- Hydrogen (H) is the lightest and most abundant chemical element in the universe, occupying the first position in the periodic table with an atomic number of 1.
- It has the simplest atomic structure of all elements, consisting of a single proton in its nucleus and, in its most common isotope protium (¹H), one electron orbiting the nucleus.
- Because of this simplicity, hydrogen is often considered the fundamental building block of matter, playing a pivotal role in the evolution of the universe.
- Hydrogen is a colorless, odorless, tasteless, and highly flammable diatomic gas (H₂) under standard temperature and pressure.
- It exists in three primary isotopic forms: protium (¹H), deuterium (²H or D, which contains one neutron), and tritium (³H or T, which contains two neutrons and is radioactive).
- In nature, hydrogen is the most plentiful element, constituting roughly 75% of the elemental mass of the universe.
- It is the primary fuel for the thermonuclear fusion reactions that power stars, including our Sun. In stellar cores, hydrogen nuclei fuse to form helium, releasing enormous amounts of energy in the form of light and heat.
- On Earth, however, hydrogen rarely exists as a free element because it is so reactive; instead, it is most commonly found in combination with other elements, such as oxygen in water (H₂O) and carbon in a vast array of organic compounds. Its chemical reactivity makes it central to both inorganic and organic chemistry, with important roles in acid–base chemistry, reduction–oxidation processes, and as a reducing agent in various industrial applications.
- Hydrogen was first formally recognized as a distinct substance by the British scientist Henry Cavendish in 1766, who described it as “inflammable air” because of its tendency to combust and produce water when burned in air—a reaction later understood as 2H₂ + O₂ → 2H₂O. French chemist Antoine Lavoisier subsequently named the element “hydrogen” in 1783, derived from the Greek words hydro (water) and genes (creator), reflecting its water-forming property.
- Today, hydrogen is produced mainly through steam methane reforming (SMR) and electrolysis of water, and it serves in the production of ammonia via the Haber–Bosch process, refining of petroleum, hydrogenation of fats and oils, and in emerging energy technologies like hydrogen fuel cells for clean energy production.
- Hydrogen’s small molecular size and low density present both challenges and opportunities. Its high energy content per unit mass makes it an attractive fuel, but its low boiling point (−252.87 °C) and tendency to diffuse through materials complicate storage and transportation. In cryogenic form, hydrogen serves as rocket fuel, famously powering NASA’s Space Shuttle main engines.
- In the context of environmental sustainability, hydrogen is considered a promising clean energy carrier because its combustion produces only water vapor, with no carbon dioxide emissions. However, the environmental benefits depend heavily on the production method—“green hydrogen” produced from renewable-powered electrolysis is favored over “grey hydrogen” from fossil fuels, which can release significant greenhouse gases unless carbon capture is implemented.
