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a metallic chemical element; symbol Na [Lat. natrium]; at. no. 11; at. wt. 22.98977; m.p. 97.81°C;; b.p. 892.9°C;; sp. gr. 0.971 at 20°C;; valence +1. Sodium is a soft, silver-white metal. Extremely reactive chemically, it is one of the alkali metalsalkali metals,
metals found in Group 1 of the periodic table. Compared to other metals they are soft and have low melting points and densities. Alkali metals are powerful reducing agents and form univalent compounds.
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 in Group 1 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. Like potassium, which it closely resembles, it oxidizes rapidly in air; it also reacts violently with water, liberating hydrogen (which may ignite) and forming the hydroxide. It must be stored out of contact with air and water and should be handled carefully. Sodium combines directly with the halogens. The metal is usually prepared by electrolysis of the fused chloride (the Downs process); formerly, the chief method of preparation was by electrolysis of the fused hydroxide (the Castner process). Metallic sodium has limited use. It is used in sodium arc lamps for street lighting; pure or alloyed with potassium, it has found use as a heat-transfer liquid, e.g., in certain nuclear reactors. It is used principally in the manufacture of tetraethyl lead (a gasoline antiknock compound) and of sodamide, NaNH2, sodium cyanide, NaCN, sodium peroxide, Na2O2, and sodium hydride, NaH. Sodium compounds are extensively used in industry and for many nonindustrial purposes. Among the most important compounds are chloride (common salt, NaCl), bicarbonate (baking soda, NaHCO3), carbonate (soda ash, or washing soda, Na2CO3), hydroxide (caustic soda, or lye, NaOH), nitrate (Chile saltpeter, NaNO3), thiosulfate (hypo, Na2S2O3·5H2O), phosphates, and boraxborax
or sodium tetraborate decahydrate
, chemical compound, Na2B4O7·10H2O; sp. gr. 1.73; slightly soluble in cold water; very soluble in hot water; insoluble in acids.
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 (Na2B4O7·10H2O). Sodium hydroxide is used wherever a cheap alkali is needed, for example, in making soapsoap,
a cleansing agent. It cleanses by lowering the surface tension of water, by emulsifying grease, and by absorbing dirt into the foam.

Ancient peoples are believed to have employed wood ashes and water for washing and to have relieved the resulting irritation with
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. Substances containing sodium impart a characteristic yellow color to a flame. Because of its activity sodium is not found uncombined in nature. It occurs abundantly and widely distributed in its compounds, which are present in rocks and soil, in the oceans, in salt lakes, in mineral waters, and in deposits in various parts of the world. Sodium compounds are found in the tissues of plants and animals. Sodium is an essential element in the diet, but some people must limit the amount of sodium in their food for medical reasons. Discovery of sodium is usually credited to Sir Humphry Davy, who prepared the metal from its hydroxide in 1807; its compounds have been known since antiquity.



(natrium), Na, a chemical element of group I in the Mendeleev periodic system. Atomic number, 11; atomic weight, 22.9898. Sodium is a soft silver-white metal that exhibits rapid surface oxidation upon exposure to air. The natural element consists of one stable isotope, 23Na.

History. Natural sodium compounds (common salt, NaCl, and sodium carbonate, Na2COs) have been known since ancient times. The name “natrium,” derived from the Arabic natrun and the Greek nitron, originally referred to natural soda. By the 18th century many other sodium compounds were known to chemists, although the metal itself was not isolated until 1807, when H. Davy obtained sodium by electrolysis of sodium hydroxide, NaOH. In Great Britain, the USA, and France, the element is called sodium (from the Spanish soda); in Italy, sodio.

Occurrence in nature. Sodium is a typical element of the upper part of the earth’s crust. The average sodium content in the lithosphere is 2.5 percent by weight; in acidic igneous rock (such as granite) it is 2.77 percent; in basic rock (such as basalt), 1.94 percent; and in ultrabasic rock (rock of the mantle), 0.57 percent. Because of the isomorphism of Na+ and Ca2+, which is determined by the closeness of their ionic radii, sodium-calcium feldspars (plagioclases) form in magmatic rock. There is marked differentiation of sodium in the biosphere: sedimentary rocks are usually deficient in sodium (0.66 percent Na in clays and shales), and most types of soil contain relatively small quantities of the element (an average of 0.63 percent). There are a total of 222 sodium minerals. Sodium is poorly retained on land masses; it is carried by rivers to the seas and oceans, where the average sodium content is 1.035 percent (sodium is the primary metallic constituent of seawater). Evaporation in coastal marine lagoons and in inland lakes of steppe and desert regions causes precipitation of sodium salts, which in turn form layers of saliferous rock. The main source minerals for sodium and sodium compounds are halite (rock salt), NaCl; Chile saltpeter, NaNO3; thenardite, Na2SO4; mirabilite, Na2SO4-10H2O; and trona, NaH (C03)2-2H2O. Annual world sodium production is estimated at 1×108 tons. Sodium is an important bioelement, and its average content in living matter is 0.02 percent; larger quantities of sodium are present in animals than in plants.

