Alkali metals are the six elements that comprise Group I in the Periodic Table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Especially when dissolved in water, these elements form strong bases (alkalis) capable of reacting with and neutralizing strong acids.
Each metal has the electron configuration of an inert (noble) gas plus one electron in the next higher s orbital. Thus, Na is 1 s 2 2 s 2 2 p 6 3 s 1 or alternatively (Ne)3 s 1 . Virtually all alkali metal compounds are ionic in nature because this outermost single electron is readily lost, forming relatively stable monovalent ions.
Sodium and potassium are abundant in Earth's crust, each comprising about 2.5 percent, and the two being the 6th and 7th most abundant elements, respectively. Other alkali metals are at least one hundred times less abundant. Francium is virtually nonexistent in the environment since all isotopes are radioactive with short half-lives.
Alkali metals are very reactive, and thus none occurs in a free state in the environment. They spontaneously react with oxygen, water, halogens , phosphorus, sulfur, and other substances; lithium even reacts with nitrogen. Reactions with water can be violent, with the evolution of hydrogen gas and formation of strongly alkaline solutions.
Compounds of various alkali metals were known in ancient times, but the great English chemist Sir Humphry Davy first isolated pure metals, purifying potassium and then sodium in 1807. Sodium is derived from "soda," a term used in the Middle Ages to characterize all alkalis, originally from the Latin sodanum , which was a headache remedy; its symbol Na is derived from the Latin word for soda, natrium. Potassium comes from the French word potasse (later the English potash ), the residue produced when wood ash solutions are evaporated (so-called pot ashes); its symbol K derives from the Latin kalium and ultimately Arabic qali , meaning "alkali." The Swedish chemist J. A. Arfedson discovered lithium during his analysis of the mineral petalite in 1817, although W. T. Brande and Davy first produced the pure metal. The name is derived from the Greek word lithos (meaning "stony").
During their flame spectrometry experiments on mineral waters in 1860, the German chemists Gustav Kirchhoff and Robert Bunsen determined the existence of cesium from the characteristic two blue lines in the spectrum. Likewise, extracts of the mineral lepidolite exhibited two dark red spectral lines from which the presence of Rb was inferred. Thus, cesium derives from the Latin caesius , meaning "heavenly blue," whereas rubidium derives from rubidus , the Latin word used to describe a very dark red color. Bunsen was able to isolate pure Rb but not Cs, later purified by C. Setterberg. Since all isotopes of Fr are radioactive, it was not discovered until 1939 at the Curie Institute in Paris by Marguerite Perey, although the Russian chemist Dimitri Mendeleev predicted its existence. Its name derives from that of France, the country where it was discovered.
Because of their metallic and alkaline properties, potassium and especially sodium are widely used in a variety of industrial processes both as metals and as compounds with various other elements. Lithium is rarely used, but does find application in lightweight alloys with magnesium. Rubidium and cesium are not commonly utilized industrially, except for some applications in electronics. Sodium and potassium are essential for life, sodium being the principal extracellular and potassium the major intracellular monovalent cations. The other alkali metals have no essential biological role.
Michael E. Maguire
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