Sodium. Chunks of the reactive metallic element sodium (Na). The shiny surfaces have been exposed by the knife cutting the metal. Sodium is a soft alkali metal that reacts rapidly with the water and oxygen in the air. This will tarnish the shiny surfaces as the oxide and hydroxide form. Sodium is not found in its elemental form in nature, due to it being highly reactive, and is mainly found in the salt (sodium chloride) of salt deposits and sea water. Sodium is obtained by the electrolysis of sodium chloride. Numerous sodium compounds are manufactured by the chemical industry and have wide-ranging uses.