Physical and chemical properties. At ordinary temperatures sodium crystallizes inoa cubic lattice; a = 4.28 angstroms (A). Atomic radius, 1.86 A; ionic radius (Na+), 0.92 A. Density, 0.968 g/cm3 (19.7°C); melting point, 97.83°C; boiling point, 882.9°C; specific heat (20°C), 1.23×103 joules per (kg .°K), or 0.295 cal/(g.deg); thermal conductivity, 1.32×102 watts per (m °K), or 0.317 cal/(cm · sec · deg); temperature coefficient of linear expansion (20°C), 7.1×10-5; specific electrical resistivity (0°C), 4.3×10-8 ohm · m, or 4.3×10-6ohm · cm. Sodium is paramagnetic, with a specific magnetic susceptibility of + 9.2×10-6 . It is extremely plastic and soft (it can easily be cut with a knife).

Sodium has a normal electrode potential of —2.74 volts ( — 2.4 volts in a melt). Sodium vapors turn flames bright yellow. The outer electron configuration is 3s1’, and all known sodium compounds are univalent. Sodium is highly chemically active. Direct reaction of sodium and oxygen yields the oxide Na2O or the peroxide Na202 (both of which are colorless crystalline substances), depending on conditions. Sodium combines with water to form sodium hydroxide, NaOH, and H2; the reaction may be accompanied by an explosion. Inorganic acids combine with sodium to produce the corresponding water-soluble salts, although sodium is relatively inert in comparison with 98–100 percent sulfuric acid.

Reaction of sodium and hydrogen begins at 200°C and results in the formation of sodium hydride, NaH, a colorless, hygroscopic crystalline substance. Sodium reacts directly with fluorine and chlorine under ordinary temperature conditions, whereas heating is necessary for reaction with bromine. No direct reaction with iodine is observed. Sodium reacts vigorously with sulfur to yield sodium sulfide. The reaction of sodium vapors with nitrogen in a silent discharge field leads to the formation of sodium nitride, Na3N; reaction with carbon at 8O0°-9O0°C yields sodium carbide, Na2C2.

Sodium dissolves in liquid ammonia (34.6 g per 100 g NH3 at 0°C) to yield ammonia complexes. Sodium amide, NaNH2, a colorless crystalline substance that is readily decomposed by water, is produced by passing gaseous ammonia through fused sodium at 300°-350°C. A large number of organosodium compounds are known; they are very similar to organolithium compounds in chemical properties but are much more reactive. Organosodium compounds are used as alkylating agents in organic synthesis.

Sodium is a constituent of many commercially important alloys. Sodium-potassium alloys containing 40–90 percent potassium (by weight) at about 25°C are silver-white liquids distinguished by their high chemical activity (they ignite upon exposure to air). The electrical conductivity and heat conductivity of liquid sodium-potassium alloys are both lower than the corresponding values for sodium and potassium. Sodium amalgams are readily obtained by adding metallic sodium to mercury; if the sodium content is more than 2.5 percent by weight, the amalgams are solid at ordinary temperatures.

Preparation and use. Sodium is usually prepared commercially by the electrolysis of fused common salt, NaCL, containing additives of KC1, NaF, CaCl2, and other substances, which lower the melting point of the salt to 575°-585°C. The electrolysis of pure NaCl would result in higher sodium losses because of evaporation, since the melting point of NaCl (801°C) and the boiling point of Na (882.9°C) are very close. Electrolysis is conducted in electrolyzers equipped with a membrane and with graphite anodes and steel or copper cathodes. The process yields chlorine in addition to sodium. The old method of preparing sodium is still used; it involves electrolysis of fused sodium hydroxide, NaOH, which is considerably more expensive than NaCl but undergoes electrolytic decomposition at a much lower temperature (320°-330°C).

