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Emission spectrum of an ultraviolet deuterium arc lamp clearly showing characteristic hydrogen emission lines (sharp peaks at 656 nm and 486 nm) and continuum emission in the ~200-400 nm region. The emission spectrum of deuterium differs from that of protium due to the influence of hyperfine interactions, though these effects alter the wavelength of the lines by mere fractions of a nanometer and are too fine to be discerned by the spectrometer used here.Hydrogen fuel cells are being investigated as mobile power sources with lower emissions than hydrogen-burning internal combustion engines. The low emissions of hydrogen in internal combustion engines and fuel cells are currently offset by the pollution created by hydrogen production. This may change if the substantial amounts of electricity required for water electrolysis can be generated primarily from low pollution sources such as solar energy or wind. Research is being conducted on H2 as a replacement for fossil fuels. It could become the link between a range of energy sources, carriers and storage. H2 can be converted to and from electricity (solving the electricity storage and transport issues), from biofuels, and from and into natural gas and diesel fuel. All of this can theoretically be achieved with zero emissions of CO2 and toxic pollutants. (See also Hydrogen economy.)

In the Haber process for the production of ammonia and the world's fifth most produced industrial compound, hydrogen is generated in situ from natural gas.

History
Discovery of H2
H2 was first produced by Theophrastus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus—by mixing metals with acids. He was unaware that the inflammable gas produced by this chemical reaction was H2. In 1671, Robert Boyle described the reaction between iron filings and dilute acids, which results in the production of H2.[2] In 1766, Henry Cavendish was the first to recognize H2 as a discrete substance, by identifying the gas from this reaction as "inflammable" and finding that the gas produces water when burned in air. Cavendish stumbled on H2 when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a compound of mercury—and not of the acid—he was still able to accurately describe several key properties of hydrogen.

Antoine Lavoisier gave the element its name and proved that water is composed of hydrogen and oxygen. One of the first uses of H2 was for balloons. The H2 was obtained by reacting sulfuric acid and metallic iron.

Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom has been central to the development of the theory of atomic structure.

Isotopes of hydrogen
In 1931, Harold C. Urey discovered deuterium, an isotope of hydrogen, by spectrographic study of the last residual milliliter after evaporation of 5 liters of cryogenically-produced liquid hydrogen. Urey was also able to concentrate deuterium in water by repeated fractional distillation. For the discovery of deuterium Urey received the Nobel Prize in Chemistry in 1934. In the same year, the discovery of the third isotope, tritium, was announced.

Electron energy levels
The ground state energy level of the electron in a hydrogen atom is 13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm.

With the Bohr Model the energy levels of hydrogen can be calculated fairly accurately. This is done by modeling the electron as revolving around the proton, much like the earth revolving around the sun, except that the sun holds earth in orbit with the force of gravity, but the proton holds the electron in orbit with the force of electromagnetism. Another difference between the Earth-Sun system and the electron-proton system is that, in this model, due to quantum mechanics the electron is allowed to only be at very specific distances from the proton. Modeling the hydrogen atom in this fashion yields the correct energy levels and spectrum. As an added feature, modeling the system fully using the reduced mass of nucleus and electron (as one would do in the two-body problem in celestial mechanics) yields an even better formula for the hydrogen spectra, and also the correct spectral shifts for the isotopes deuterium and tritium, which are induced by changes only in this parameter.

The electronic ground state energy level is split into hyperfine structure levels because of magnetic effects due to the quantum mechanical spin of the electron and proton. The energy of the atom when the proton and electron spins are aligned is 5.9 x 10-6 eV higher than when they are not aligned. The transition from the upper to lower levels can occur through emission of a photon through a magnetic dipole transition. A photon of this energy has a frequency of 1420.4 MHz and a wavelength of 21.1 cm. Astronomers observe this radiation with radio telescopes in order to map the distribution of hydrogen in the Galaxy.

Occurrence

NGC 604, a giant H II region in the Triangulum Galaxy.Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. [3] This element is found in great abundance in stars and gas giant planets. However, it is very rare in the Earth's atmosphere (1 ppm by volume). Its scarcity is due to the fact that hydrogen is the lightest gas, allowing it to escape Earth's gravity. When compounds are included, though, hydrogen is the tenth most abundant element on Earth. 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 including coal, natural gas, and other fossil fuels. Methane (CH4) is an increasingly important source of hydrogen.

Throughout the universe, hydrogen is mostly found in the plasma state whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora.