With the expected increasing significance of hydrogen as a universal chemical and as an energy vector, its physical and thermodynamic properties are undergoing extensive investigation. To provide a basis of understanding for the themes covered in the remainder of the book, this chapter briefly describes the fundamental properties of hydrogen.

Discovery and occurrence

Named by a French chemist, Lavoisier, hydrogen (H) is the first chemical element of the periodic table of elements with an atomic number of one. At standard temperature and pressure, hydrogen is a colourless, tasteless, odourless and easily flammable gas. With its atomic mass of 1.00797 g/mol, hydrogen is the lightest element. The British scientist, Henry Cavendish, was the first to identify H as a distinct element in 1766, publishing precise values for its specific weight and density (NHA, 2007).

Hydrogen is also one of the most abundant chemical elements in the Universe (70–80 wt.% H2 content); more than 50 wt.% of the Sun consists of hydrogen. However, on Earth it mostly occurrs naturally in the form of chemical compounds, most frequently water and hydrocarbons. As a gas in its free state, hydrogen is very rare (1 ppm by volume in the Earth's atmosphere), owing to its light weight, and it can only be found in natural gas and some volcanic gases, as well as trapped in small quantities in some minerals and rocks (Ullmann, 2003).