Lithium. (Source: © Björn Wylezich / stock.adobe.com)

A chemical element with the atomic number 3. A soft, silvery lustrous and very light alkaline metal. Reacts violently with water and air. It is abundant on Earth in compounds or dissolved in seawater. Lithium is important for thermonuclear fusion because the capture of a neutron by lithium produces tritium, an essential component of fusion fuel.