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Element Thallium, Tl, Poor Metal

About Thallium

Thallium occupies a remarkable intermediate position between various other elements. By reason of the physical properties of the free element it is allied to lead, for, like this, it is soft, ductile, and has a high density. Its hydroxide, which is readily soluble in water, procures it a position along with the alkali metals, with which it is isomorphous in various compounds; its sparingly soluble halogen compounds bring it near to silver, copper, and mercury, and in another series of compounds it exhibits relations to the trivalent elements aluminium and iron.

Thallium was discovered by means of the spectroscope; all its compounds on being heated in the Bunsen flame, in which they quickly volatilise, give a green coloration which on being examined with the spectroscope appears as a single bright green line.

Thallium occurs only in small quantity in nature, but, like all the elements which can be detected in small amounts, it has been found to be fairly widely distributed. It is obtained as a by-product from the flue-dust in sulphuric acid works in which pyrites containing thallium is employed, and also, in association with zinc, from zinc ores. In the latter way it could be obtained in fairly large amounts if there were any demand for it.

As has already been mentioned, metallic thallium is very similar to lead, but is still softer. Its density is 11.9, its melting point 290°. It makes a grey mark on paper, but this soon disappears owing to oxidation. Fresh surfaces of the metal, which have an almost silver-white appearance, quickly tarnish in the air through oxidation. In the potential series it stands between cadmium and iron, and is therefore a metal which readily replaces hydrogen from dilute acids. As a matter of fact, it dissolves in dilute acids which do not form sparingly soluble salts, e.g. sulphuric acid and nitric acid, and is precipitated in the metallic state from its solutions by zinc and cadmium.

Thallium forms two kinds of elementary ions, monovalent monothallion, Tl, and trivalent trithallion, Tl•••. The former conditions the similarity of thallium to the alkali metals, the latter that to aluminium.

Thallium History

Thallium was discovered independently by Crookes in England and Lamy in France. Each of these scientists observed a new green line in the spectrum of some material he was examining, attributed it to a new element, and succeeded in isolating it. Crookes, who in March 1861 was the first to make the discovery, was engaged in extracting selenium from a deposit obtained from a sulphuric acid factory at Tilkerode in the Harz. At first he suspected that the new element was a metalloid belonging to the sulphur group, and called it thallium in allusion to the green colour of its spectrum line (Latin, thallus, a budding twig). His early work on thallium was greatly hampered by lack of material, but he eventually discovered the metallic nature of thallium, and in May 1862 was able to exhibit a few grains of the metallic element in powder form. Lamy made his discovery in April 1862, when examining the lead chamber deposits from the sulphuric acid factory of M. F. Kuhlmann at Loos, where Belgian pyrites were burnt. Being more fortunate than Crookes in having considerable material at his disposal, Lamy was able very quickly to establish the metallic nature of thallium and to exhibit a lump of the metal in May 1862; and before the end of the year he was able to isolate several hundred grams of thallium and give a fairly complete account of the physical and chemical properties of the element. He showed that thallium forms more than one series of compounds, and that one series closely resembles the corresponding series of compounds of the alkali metals. Further, he found that in this series of compounds the equivalent of thallium is about 204, and (with the assistance of Regnault) showed by the application of Dulong and Petit's Law that this number also represents the atomic weight of thallium. During the same year Kuhlmann prepared a number of organic thallium salts, and de la Provostaye indicated the isomorphism of several of them with the corresponding salts of potassium. During the next few years a considerable amount of work on thallium was published, notably by Crookes, Lamy, Descloizeaux, Kuhlmann, Werther, Bottger, Nicklks, Willm, and Carstanjen.

