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Hydrogen ( French for water-maker, from Greek hudôr, "water" and gennen, "generate") was first recognized as a distinct substance in 1776 by Henry Cavendish. Cavendish stumbled upon it when experimenting with acids and mercury. Altough he wrongly assumed that hydrogen was a compound of mercury (and not of the acid), he was still able to describe many of hydrogen's properties quite accurately. Antoine Lavoisier gave the element its name and proved that water was made of hydrogen and oxygen. One of its first uses was for balloons. The hydrogen was obtained by mixing sulfuric acid and iron.
Deuterium, one isotope of hydrogen, was discovered by Harold C. Urey by distilling a sample of water multiple times. Urey received a Nobel prize for his discovery in 1934. In the same year, the third isotope, tritium, was discovered.Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is found in great abundance in stars and gas giant planets. Relative to its great abundance elsewhere, hydrogen is very rare in the earth's atmosphere (1 ppm by volume). The most common source for this element on earth is water which is composed two parts hydrogen to one part oxygen (H2O). Other sources include most forms of organic matter (currently all known life forms), coal, fossil fuels and natural gas. Methane ( CH4), which is a byproduct of organic decay, is an increasingly important source of hydrogen.
Hydrogen can be prepared in several different ways: steam on heated carbon, hydrocarbon decomposition with heat, reaction of a strong base in an aqueous solution with aluminium, water electrolysis, or displacement from acids with certain metals.
Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700-1100 °C), steam reacts with methane to yield carbon monoxide and hydrogen.
Additional hydrogen can be recovered from the carbon monoxide through the water-gas shift reaction:
The lightest of all gases, hydrogen combines with most other elements to form compounds. Hydrogen has an electronegativity of 2.2, so it forms compounds where it is the more non-metallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H- ions or just as a solute within the other element (as in palladium hydride ). The latter tend to be covalent, since the H+ ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like hydronium (H3O+) as the protons latch on to something.
Hydrogen combines with oxygen to form water, H2O, and releases a lot of energy in doing so, burning explosively in air. Deuterium oxide, or D2O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and the study of the properties of these compounds is called organic chemistry.
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