HYDROGEN

Hydrogen is the natural element in the Universe and is a major constituent of stars. It is relatively much less common on Earth but nevertheless forms nearly 1% by mass of the oceans and crust, principally as water and in hydroxide and hydrates minerals of the crust.

The dihydrogen molecule H2 is the stable form of the element under simple conditions, although atomic hydrogen may be made in the gas phase at high temperatures, and hydrogen may convert a metallic liquid or solid at extremely high pressures. At 1 bar pressure, dihydrogen condenses to a liquid at 20 K and solidifies at 14 K, these being the minimum boiling and melting points for any substance except helium. The H-H bond has a dissociation enthalpy of 436 kJ mol-1 and a length of 74 pm. That is the shortest bond known, and one of the strongest single covalent bonds.

Although it is thermodynamically capable of reacting with many compounds and elements, these reactions usually have a large kinetic barrier and needs elevated temperatures and/or the use of catalysts.

Hydrides of metals

Not every metallic elements form hydrides. Those that do will be classified as follows.

• Highly electropositive metals have solid hydrides regarded as containing the H- ion. They have structures same to halides, although the ionic character of hydrides is undoubtedly much lower. As like LiH (rocksalt structure) and MgH2 (rutile structure ;).

• Some f- and d- block elements form hydrides that are often metallic in nature, and of variable (nonstoichiometric) composition. As like TiH2 and CeH2+x.

• Some heavier p-block metals form molecular hydrides similar to those of nonmetals in the same group, stannane (SnH4) being and digallane (Ga2H6), both of very low stability.