Introduction group VIa

Group VIA includes following elements: Oxygen - [O], Sulfur - [S], Selenium - [Se], Tellurium - [Te], Polonium - [Po].

Oxygen

Oxygen is the most abundant element on the earth’s surface: it occurs both as the free element and combined in innumerable compounds, and comprises 23% of the atmosphere by weight, 46% of the lithosphere and more than 85% of the hydrosphere (~85.8% of the oceans and 88.81% of pure water). It is also, perhaps paradoxically, by far the most abundant element on the surface of the moon where, on average, 3 out of every 5 atoms are oxygen (44.6% by weight).

The “discovery” of oxygen is generally credited to C. W. Scheele and J. Priestley (independently) in 1773-1774, though several earlier investigators had made pertinent observations without actually isolating and characterizing the gas. Scheele, a pharmacist in Uppsala, Sweden, prepared oxygen at various times between 1771-1773 by heating KNO3, Mg(NO3)2, Ag2CO3, HgO and a mixture of H3AsO4 and MnO2. He called the gas “vitriol air” and reported that it was colourless, odourless and tasteless, and supported combustion better than common air, but the results did not appear until 1777 because of his publisher’s negligence. Priestley’s classic experiment of using a “burning glass” to calcine HgO confined in a cylinder inverted over liquid mercury was first performed in Colne, England, on 1 August 1774; he related this to A. L. Lavoisier and others at a dinner party in Paris in October 1774 and published the results in 1775 after he had shown that the gas was different from nitrous oxide. Priestley’ s ingenious experiments undoubtedly established oxygen as a separate substance (“dephlogisticated air”) but it was Lavoisier’s deep insight which recognized the new gas as an element and as the key to our present understanding of the nature of combustion. This led to the overthrow of the phlogiston theory and laid the foundations of modem chemistry. Lavoisier named the element “oxygene” in 1777 in the erroneous belief that it was an essential constituent of all acids.

Sulfur

Sulfur occurs uncombined in many parts of the world and has therefore been known since prehistoric times. Indeed, sulfur and carbon were the only two non-metallic elements known to the ancients. References to sulfur occur throughout recorded history from the legendary destruction of Sodom and Gomorrah by brimstone to its recent discovery (together with H2SO4) as a major component in the atmosphere of the planet Venus. The element was certainly known to the Egyptians as far back as the sixteenth century BC and Homer refers to its use as a fumigant. Pliny the Elder mentioned the occurrence of sulfur in volcanic islands and other Mediterranean locations, spoke of its use in religious ceremonies and in the fumigation of houses, described its use by fullers, cotton-bleachers, and match-makers, and indicated fourteen supposed medicinal virtues of the element.

Gunpowder, which revolutionized military tactics in the thirteenth century, was the sole known propellant for ammunition until the midnineteenth century when smokeless powders based on guncotton (1846), nitroglycerine (1846), and cordite (1889) were discovered. Gunpowder, an intimate mixture of saltpeter (KNO3), powdered charcoal and sulfur in the approximate ratios 75:15:10 by weight, was discovered by Chinese alchemists more than 1000 years ago. The earliest known recipe for explosive gunpowder (as distinct from incendiary mixtures and fireworks) appeared in a Chinese military manual of AD 1044 and its use in a gun (bombard) dates from at least as early as 1128. Arab and European formulae and technology were derived from this. The first use of gunpowder in a major compaign in the West was at the Battle of Crecy (26 August 1346), but the guns lacked all power of manoeuvre and the devastating victory of Edward III was due chiefly to the long-bow men whom the French were also encountering for the first time. By 1415, however, gunpowder was decisive in Henry V’s siege of Harfleur, and its increasing use in mobile field guns, naval artillery, and hand-held firearms was a dominant feature of world history for the next 500 y. Parallel with these activities, but largely independent of them, was the European development of the alchemy and chemistry of sulfur, and the growth of the emerging chemical industry based on sulfuric acid.

Selenium. Tellurium. Polonium

Tellurium was the first of these three elements to be discovered. It was isolated by the Austrian chemist F. J. Muller von Reichenstein in 1782 a few years after the discovery of oxygen by J. Priestley and C. W. Scheele, though the periodic group relationship between the elements was not apparent until nearly a century later. Tellurium was first observed in ores mined in the gold districts of Transylvania; Muller called it metallum problematicum or aurum paradoxum because it showed none of the properties of the expected antimony. The name tellurium (Latin tellus, earth) is due to another Austrian chemist, M. H. Klaproth, the discoverer of zirconium and uranium.

Selenium was isolated some 35 y after tellurium and, since the new element resembled tellurium, it was named from the Greek - selene, the moon. The discovery was made in 1817 by the Swedish chemist J. J. Berzelius (discoverer of Si, Ce and Th) and J. G. Gahn (discoverer of Mn); they observed a reddishbrown deposit during the burning of sulfur obtained from Fahlun copper pyrites, and showed it to be volatile and readily reducible to the new element.

The discovery of polonium by Marie Curie in 1898 is a story that has been told many times. The immense feat of processing huge quantities of uranium ore and of following the progress of separation by the newly discovered phenomenon of radioactivity (together with her parallel isolation of radium by similar techniques), earned her the Nobel Prize for Chemistry in 1911. She had already shared the 1902 Nobel Prize for Physics with H. A. Becquerel and her husband P. Curie for their joint researches on radioactivity. Indeed, this was the first time, though by no means the last, that invisible quantities of a new element had been identified, separated, and investigated solely by means of its radioactivity. The element was named after Marie Curie's home country, Poland.

Selenium and tellurium are comparatively rare elements, being sixty-sixth and seventythird respectively in order of crustal abundance; polonium, on account of its radioactive decay, is exceedingly unabundant. Selenium comprises some 0.05 ppm of the earth's crust and is therefore similar to Ag and Hg, which are each about 0.08 ppm, and Pd (0.015 ppm). Tellurium, at about 0.002 ppm can be compared with Au (0.004 ppm) and Ir (0.001 ppm). Both elements are occasionally found native, in association with sulfur, and many of their minerals occur together with the sulfides of chalcophilic metals e .g. Cu, Ag, Au, Zn, Cd, Hg; Fe, Co, Ni; Pb, As, Bi. Sometimes the minerals are partly oxidized, e.g. MSeO3·2H2O (M = Ni, Cu, Pb); PbTeO3, Fe2(TeO3)3·2H2O, FeTeO4, Hg2TeO4, Bi2TeO4(OH)4, etc. Selenolite, SeO2, and tellurite, TeO2, have also been found.

Polonium has no stable isotopes, all 27 isotopes being radioactive; of these only 210Po occurs naturally, as the penultimate member of the radium decay series:

21082Pb (β, 22.3 y) → 21083Bi (β, 5.01 d) → 21084Po (α, 138.38 d) → 20682Pb

Because of the fugitive nature of 210Po, uranium ores contain only about 0.1 mg Po per tonne of ore (i.e. ppm). The overall abundance of Po in crustal rocks of the earth is thus of the order of 3·10-10ppm.