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Element Fact
Cesium is used in atomic clock. As in 1999, the best clock is accurate to a second in 20 million years.

Hydrogen, H

hydrogen sample

Atomic number: 1
Atomic mass: 1.00794(7)
Natural abundance: 1520 ppm
1H (1.0078250) - 99.985%
2H (2.0141017) - 0.015%
3H (3.0160492) - 0.00%

History and Production

Derive from the Greek word Hydros genes, meaning "water forming". Discovered in 1766 by H. Cavendish at London, England. Produced mainly from the catalytic steam-hydrocarbon process using natural gas or oil-refinary feedstock.

H production

It is also produced as a byproduct of the brine elctrolysis process for the manufacture of chlorine and sodium hydroxide. It is used in large quantities in the production of ammonia (Haber process) and in the hydrogenation of fats and oils. It is also used in the reduction of metallic ores, and in a smaller quantities, as a rocket fuel and welding. Hydrogen has the potential to use as alternative clean energy to replace natural gas and fossil fuel.

The isotope 2H or deuterium (D) is also used, in the form of heavy water (D2O), as a moderator to slow down neutrons. Only about 1 ml of heavy water can be produced when 100 gallon of ordinary water is electrolysed into hydrogen and oxygen. It has a density of 1.107 g/cm3 and a boiling point of 101.4 C. One atom in every 1018 atoms of ordinary hydrogen is thought to be another isotope 3H or tritium, symbol T. However, it can be made artificially by bombardment of deuterium by deuterons.

Physical Data

Neutral, odorless, colorless gas. Exists as diatomic molecule, H2 in room temperature. It is the most abundant of all elements in the universe, which makes up more than 90% of all the atoms.

Interatomic distance: 74.6 pm
Melting point: -259.34C
Boiling point: -252.87C
Thermal conductivity/Wm-1K-1: 0.1815 (27C)
Density/kgm-3: 70.6 (-262C), 70.8 (b.p.), 0.08988 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 218 kJ/mol
Gibbs free energy of formation: 203.3 kJ/mol
Entropy: 114.7 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: 1s1
Term symbol: 2S1/2
Electron affinity: 72.7688 kJ/mol   Electronegativity (Pauline): 2.20
Ionization energy (first, second, third): 1312, -, - kJ/mol

Chemical property

The chemical behavior of hydrogen resembles the Group I (alkali metals) and Group VII (halogens) elements. It combines directly with most of the elements: with metals it forms hydride (H-) anions and covalently bonded to nonmetals (such as HCl, CH4). The properties of most acids in aqueous solutions arise from the presence of H+ ions (believed to be in combination with a water molecule to give hydroxonium ions, [H3O]+).
Chemically, deuterium behave the same as normal hydrogen. However, it differs slightly in properties which depend upon mass, for instance, rate of diffusion.

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