Hydrogen (symbol H, from the Greek ὕδωρ, hýdor, "water", plus the root γεν-, ghen-, "generate", therefore "water generator") is the first chemical element of the periodic table (atomic number 1 ) and the lightest. It is the most abundant element in the observable universe and its most common isotope, the great uncle, consists of a proton, which forms the nucleus, and an electron. Being the simplest atom, it has been studied in depth by quantum mechanics.
In the free state, at atmospheric pressure and room temperature (298 K), it is found in the form of diatomic gas having the formula H2, colorless, odorless, tasteless and highly flammable, with a boiling point of 20.27 K and a melting point 14.02 K. In the bound state it is present in water (11.19%) and in all organic compounds and living organisms; moreover it is occluded in some rocks, such as granite, and forms compounds with most of the elements, often also by direct synthesis.
It is the main constituent of the stars, where it is present in the plasma state and represents the fuel of thermonuclear reactions, while on Earth it is scarcely present in the free and molecular state and must therefore be produced for its various uses; in particular it is used in the production of ammonia, in the hydrogenation of vegetable oils, in aeronautics (formerly in airships), as an alternative fuel and more recently as an energy reserve in fuel cells.
Gaseous diatomic hydrogen H2 was first described formally by Theophrastus Von Hohenheim (known as Paracelsus, 1493-1541), who obtained it artificially by mixing metals with strong acids. Paracelsus did not realize that the flammable gas obtained in these chemical reactions was made up of a new chemical element, later called hydrogen. In 1671, Robert Boyle rediscovered and described the reaction that took place when iron filings and diluted acids were mixed, and which generated H2.
In 1766, Henry Cavendish was the first to recognize gaseous molecular hydrogen H2 as a discrete substance, identifying the gas produced in the metal-acid reaction as "flammable air" and discovering that the combustion of the gas generated water. Cavendish used acids and mercury in these experiments and erroneously came to the conclusion that dihydrogen was a substance freed from mercury and not acid, but he was able to accurately describe many fundamental properties of hydrogen and dihydrogen. Traditionally, Cavendish is regarded as the discoverer of hydrogen.
In 1783, Antoine Lavoisier gave the element the name of "hydrogen" (in French Hydrogène, from the Greek ὕδωρ, ὕδᾰτος, "water" and γένος-ου, "generator") when he tried (together with Laplace) Cavendish's discovery that the combustion of hydrogen generated water.
One of the first uses of hydrogen was as a filler gas for balloons and later for other types of airships. Famous is the tragedy of the Hindenburg airship, which took place despite the fact that the engineers had coated the structure of the airship so as not to cause sparks, since the flammability of the gas was known. That was a particular case of use, since helium was not available, almost as light but inert gas. At the time, molecular hydrogen was obtained from the reaction of sulfuric acid with iron.
Hydrogen is the only element whose best-known isotopes are given specific names: great-uncle, the most common isotope, has no neutrons; deuterium has one neutron and tritium (radioactive) has two neutrons. The two stable isotopes are great-uncle (1H) and deuterium (2H, D). The 4H, 5H and 6H isotopes were also observed.
Dihydrogen is a substance with formula H2, its molecules being made up of two hydrogen atoms. In condi