PTL:Hydrogen
From ChemEd Collaborative
Hydrogen, the lightest element, exists as diatomic molecules. In the solid state the element has a hexagonal closest packed structure.
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Discovered
Hydrogen was prepared many years before it was recognized as a distinct substance by Cavendish in 1766. It was given its name by Lavoisier in 1783.
Name
The name is derived from the Greek words, hydro and genes, meaning water forming; the symbol H is derived from the name.
Characteristics
Hydrogen is the most abundant element in the universe.
The element has three isotopes:
H = hydrogen (H, protium), 1.0078 amu
D = deuterium, D, 2.0141 amu
T = tritium, T, 3.0160 amu (radioactive)
Hydrogen is the most abundant of all elements in the universe. It has been estimated that hydrogen makes up more than 90% of all the atoms or three-quarters of the mass of the universe. It is found in the sun and most stars, and plays an important part in the proton-proton reaction and carbon-nitrogen cycle, which accounts for the energy of the sun and stars.
Found
Hydrogen is a major constituent of some planets, such as Jupiter; the core of this planet may consist of solid hydrogen, at least in part, and liquid hydrogen may be present elsewhere on the planet.
On earth, hydrogen occurs chiefly in combination with oxygen in water, but it is also present in organic matter, such as living plants, petroleum, coal, etc. It is the lightest of all gases, and combines with other elements, sometimes explosively, to form compounds (reaction of hydrogen with oxygen).
Prepared
Hydrogen is prepared by the action of steam on heated carbon,
C(s) + H2O(g) 
H2(g) + CO(g)
decomposition of certain hydrocarbons with steam,
CH4(g) + H2O(g) 3H2(g) + CO(g)
by the electrolysis of water,
2H2O(liq) + electrical energy 2H2(g) + O2(g)
Reactions
by the displacement from acids by certain metals,
Zn + 2HCl H2(g) + ZnCl2
or by the action of sodium or potassium hydroxide on aluminum.
Al + NaOH H2(g) + Al(OH)3
Uses
H2 gas is generally synthesized by the steam reformation of hydrocarbons.
Liquid H2 is used as a fuel in the [Space Shuttle].
Liquid hydrogen is important in cryogenics and in the study of super-conductivity as its boiling point is only a few degrees above absolute zero.
Great quantities of hydrogen are required commercially for the fixation of nitrogen from the air in the Haber ammonia process and for the hydrogenation of fats and oils. It is also used in large quantities in methanol production, in hydrodealkylation, hydrocracking, and hydrodesulfurization.
It is used as a [rocket fuel], for welding, for production of hydrochloric acid, for the reduction of metallic ores, and for filling balloons. At one time hydrogen was used for [dirigibles], but now the inert gas helium is preferred because it is inflammable. Production of hydrogen in the U.S. alone now amounts to hundreds of millions of cubic feet per day.
Isotopes
A stable isotope, deuterium, with an atomic weight of 2 was discovered by Urey and G. M. Murphy in 1932, although its existence had been suspected for some years before. Two years later an unstable isotope, tritium, with an atomic weight of 3 was discovered.
Tritium is radioactive and has a half-life of 12.33 years. There are about 6000 ordinary hydrogen atoms for each atom of deuterium. Tritium atoms are also present in nature, but in much smaller proportion. Tritium is readily produced in nuclear reactors, and is used in the production of hydrogen bombs. It is also used as a radioactive agent in making luminous paints.
Water whose molecules consist of two atoms of deuterium and one of oxygen is known as heavy water; it was also discovered by Urey and was first obtained in nearly pure form by Lewis. Heavy water, which is used as a moderator to slow down neutrons, is available without permit at a cost of 6 cents to 1 dollar/gm, depending on quantity and purity.
Emission Spectrum
This is a picture of the visible emission spectrum for hydrogen.
