Sodium: historical information
Until the 18th century no distinction was made between potassium and sodium. This was because early chemists did not recognise that "mineral alkali" (Na2CO3, sodium carbonate, coming from deposits in the earth) and "vegetable alkali" (K2CO3, potassium 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).
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.]