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{{Infobox sodium}}

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{{for|sodium in the diet|Edible salt}}

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==Overview==

'''Sodium''' ([[International Phonetic Alphabet|IPA]]: {{IPA|/ˈsəʊdiəm/}}) is a [[chemical element]] which has the symbol '''Na''' (Latin: natrium), atomic number 11, atomic mass 22.9898 g/mol, common [[oxidation number]] +1. Sodium is a soft, [[silver]]y 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]], <sup>23</sup>Na. Sodium was first isolated by Sir [[Humphry Davy]] in 1807 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]]. Sodium is present in great quantities in the earth's oceans as [[sodium chloride]]. 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.” 

== Notable characteristics ==
[[Image:Flametest--Na.swn.jpg|thumb|left|200px|The [[flame test]] for sodium displays a brilliantly bright yellow emission due to the so called "sodium D-lines" at 588.9950 and 589.5924&nbsp;nanometers.]]
Compared with other alkali metals, sodium is generally less reactive than [[potassium]] and more so than [[lithium]], in accordance with "[[periodic law]]": for example, their reaction in water, chlorine gas, etc.; the reactivity of their [[nitrate]]s, [[chlorate]]s, [[perchlorate]]s, etc. The density of elements generally increase with increasing [[atomic number]], but potassium is less dense than sodium.

Owing to its high reactivity, sodium is found in nature only as a compound and never as the free element. Sodium reacts [[Exothermic reaction|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]] and highly flammable hydrogen gas. These are extreme hazards (see ''Precautions'' section below). When burned in air, sodium forms [[sodium peroxide]] Na<sub>2</sub>O<sub>2</sub>, or with limited [[oxygen]], the oxide Na<sub>2</sub>O</sub> (unlike [[lithium]], the [[nitride]] is not formed). If burned in oxygen under pressure, [[sodium superoxide]] NaO<sub>2</sub> will be produced.

When sodium or its compounds are introduced into a flame it will contribute a bright yellow.

Sodium ions are necessary for regulation of blood and body fluids, transmission of nerve impulses, heart activity, and certain metabolic functions. Interestingly, sodium is needed by animals, which maintain high concentrations in their blood and extracellular fluids, but the ion is not needed by plants. A completely plant-based diet, therefore, will be very low in sodium. This requires some herbivores to obtain their sodium from salt licks and other mineral sources. The animal need for sodium is probably the reason for the highly-conserved ability to [[taste]] the sodium ion as "salty." Receptors for the pure salty taste respond best to sodium, and otherwise only to a few other small monovalent cations (Li<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, and to some extent also K<sup>+</sup>). [[Calcium chloride]] also tastes somewhat salty, but also quite bitter.

The most common sodium salt, sodium chloride (table salt), used for seasoning (for example the English word "salad" refers to salt) and warm-climate food preservation, such as pickling and making jerky (the high osmotic content of salt inhibits bacterial and fungal growth). As such, salt has been an important commodity in human activities (the English word ''salary'' refers to ''salarium'', the perquisite ("perk") given to Roman soldiers for the purpose of buying salt). The human requirement for sodium in the diet is less than 500&nbsp;mg per day, which is typically less than a tenth as much as many diets "seasoned to taste." Most people consume far more sodium than is physiologically needed. For certain people with salt-sensitive blood pressure, this extra intake may cause a negative effect on health.

== Precautions ==<!-- This section is linked from [[Potassium]] -->
Extreme care is required in handling elemental/metallic sodium. Sodium is potentially explosive in water (depending on quantity) and is a caustic poison, since it is rapidly converted to sodium hydroxide on contact with moisture. The powdered form may combust spontaneously in air or oxygen. Sodium must be stored either in an inert (oxygen and moisture free) atmosphere (such as [[nitrogen]] or [[argon]]), or under a liquid hydrocarbon such as [[mineral oil]] or [[kerosene]].

The reaction of sodium and water is a familiar one in chemistry labs, and is reasonably safe if amounts of sodium smaller than a pencil eraser are used and the reaction is done behind a plastic shield by people wearing eye protection. However, the sodium-water reaction does not scale up well, and is treacherous when larger amounts of sodium are used. Larger pieces of sodium melt under the heat of the reaction, and the molten ball of metal is buoyed up by [[hydrogen]] and may appear to be stably reacting with [[water]], until splashing covers more of the reaction mass, causing thermal runaway and an explosion which scatters molten sodium, lye solution, and sometimes flame. (18.5 g explosion [http://video.google.de/videoplay?docid=-2158222101210607510&q=sodium]) This behavior is unpredictable, and among the alkali metals it is usually sodium which invites this surprise phenomenon, because [[lithium]] is not reactive enough to do it, and [[potassium]] is so reactive that chemistry students are not tempted to try the reaction with larger [[potassium]] pieces.

Sodium is much more reactive than [[magnesium]]; a [[reactivity]] which can be further enhanced due to sodium's much lower melting point. When sodium catches fire in air (as opposed to just the hydrogen gas generated from water by means of its reaction with sodium) it more easily produces temperatures high enough to melt the sodium, exposing more of its surface to the air and spreading the fire.

