Lithium (Greek lithos, stone); atomic weight 6.94. It is the lightest of all metals with a density only half that of water and was discovered in 1817 by Arfvedson in Sweden. The free form of lithium does not occur naturally but is produced by an electrolytic process from lithium chloride. Pure lithium is silvery like sodium and potassium and reacts with water, but less vigorously than sodium. Lithium imparts a bright red (carmine) colour to a flame. Lithium chloride is one of the most hygroscopic substances known (hence its use in air conditioning and industrial drying systems) and so has to be handled carefully when preparing accurate solutions.

Trace amounts of lithium are found in water from some springs and prior to 1950 it was used as a 'salt substitute' in the drink 7-Up. Some mineral waters have been found to have lithium concentrations approaching 1 mmol/l and levels of 0.1 mmol/l have been measured in water samples taken in Zanesville, Ohio (Journal of the American Medical Association 1970, 211: 1012).

For a recent review (and over 400 references) see Birch 1999.