The element hydrogen is believed to be the most abundant element in the universe. It is the first element in the Periodic Table, containing just one proton in its nucleus. As an element, hydrogen exists as a diatomic molecule: H2 and is a gas at standard temperature and pressure.
Properties of Hydrogen
Hydrogen is a colourless, odourless, flammable gas. It is also much lighter than air, so a large proportion of elemental hydrogen produced on earth is lost from the atmosphere. Hydrogen has three common isotopes: Protium has one proton and no neutrons in its nucleus, so it has a mass number of 1; it makes up 99.985% of all hydrogen. Deuterium (D) has one proton and one neutron in its nucleus, and is sometimes called ‘heavy hydrogen’. Tritium has one proton and two neutrons in its nucleus, and is a radioactive isotope.
Compounds of Hydrogen
On earth, hydrogen is the third most abundant element in terms of number of atoms (behind oxygen and silicon), but ninth in terms of mass. Virtually all of this hydrogen is found combined in molecules, the most abundant being water (H2O). Hydrogen forms a large number of other compounds, however, being assigned an oxidation number of +1 or -1 depending on the electronegativity of the element to which it is bound.
Covalent Hydrogen Compounds
In its +1 oxidation state, hydrogen is found in binary compounds such as hydrogen chloride (HCl), hydrogen sulphide (H2S), and ammonia (NH3) in which it is covalently bonded to a more electronegative atom. Other compounds of hydrogen include acids like sulphuric acid (H2SO4) and nitric acid (HNO3); basic hydroxides like sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2); and acid salts like sodium hydrogen phosphate (NaH2PO4) and sodium hydrogen carbonate (NaHCO3). Additionally it is present in many organic compounds, covalently bonded to carbon (CH4, C2H6 etc).
Metal Hydrides
In its -1 oxidation state, hydrogen is found in compounds with metals, called hydrides. These are ionic when hydrogen is combined with alkali or alkaline-earth metals like sodium or potassium. It also forms covalent bonds with elements silicon and antimony. With transition metals, lathanides or actinides, the hydrogen seems to dissolve into the structure of the metal, giving a more brittle substance which is still a conductor or semiconductor.
Uses of Hydrogen
Large quantities of hydrogen are used in the Haber Process, which turns nitrogen from the air into ammonia used in fertilizers. It is also used in the hydrogenation of fats and oils, the production of methanol, hydrocracking and metal refining. It is also an important rocket fuel, used by NASA to launch the space shuttles. Deuterium is used in nuclear power stations as a moderator to slow neutrons down.
The Hydrogen Economy
The potential of hydrogen as a clean source of energy has led many people to talk about a ‘hydrogen economy’ to replace the present, hydrocarbon-based economy. Cars powered by fuel cells which use the energy stored in hydrogen, and only emitting water vapour are regularly promised, but unfortunately the technology has a long way to go. The main problem is the energy needed to produce the hydrogen – either from water, or from hydrocarbons themselves, which seems to defeat the object.
Sources:
John B Russell, “General Chemistry”, 1992, McGraw-Hill
“Hydrogen”, Chemicool Periodic Table, chemicool.com, Accessed 9th April 2010
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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