Sodium: the essentials

Sodium is a Group 1 element (or IA in older labelling styles). Group 1 elements are often referred to as the "alkali metals". The chemistry of sodium is dominated by the +1 ion Na+. Sodium salts impart a characteristic orange/yellow colour to flames and orange street lighting is orange because of the presence of sodium in the lamp.

Soap is generally a sodium salt of fatty acids. The importance of common salt to animal nutrition has been recognized since prehistoric times. The most common compound is sodium chloride, (table salt).

Table: basic information about and classifications of sodium.

Sodium: historical information

Sodium was discovered by Sir Humphrey Davy at 1807 in England. Origin of name: from the English word "soda" (the origin of the symbol Na comes from the Latin word "natrium").

Until the 18th century no distinction was made between potassium and sodium. This was because early chemists did not recognise that "vegetable alkali" (K2CO3, potassium carbonate, coming from deposits in the earth) and "mineral alkali" (Na2CO3, sodium carbonate, derived from wood ashes) are distinct from each other. Eventually a distinction was made.

Sodium was first isolated in 1807 by Sir Humphry Davy, who made it by the electrolysis of very dry molten sodium hydroxide, NaOH. Sodium collected at the cathode. Davy isolated potassium by a similar procedure, also in 1807. Shortly after, Thenard and Gay-Lussac isolated sodium by reducing sodium hydroxide with iron metal at high temperatures.

Sodium is one of the elements which has an alchemical symbol, shown below (alchemy is an ancient pursuit concerned with, for instance, the transformation of other metals into gold).

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Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for sodium is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Dalton's symbol for sodium

Sodium: physical properties

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Sodium: orbital properties

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Isolation: sodium would not normally be made in the laboratory as it is so readily available commercially. All syntheses require an electrolytic step as it is so difficult to add an electron to the poorly electronegative sodium ion Na+.

Sodium is present as salt (sodium chloride, NaCl) in huge quantities in underground deposits (salt mines) and seawater and other natural waters. It is easily recovered as a solid by drying.

Sodium chloride has a high melting point (> 800°C) meaning that it sould be expensive to melt it in order to carry out the electrolysis. However a mixture of NaCl (40%) and calcium chloride, CaCl2 (60%) melts at about 580°C and so much less energy and so expense is required for the electrolysis.

cathode: Na+(l) + e- → Na (l)

anode: Cl-(l) → 1/2Cl2 (g) + e-

The electrolysis is carried out as a melt in a "Downs cell". In practice, the electrolysis process produces calcium metal as well but this is solidified in a collection pipe and returned back to the melt.

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sodium atomic number