معرفی عناصر گروه 2
Introduction group IIa
The Group 2a (beryllium Be, magnesium Mg, calcium Ca, strontium Sr, barium Ba, radium Ra) or alkaline earth metals exemplify and continue the trends in properties noted for the alkali metals.
The discovery of beryllium in 1798 followed an unusual train of events. The mineralogist R.-J. Haily had observed the remarkable similarity in external crystalline structure, hardness and density of a beryl from Limoges and an emerald from Peru, and suggested to L.N. Vauquelin that he should analyse them to see if they were chemically identical. As a result, Vauquelin showed that both minerals contained not only alumina and silica as had previously been known, but also a new earth, beryllia, which closely resembled alumina but gave no alums, apparently did not dissolve in an excess of KOH and had a sweet rather than an astringent taste. Caution: beryllium compounds are now known to be extremely toxic, especially as dusts or smokes; it seems likely that this toxicity results from the ability of Be to displace Mg from Mg-activated enzymes due to its stronger coordinating ability.
Both beryl and emerald were found to be essentially Be3Al2Si6O18, the only difference between them being that emerald also contains about 2% Cr, the source of its green color. The combining weight of Be was 4.7 but the similarity between Be and Al led to considerable confusion concerning the valency and atomic weight of Be (2 x 4.7 or 3 x 4.7); this was not resolved until Mendeleev 70 years later stated that there was no room for a tervalent element of atomic weight 14 near nitrogen in his periodic table, but that a divalent element of atomic weight 9 would fit snugly between Li and B. Beryllium metal was first prepared by F. Wishler in 1828 (the year he carried out his celebrated synthesis of urea from NH4CNO); he suggested the name by allusion to the mineral. The metal was independently isolated in the same year by A.B. Bussy using the same method reduction of BeCl2 with metallic K. The first electrolytic preparation was by P. Lebeau in 1898 and the first commercial process (electrolysis of a fused mixture of BeF2 and BaF2) was devised by A. Stock and H. Goldschmidt in 1932.
Compounds of Mg and Ca, like those of their Group 1a neighbours Na and K, have been known from ancient times though nothing was known of their chemical nature until the seventeenth century. Magnesian stone was the name given to the soft white mineral steatite (otherwise called soapstone or talc) which was found in the Magnesia district of Thessally, whereas calcium derives from the Latin calx, calcis - lime. The Romans used a mortar prepared from sand and lime (obtained by heating limestone, CaCO3) because these lime mortars withstood the moist climate of Italy better than the Egyptian mortars based on partly dehydrated gypsum (CaSO4 - 2H20); these had been used, for example, in the Great Pyramid of Gizeh. The names of the elements themselves were coined by H. Davy in 1808 when he isolated Mg and Ca, along with Sr and Ba by an electrolytic method following work by J. J. Berzelius and M. M. Pontin: the moist earth (oxide) was mixed with one-third its weight of HgO on a platinum plate which served as anode; the cathode was a platinum wire dipping into a pool of mercury and electrolysis gave an amalgam from which the desired metal could be isolated by distilling off the mercury.
A mineral found in a lead mine near Strontian, Scotland, in 1787 was shown to be a compound of a new element by A. Crawford in 1790. This was confirmed by T. C. Hope the following year and he clearly distinguished the compounds of Ba, Sr and Ca, using amongst other things their characteristic flame colorations: Ba yellow-green, Sr bright red, Ca orange-red. Barium-containing minerals had been known since the seventeenth century but the complex process of unravelling the relation between them was not accomplished until the independent work of C. W. Scheele and J. G. Gahn between 1774 and 1779: heavy spar was found to be BaSO4 and called barite or barytes, whence Scheele's new base baryta (BaO) from which Davy isolated barium in 1808.
Radium, the last element in the group, was isolated in trace amounts as the chloride by Pierre and Marie Curie in 1898 after their historic processing of tonnes of pitchblende. It was named by Mariee Curie in allusion to its radioactivity; the element itself was isolated electrolytically via an amalgam by M. Curie and A. Debierne in 1910 and its compounds give a carmine-red flame test.
