معرفی عناصر گروه 3
Introduction group IIIa
Group 3a will include the following elements: boron B, aluminium Al, gallium Ga, indium In and thallium Tl.
Boron is a unique and exciting element. Over the years it has proved a constant challenge and stimulus not only to preparative chemists and theoreticians, but also to industrial chemists and technologists. It is the only non-metal in Group 3a of the periodic table and shows many similarities to its nelghbour, carbon, and its diagonal relative, silicon. Thus, like carbon and silicon, it shows a marked propensity to form covalent, molecular compounds, but it differs sharply from them in having one less valence electron than the number of valence orbitals, a situation sometimes referred to as "electron deficiency". This has a dominant effect on its chemistry. Borax was known in the ancient world where it was used to prepare glazes and hard (borosilicate) glasses. Sporadic investigations during the eighteenlh century led ultimately to the isolalion of very impure boron by H. Davy and by J. L. Gay Lussac and L. J. Thenard in 1808, but it was not until 1892 that H. Moissan obtained samples of 95-98% purity by reducing B2O3 with Mg.
Aluminium derives its name from alum, the double sulfate KAl(SO4)2-12H20, which was used medicinally as an astringent in ancient Greece and Rome . Humphry Davy was unable to isolate the metal but proposed the name "alumium" and then "aluminum"; this was soon modified to aluminium and this form is used throughout the world except in North America where the ACS decided in 1925 to adopt "aluminum" in its publications. The impure metal was first isolated by the Danish scientist H. C. Oersted using the reaction of dilute potassium amalgam on AlCl3. This method was improved in 1827 by H. Wohler who used metallic potassium, but the first commercially successful process was devised by H. St.C. Deville in 1854 using sodium. In the same year both he and R. W. Bunsen independently obtained metallic aluminium by electrolysis of fused NaAlCl4.
Gallium was predicted as eka-aluminium by D. I. Mendeleev in 1870 and was discovered by P. E. Lecoq de Boisbaudran in 1875 by means of the spectroscope; de Boisbaudran was, in fact, guided at the time by an independent theory of his own and had been searching for the missing element for some years. The first indications came with the observation of two new violet lines in the spark spectrum of a sample deposited on zinc, and within a month he had isolated 1 g of the metal starting from several hundred kilograms of crude zinc blende ore. The element was named in honour of France (Latin Gallia) and the striking similarity of its physical and chemical properties to those predicted by Mendeleev did much to establish the general acceptance of the Periodic Law; indeed, when de Boisbaudran first stated that the density of Ga was 4.7g/cm3 rather than the predicted 5.9g/cm3, Mendeleev wrote to him suggesting that he redetermine the figure (the correct value is 5.904 g/cm3).
Indium and thallium were also discovered by means of the spectroscope as their names indicate. Indium was first identified in 1863 by F. Reich and H.T. Richter and named from the brilliant indigo blue line in its flame spectrum (Latin indicum). Thallium was discovered independently by W. Crookes and by C. A. Lamy in the preceding year 1861-1862 and named after the characteristic bright green line in its flame spectrum.
