Lightness is the outstanding and best known characteristic of aluminium. The metal has an atomic weight of 26.98 and a specific gravity of 2.70, approximately one-third the weight of other commonly used metals ; with the exception of titanium and magnesium. As with most metals the density decreases with increasing temperature. The addition of other metals in the amounts commonly used in aluminium alloys does not appreciably change the density (plus 3%, minus 2%), except in the case of Lithium alloys where the density of the alloy is reduced by up to 15%.
Weight is important for all applications involving motion. Saving weight results in more payload or greater economy of operation. Saving weight also saves energy, reduces vibration forces, improves the performance of reciprocating and moving parts, reduces tiredness when using manually operated equipment, offers lower shipping, handling and erection costs. Low weight combined with the high strength possible with special alloys has placed aluminium as the major material for aircraft construction for the past sixty years. Although purchased on a weight basis, metals are generally used on a volume basis, it is therefore important to compare the cost of aluminium with other materials on this basis.