(Symbol Na. ) A very light, soft, metallic element, atomic number 11, atomic mass 23; one of the most common metals on earth. It is alkaline, very reactive, and, due to its high activity, sodium is almost always found in chemical combination with other elements. It is a major component of minerals, ocean water, and marine aerosol particles. Sodium is present in atomic form in the upper atmosphere due to the ablation of meteors and forms a sodium layer at the approximately 80-km altitude. Sodium has a very strong, yellow, optical transition at 593 nm (sodium D-line). Emission from this line is seen in the airglow, following the reaction of sodium with ozone. The transition has also been used in ground-based lidar sounding applications to detect the passage of gravity waves.