Sodium is extremely abundant, composing about 3 percent of the Earth's crust, but it is never found in its pure form as sodium metal. This is because of its extreme reactivity, with exposure to air, water and many other substances causing often rapid and very energetic reactions. As such, it is found naturally only in compounds with other elements, such as sodium chloride, also known as table salt, sodium borate, also known as borax, and sodium carbonate, also known as soda.
Pure sodium is a relatively light, silvery and soft metal. This means it is found more frequently in the lighter granites that compose the continental crusts and less frequently in the heavier basalts of oceanic crusts. Pure sodium is produced via chemical processes from sodium compounds, but it is both dangerous and difficult to handle and store. It is the second-lightest alkali metal after lithium. It begins tarnishing immediately upon exposure to air, reacting to both the oxygen and water vapor. It reacts explosively when exposed to liquid water, and it burns rapidly when sufficiently heated in air.
Sodium is an extremely important element for life. One of its ionic compounds, sodium chloride, is an essential solute in the fluids of every life form.