Sodium and sodium alloys are widely used as heat-transfer agents in processes that require uniform heating in the range from 450° to 650°C—for example, in aircraft engine valves and particularly in nuclear power plants. In the latter case sodium-potassium alloys are used as liquid-metal heat-transfer agents (both elements have small thermal neutron absorption cross sections: 0.49 barn for Na). Such alloys have high boiling points and heat-transfer coefficients and do not react with structural materials at the high temperatures generated in nuclear power reactors. The compound NaPb (10 percent Na by weight) is used in the manufacture of tetraethyl lead, a highly effective antiknock compound. Sodium is used as a hardener in lead-based alloys (0.73 percent Ca, 0.58 percent Na, 0.04 percent Li), which are used in the manufacture of axle bearings for railroad cars. In metallurgy sodium is used as an active reducing agent in the preparation of certain rare metals (titanium, zirconium, and tantalum) by methods of thermal reduction. Sodium also takes an active part in the organic synthesis reactions of reduction, condensation, and polymerization.

Care must be taken when handling sodium because of its extreme chemical activity. Contact with water is particularly dangerous, since it may result in fire and explosion. Individuals working with sodium should wear protective goggles and heavy rubber gloves. Severe burns may be caused if sodium comes in contact with damp skin or clothing.


Sodium in the body. Sodium is one of the main elements that participate in the mineral metabolism of animals and humans. Quantities of sodium are usually present in extracellular fluids (10 millimoles per kg in human erythrocytes and 143 millimoles per kg in blood serum). Sodium plays an active role in maintaining osmotic pressure and acid-base equilibrium and in conducting nerve impulses. The daily human NaCl requirement is 2–10 g, depending on the amount of sodium chloride lost through perspiration. The sodium ion concentration in the body is primarily regulated by aldosterone, a hormone secreted by the adrenal cortex. In spite of the relatively high sodium content in plant tissue (about 0.01 percent raw weight), very few studies have been carried out on the role of sodium in plant growth. In halophytes (species that grow on strongly saline soils), sodium produces high osmotic pressure in the cell sap, thereby promoting the extraction of water from the soil.


Sodium in medicine. The sodium preparations most widely used in medicine are sodium sulfate; sodium chloride, NaCl (for bleeding, loss of fluid, vomiting, and so on); sodium borate, Na2B4O7.IOH2O (as an antiseptic); sodium bicarbonate, NaHCO3 (as an expectorant, and also as a rinse in the treatment of rhinitis and laryngitis); sodium thiosulfate, Na2S203 · 5H2O (as anti-inflammatory, detoxicant, and desensitizing agent); and sodium citrate, Na3C6H507·5½H2O (as an anticoagulant).

The artificially prepared radioactive sodium isotopes 22Na (half-life T1/2 = 2.64 yr) and 24Na (T1/2 = 15 hr) are used to determine the blood flow rate in individual sections of the circulatory system in cases of cardiovascular and pulmonary diseases, and also endarteritis obliterans. Radioactive solutions of sodium salts (for example, 24NaCl) are also used to determine vascular permeability and to study the overall metabolic sodium content in the body, water-salt metabolism, intestinal absorption, nervous system activity, and certain other phenomena.



Sittig, M. Natrii, ego proizvodstvo, svoistva i primenenie. Moscow, 1961. (Translated from English.)
Ullmanns Encyklopädie der technischen Chemie, 3rd ed., vol. 12. Munich-Berlin, 1960.
Reformatskii, I. A. “Natrii.” In Kratkaia khimicheskaia entsiklopediia, vol. 3. Moscow, 1964.
Ripan, R., and I. Ceteanu. Neorganicheskaia khimiia, vol. 1. Moscow, 1971. (Translated from Rumanian.)
Prosser, C. L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)


A metallic element of group 1, symbol Na, with atomic number 11, atomic weight 22.98977; silver-white, soft, and malleable; oxidizes in air; melts at 97.6°C; used as a chemical intermediate and in pharmaceuticals, petroleum refining, and metallurgy; the source of the symbol Na is natrium.


a. a very reactive soft silvery-white element of the alkali metal group occurring principally in common salt, Chile saltpetre, and cryolite. Sodium and potassium ions maintain the essential electrolytic balance in living cells. It is used in the production of chemicals, in metallurgy, and, alloyed with potassium, as a cooling medium in nuclear reactors. Symbol: Na; atomic no.: 11; atomic wt.: 22.989768; valency: 1; relative density: 0.971; melting pt.: 97.81±0.03°C; boiling pt.: 892.9°C
b. (as modifier): sodium light
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