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A metallic element (symbol Na - Latin: natrium).
It was first isolated by Sir Humphry Davy in 1807. It's a soft, bright, silvery, reactive metal tarnishing quickly on explosive to air, and reacting violently with water to form sodium hydroxide.
Important sodium compounds include common salt (sodium chloride NaC1), sodium carbonate (Na2C03, washing soda), and hydrogencarbonate (NaHC03), bicarbonate of soda), sodium hydroxide or caustic soda (Na0H), sodium nitrate or Chile saltpetre (NaN03), fertiliser); sodium thiosulphate or hypo (Na2S203, photographic fixer).
In the human body, it works with potassium and chloride to regulate the body's fluid balance and maintain normal blood pressure. It's also necessary for stimulating nerve and muscle activity.
Its best sources are to be found in salt, prawns, much processed food (such as baked beans in tomato sauce) and hard cheese.