Sodium ( ) is an element which has the symbol Na (Latin natrium, from Arabic natrun), atomic number 11, atomic mass 23 amu, and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" (formerly known as ‘group IA'). It has only one stable isotope, ²³Na. Sodium was first isolated by Sir Humphry Davy in 1806 by passing an electric current through molten sodium hydroxide. Sodium quickly oxidizes in air and is violently reactive with water, so it must be stored in an inert medium, such as kerosene or mineral oil. Sodium is present in great quantities in the Earth's oceans as sodium chloride (common salt). It is also a component of many minerals, and it is an essential element for animal life. As such, it is classified as a “dietary inorganic macro-mineral.”

Characteristics

At room temperature, sodium metal is so soft that it can be easily cut with a knife. In air, the bright silvery luster of freshly exposed sodium will rapidly tarnish. The density of alkali metals generally increases with increasing atomic number, but sodium is denser than potassium.

Chemical properties

Compared with other alkali metals, sodium is generally less reactive than potassium and more reactive than lithium, in accordance with "periodic law": for example, their reaction in water, chlorine gas, etc.;

Sodium reacts exothermically with water: small pea-sized pieces will bounce across the surface of the water until they are consumed by it, whereas large pieces will explode. While sodium reacts with water at room temperature, the sodium piece melts with the heat of the reaction to form a sphere, if the reacting sodium piece is large enough. The reaction with water produces very caustic sodium hydroxide (lye) and highly flammable hydrogen gas. These are extreme hazards (see Precautions section below). When burned in air, sodium forms sodium peroxide Na₂O₂, or with limited oxygen, the oxide Na₂O (unlike lithium, the nitride is not formed). If burned in oxygen under pressure, sodium superoxide NaO₂ will be produced. In chemistry, most sodium compounds are considered soluble but nature provides examples of many insoluble sodium compounds such as the feldspars. There are other insoluble sodium salts such as sodium bismuthate NaBiO₃, sodium octamolybdate Na₂Mo₈O₂₅• 4H₂O, sodium thioplatinate Na₄Pt₃S₆, sodium uranate Na₂UO₄. Sodium meta-antimonate's 2NaSbO₃•7H₂O solubility is 0.3g/L as is the pyro form Na₂H₂Sb₂O₇• H₂O of this salt. Sodium metaphosphate NaPO₃ has a soluble and an insoluble form.