Sodium is a chemical element; it has symbol Na (from Neo-Latin natrium) and atomic number 11.

Sodium is the sixth most abundant element in the Earth's crust and exists in numerous minerals such as feldspars, sodalite, and halite (NaCl).

[11] The melting (98 °C) and boiling (883 °C) points of sodium are lower than those of lithium but higher than those of the heavier alkali metals potassium, rubidium, and caesium, following periodic trends down the group.

[22] Sodium compounds are of immense commercial importance, being particularly central to industries producing glass, paper, soap, and textiles.

[24] In compounds, sodium is usually ionically bonded to water and anions and is viewed as a hard Lewis acid.

[26] Sodium tends to form water-soluble compounds, such as halides, sulfates, nitrates, carboxylates and carbonates.

The main aqueous species are the aquo complexes [Na(H2O)n]+, where n = 4–8; with n = 6 indicated from X-ray diffraction data and computer simulations.

[31] Sodium content of samples is determined by atomic absorption spectrophotometry or by potentiometry using ion-selective electrodes.

Because of the high polarity of the C-Na bonds, they behave like sources of carbanions (salts with organic anions).

[45] The chemical abbreviation for sodium was first published in 1814 by Jöns Jakob Berzelius in his system of atomic symbols,[46][47] and is an abbreviation of the element's Neo-Latin name natrium, which refers to the Egyptian natron,[42] a natural mineral salt mainly consisting of hydrated sodium carbonate.

As early as 1860, Kirchhoff and Bunsen noted the high sensitivity of a sodium flame test, and stated in Annalen der Physik und Chemie:[49] In a corner of our 60 m3 room farthest away from the apparatus, we exploded 3 mg of sodium chlorate with milk sugar while observing the nonluminous flame before the slit.

After a while, it glowed a bright yellow and showed a strong sodium line that disappeared only after 10 minutes.

Many other stars appear to have a sodium absorption line, but this is actually caused by gas in the foreground interstellar medium.

The introduction of the Hall–Héroult process for the production of aluminium by electrolysing a molten salt bath ended the need for large quantities of sodium.

[77] Liquid sodium is used as a heat transfer fluid in sodium-cooled fast reactors[79] because it has the high thermal conductivity and low neutron absorption cross section required to achieve a high neutron flux in the reactor.

[80] The high boiling point of sodium allows the reactor to operate at ambient (normal) pressure,[80] but drawbacks include its opacity, which hinders visual maintenance, and its strongly reducing properties.

[92] The Committee to Review the Dietary Reference Intakes for Sodium and Potassium, which is part of the National Academies of Sciences, Engineering, and Medicine, has determined that there isn't enough evidence from research studies to establish Estimated Average Requirement (EAR) and Recommended Dietary Allowance (RDA) values for sodium.

[94] High sodium consumption is unhealthy, and can lead to alteration in the mechanical performance of the heart.

[103] The sodium ion (Na+) is an important electrolyte in neuron function, and in osmoregulation between cells and the extracellular fluid.

[104] The difference in extracellular and intracellular ion concentration, maintained by the sodium-potassium pump, produce electrical signals in the form of action potentials that supports cardiac muscle contraction and promote long distance communication between neurons.

[105] In humans, unusually low or high sodium levels in the blood is recognized in medicine as hyponatremia and hypernatremia.

[106] In C4 plants, sodium is a micronutrient that aids metabolism, specifically in regeneration of phosphoenolpyruvate and synthesis of chlorophyll.

[107] In others, it substitutes for potassium in several roles, such as maintaining turgor pressure and aiding in the opening and closing of stomata.

[108] Excess sodium in the soil can limit the uptake of water by decreasing the water potential, which may result in plant wilting; excess concentrations in the cytoplasm can lead to enzyme inhibition, which in turn causes necrosis and chlorosis.

In the case of massive (non-molten) pieces of sodium, the reaction with oxygen eventually becomes slow due to formation of a protective layer.

[115] Other effective agents include Lith-X, which has graphite powder and an organophosphate flame retardant, and dry sand.

[119] Pool-type sodium fires are prevented using diverse design measures called catch pan systems.

In a technical report for the United States Fire Administration,[114] R. J. Gordon writes (emphasis in original) Molten sodium is extremely dangerous because it is much more reactive than a solid mass.

Any amount of water introduced into a pool of molten sodium is likely to cause a violent explosion inside the liquid mass, releasing the hydrogen as a rapidly expanding gas and causing the molten sodium to erupt from the container.

Except for soda ash, most of the powdered agents that are used to extinguish small fires in solid pieces or shallow pools will sink to the bottom of a molten mass of burning sodium – the sodium will float to the top and continue to burn.