), present in metallic hydrides such as NaH or LiH, thus adopting the stable configuration of helium [He].
The oxidation state 0 corresponds to dihydrogen H₂, its natural molecular form, where two hydrogen atoms share a pair of electrons forming a simple covalent bond.
The electronic configuration of hydrogen, with its single 1s electron, gives it a unique and ambiguous position in the periodic table. This structure gives it exceptional characteristic properties: hydrogen can both lose its electron (like alkali metals) or gain one (like halogens), giving it dual chemical behavior. However, unlike alkali metals, hydrogen is a non-metal under normal conditions and mainly forms covalent bonds rather than ionic bonds. The H⁺ ion is extremely small (just a proton) and never exists isolated in solution; it is always associated with water molecules forming the hydronium ion H₃O⁺.
Dihydrogen H₂ is a colorless, odorless, extremely light gas (the lightest molecule that exists), and highly flammable. Its combustion with oxygen produces only water, making it an ideal clean fuel. Hydrogen forms covalent bonds with virtually all non-metals and can form hydrogen bonds, weak but essential interactions for many biological and chemical phenomena.
The importance of hydrogen is absolutely fundamental and universal: hydrogen is the most abundant element in the universe (about 75% of the baryonic mass), the main constituent of stars where it fuses to form helium and release solar energy; it is essential to all organic chemistry and life, present in water H₂O, in all organic compounds, acids, and bases; hydrogen bonds stabilize the structure of DNA, proteins, and determine the unique properties of water; industrially, hydrogen is massively used in the Haber-Bosch process to produce ammonia NH₃ (the basis of fertilizers), in petroleum refining (hydrogenation), methanol production, and the synthesis of many chemicals; hydrogen is considered the energy carrier of the future for a decarbonized economy: fuel cells convert hydrogen into electricity with water as the only byproduct, hydrogen-powered vehicles are developing, and hydrogen can store excess renewable energy; in metallurgy, hydrogen serves as a reducing agent; three natural isotopes exist: protium ¹H (99.98%), deuterium ²H or D (0.02%, used as a tracer and in nuclear fusion), and tritium ³H or T (radioactive, used in dating and fusion research).
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.
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.