Few common fire extinguishers work on sodium fires. [[Water]], of course, exacerbates sodium fires, as do water-based foams. CO<sub>2</sub> and Halon are often ineffective on sodium fires, which reignite when the extinguisher dissipates. Among the very few materials effective on a sodium fire are Pyromet and Met-L-X. Pyromet is a NaCl/(NH<sub>4</sub>)<sub>2</sub>HPO<sub>4</sub> mix, with flow/anti-clump agents. It smothers the fire, drains away heat, and melts to form an impermeable crust. This is the standard dry-powder canister fire extinguisher for all classes of fires. Met-L-X is mostly sodium chloride, NaCl, with approximately 5% [[Saran (plastic)|Saran plastic]] as a crust-former, and flow/anti-clumping agents. It is most commonly hand-applied, with a scoop. Other extreme fire extinguishing materials include [[Lith<sup>+</sup>]], a graphite based dry powder with an organophosphate flame retardant; and [[Na<sup>+</sup>]], a Na<sub>2</sub>CO<sub>3</sub>-based material.

Because of the reaction scale problems discussed above, disposing of large quantities of sodium (more than 10 to 100 grams) must be done through a licensed hazardous materials disposer. Smaller quantities may be broken up and neutralized carefully with [[ethanol]] (which has a much slower reaction than water), or even [[methanol]] (where the reaction is more rapid than ethanol's but still less than in water), but care should nevertheless be taken, as the caustic products from the ethanol or methanol reaction are just as hazardous to eyes and skin as those from water. After the alcohol reaction appears complete, and all pieces of reaction debris have been broken up or dissolved, a mixture of alcohol and water, then pure water, may then be carefully used for a final cleaning. This should be allowed to stand a few minutes until the reaction products are diluted more thoroughly and flushed down the drain. The purpose of the final water soak and wash of any reaction mass which may contain sodium is to ensure that alcohol does not carry unreacted sodium into the sink trap, where a water reaction may generate hydrogen in the trap space which can then be potentially ignited, causing a confined sink trap explosion.

== Physiology and sodium ions ==
{{main|action potential}}

Sodium ions play a diverse and important role in many physiological processes. Excitable animal cells, for example, rely on the entry of Na<sup>+</sup> to cause a [[depolarization]]. An example of this is [[signal transduction]] in the human [[central nervous system]], which depends on sodium ion motion across the nerve cell membrane, in all nerves.

Some potent [[neurotoxin]]s, such as [[batrachotoxin]], increase the sodium ion permeability of the [[cell membrane]]s in nerves and muscles, causing a massive and irreversible [[depolarization]] of the membranes, with potentially fatal consequences. However, drugs with smaller effects on sodium ion motion in nerves may have diverse pharmacological effects which range from anti-depressant to anti-seizure actions.

{{main|hyponatremia|hypernatremia|diuretic|vasopressin}}

Sodium is the primary [[cation]] (positive ion) in extracellular fluids in animals and humans. These fluids, such as blood plasma and extracellular fluids in other tissues, bathe cells and carry out transport functions for nutrients and wastes. Sodium is also the principal cation in seawater, although the concentration there is about 3.8 times what it is normally in extracellular body fluids.

Although the system for maintaining optimal salt and water balance in the body is a complex one, one of the primary ways in which the human body keeps track of loss of body water is that [[osmoreceptor]]s in the [[hypothalamus]] sense a balance of sodium and water concentration in extracellular fluids. Relative loss of body water will cause sodium concentration to rise higher than normal, a condition known as [[hypernatremia]]. This ordinarily results in thirst. Conversely, an excess of body water caused by drinking will result in too little sodium in the blood ([[hyponatremia]]), a condition which is again sensed by the [[hypothalamus]], causing a decrease in [[vasopressin]] hormone secretion from the [[posterior pituitary]], and a consequent loss of water in the urine, which acts to restore blood sodium concentrations to normal.

Severely dehydrated persons, such as people rescued from ocean or desert survival situations, usually have very high blood sodium concentrations. These must be very carefully and slowly returned to normal, since too-rapid correction of hypernatremia may result in brain damage from cellular swelling, as water moves suddenly into cells with high [[osmolar]] content.

Because the [[hypothalamus]]/[[osmoreceptor]] system ordinarily works well to cause drinking or urination to restore the body's sodium concentrations to normal, this system can be used in medical treatment to regulate the body's total fluid content, by first controlling the body's sodium content. Thus, when a powerful [[diuretic]] drug is given which causes the kidneys to excrete sodium, the effect is accompanied by an excretion of body water (water loss accompanies sodium loss). This happens because the kidney is unable to efficiently retain water while excreting large amounts of sodium. In addition, after sodium excretion, the [[osmoreceptor]] system may sense lowered sodium concentration in the blood, and then directs compensatory urinary loss of water, in order to correct the [[hyponatremia]], or (low-blood-sodium) state.

== See also ==
{{alkalimetals}}

== References ==
{{reflist|2}}
*Rebecca J. Donatelle. Health, The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.

== External links ==
*[http://www.webelements.com/webelements/elements/text/Na/index.html WebElements.com – Sodium]
*[http://www.theodoregray.com/PeriodicTable/Elements/011/index.html The Wooden Periodic Table Table's Entry on Sodium]
*[http://www.americanheart.org/presenter.jhtml?identifier=4708 Dietary Sodium]
*[http://ie.lbl.gov/education/parent/Na_iso.htm Sodium isotopes data from ''The Berkeley Laboratory Isotopes Project's'']

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[[Category:Desiccants]]
[[Category:Dietary minerals]]
[[Category:Sodium|Sodium]]
[[Category:Reducing agents]]

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[[ar:صوديوم]]
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[[zh-min-nan:Na (goân-sò͘)]]
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[[tr:Sodyum]]
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