At the time when thallium was discovered, the Periodic Classification had not been formulated, and it proved a difficult matter to place thallium in the classification generally accepted at that period for the metals. A number of thallous compounds were observed to resemble closely the corresponding compounds of lead in their physical properties; and even more remarkable was the resemblance between the actual elements themselves, lead and thallium. Other thallous salts, however, were observed to be isomorphous with, and to resemble quite closely, the corresponding salts of potassium. Moreover, the spectrum of thallium was extremely simple and very easy to observe, resembling in this respect the spectra of sodium, potassium, and the then newly discovered elements rubidium and caesium. It is therefore not surprising that Dumas should have reported to the French Academy that "le thallium offre une reunion de proprietes contradictoires qui autoriserait a l'appeler le metal paradoxal, l'ornithorynque des metaux."

To classify thallium with the alkali metals is clearly unsatisfactory, since the element itself is decidedly different in properties from sodium, potassium, etc., and, moreover, it forms a series of salts in which it is tervalent; to classify it with lead is worse, since the only analogies between lead and thallium salts are physical analogies between salts of univalent thallium and bivalent lead. With the publication of the Periodic Classification, Mendeleeff pointed out that thallium should really be classified with aluminium and indium, and made out a very good case for this contention. Recalling the fact that when the elements are arranged in the increasing order of their atomic weights, aluminium comes between magnesium in Group II. and silicon in Group IV., while thallium comes between mercury in Group II. and lead in Group IV., the case may be stated almost exactly as it was given by Mendeleeff. Only the highest oxide of mercury shows any analogy with magnesium oxide; only the highest oxide of lead shows any analogy with silica. In the same way, only the highest oxide of thallium shows any analogy with alumina. Magnesium and mercuric oxides are basic oxides and give rise to salts of the type MX2; aluminium and thallic oxides are more feeble bases and give rise to salts of the type MX3; silicic and plumbic oxides are feeble acidic oxides. Thallium gives, independently of thallic oxide, a lower oxide which is a powerful base; aluminium forms no lower basic oxide. Strictly analogous is the existence of basic oxides of mercury and lead lower than mercuric and plumbic oxides, and the non-existence of lower oxides of magnesium and silicon. The higher oxides of the formulae HgO, Tl2O3, PbO2, and Bi2O5 are peroxides relatively to the lower oxides of the formulae Hg2O, Tl2O, PbO, and Bi2O3, and give off oxygen when heated. The higher chloride of mercury is stable, that of thallium unstable, the tetrachloride of lead very unstable, and pentachloride of bismuth non-existent. Bismuth trichloride is decomposed by water, and lead (di)chloride when heated in steam, but thallous chloride is stable. Thallium is less volatile than mercury but more so than lead. The similarity between thallous and alkali salts is no more remarkable than that between the plumbous salts and the salts of the alkaline-earth metals, or that between bismuth salts and the salts of certain elements of Group III. There is, then, no more difficulty in placing thallium in Group III. than there is in placing mercury in Group II., lead in Group IV., or bismuth in Group V.

Thallium Occurrence

Poor metal Thallium crustal abundance is 7x10-5 mass %, which is hundredfold of gold abundance and tenfold of that of silver. Thallium is a trace element; its own minerals are very rare, but it vastly occurs as isomorphic component in a great number of other minerals, replacing copper, arsenic and silver in sulphide ores as well as potassium, rubidium and, less frequently, other alkali metals, in alumosilicates and chlorides.

Thallium is very abundant in leucite KAlSi2O6 and in orthoclase KAlSi3O8. It's content in lepidolite K2Li1.5Al1.5[Si3AlO10][F, OH]2 and in zinnwaldite is approximately 10-3-10-1%. Relatively large amount of thallium is found in pollucite - 10-2%. Thallium concentration in sulphide minerals is around 10-3%. This element has been found in deposits of zinc blende (sphalerite), and galena (lead glance). In hydrothermal sulphide, complex and lead-zinc ores the concentration may excess 0.1%.

The average abundance is 10-5% in soils, 10-9% in sea water, and 4x10-5% in animal tissues. Some plants such as beet, grapes, oak tree, beech tree as well as sea creatures and seaweed can extract environmental thallium and accumulate it. That is the reason of high thallium concentration in coals - 10-3-10-2%.


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