Atomic Weight of Tellurium

The atomic weight of tellurium is one which has been the subject of much investigation on account of the position of the element in the Periodic Table. Tellurium must on common-sense grounds be classed in Group VI along with sulphur and selenium, rather than with the halogens in Group VII, in spite of the fact that if the atomic weights were alone considered tellurium would be ranked with the halogens and iodine with sulphur and selenium. Mendeleef and Brauner expressed the opinion that this apparent anomaly in the table was due to the presence in the tellurium of a small amount of an element, similar to tellurium but of higher atomic weight, and this opinion has been upheld by Bettel and by Browning and Flint. Many other investigators, however, consider tellurium to be homogeneous, and all attempts at a separation have led to negative results.

The work of Aston and others has shown that a property of an element which is more fundamental than its atomic weight is its atomic number, or the number of resultant positive charges on the nucleus of

the atom. When arranged in order of atomic number, tellurium precedes iodine.

Measurements of the wave-lengths of lines in the high frequency spectra of iodine and tellurium by the primary ray method have given

λ = 0.437×10^-8 and λ = 0.388×10^-8 cm.

for the wave-lengths of the α₁ and β₁ lines in the spectrum of iodine, the corresponding values for tellurium being

λ = 0.456×10^-8 and λ = 0.04×10^-8 cm.

These numbers satisfy Moseley's formula connecting the frequency with the atomic number if the atomic numbers of tellurium and iodine are respectively 52 and 53.

The atomic weight of tellurium was determined for the first time by Berzelius in 1812, the value obtained being 129.2. The tellurium used, however, was impure, a fact only recognised in 1817. In 1833 Berzelius repeated his determinations, converting tellurium into its dioxide; this time he obtained the value 128.34.

In 1858 von Hauer prepared and analysed the double bromide of potassium and tellurium, K₂TeBr₆, and obtained the value 127.8.

Wills in 1879 also adopted the preceding method and as a result of five experiments obtained the mean value 127.1. Wills also used the oxidation method of Berzelius, first using nitric acid as the oxidising agent and obtaining as the mean of five experiments the value 128.1, then using aqua regia, the mean of four experiments giving the value 128.2. With aqua regia the results were very much more consistent than with nitric acid. Many other workers have repeated these determinations at later dates, obtaining results slightly differing among themselves, but all higher than the atomic weight of iodine.

Brauner, after trying various unsatisfactory methods, prepared and analysed pure tellurium tetrabromide, obtaining the value 127.54.

A method involving the analysis of the basic nitrate, 2TeO₂.HNO₃, which is conveniently prepared by dissolving tellurium in a slight excess of nitric acid and evaporating the solution to crystallising-point, has led to concordant results being obtained by Kothner and by Norris and his co-workers. On careful ignition of the salt the dioxide is obtained. The method was criticised by Dudley and Bowers as giving very irregular results, since tellurium may crystallise from nitric acid solution as tellurous acid, tellurium dioxide and basic nitrate, the exact conditions for the separation of any one of these appearing to be very elusive. Good results have been obtained, however, by Stahler and Tesch, who first very carefully purified the tellurium used by fractional distillation, which removes most impurities except antimony; the latter was removed by conversion to tellurium tetrachloride and fractional distillation in a stream of chlorine. The tetrachloride, dissolved in hydrochloric acid and precipitated by sulphur dioxide, yielded an amorphous product which on further distillation gave tellurium spectroscopically free from impurities. This was converted into the basic nitrate by dissolving in nitric acid and heating in a stream of dry air. On decomposition of the product by heat, tellurium dioxide was obtained, which on analysis gave the value 127.513 ± 0.003 for the atomic weight of the element.

Dudley and Bowers, having first obtained pure tellurium by fractional precipitation with hydrazine hydrochloride, synthesised the tetra- bromide and obtained a mean value of 127.479 for the atomic weight.

Gutbier and Wagenknecht in 1905 carried out a series of determinations based on the reduction of tellurium dioxide to the metal. The reduction was effected by two distinct methods, but the results in both cases were practically the same. In the first series of experiments the dioxide was mixed with silver and powdered quartz and reduced in a current of hydrogen. In the second series the reduction was effected by means of hydrazine hydrochloride. The mean value for the atomic weight from both series of determinations was 127.62.

Marckwald, after a research with unsatisfactory material, prepared some pure tellurium dioxide from telluric acid which had been crystallised several hundred times. The dioxide was analysed volumetrically by oxidising to telluric acid by means of potassium permanganate, excess of the latter being determined by means of oxalic acid. The mean result obtained was 127.61.

Lenher determined the atomic weight of tellurium by decomposing weighed amounts of potassium telluribromide, K₂TeBr₆, by means of chlorine and hydrogen chloride, and weighing the potassium chloride which remained. Using tellurium ores from three different sources, the oxide obtained was converted into the double bromide by the action of hydrobromic acid and potassium bromide; the telluribromide formed was crystallised repeatedly from water. As a mean of sixteen concordant experiments Lenher obtained the value 127.55.

The apparent simplicity of the foregoing method and the close agreement of the results obtained led Dennis and Anderson to employ it in their investigations. These workers found, however, that it was impossible to free the potassium telluribromide from water without causing partial decomposition of the salt. They then prepared hydrogen telluride by electrolysis and oxidised it by passing it into nitric acid, calculating the atomic weight of tellurium from titration of the resulting dioxide with potassium permanganate. The average result was 127.50.

The method of Bruylants and Desmet was similar to the foregoing. Tellurium prepared from hydrogen telluride was treated with nitric acid and the resulting dioxide separated by evaporation, calcined, and dissolved in aqueous sodium hydroxide. The tellurium was then estimated volumetrically either in alkaline or just acid solution. The mean of twelve estimations in alkaline solution gave the value 127.8, and of nine estimations in acid solution the value 127.65.

Hydrogen telluride was also the starting-point in the investigation of Bruylants and Michielsen. The gas after careful purification was decomposed into its elements at 200° to 220° C., the tellurium weighed as such and the hydrogen oxidised to water by means of cupric oxide. The value obtained was 127.8.

Baker and Bennett adopted a method apparently not used by any other investigator, but which is applicable to elements which lie on the border line between metals and non-metals. Tellurium dioxide was reduced to the element by heating with sulphur in a current of nitrogen, the sulphur being oxidised to sulphur dioxide. From the ratio TeO₂:SO₂, the value obtained for the atomic weight was 127.609. This result, however, is high, and it was found that some sulphur trioxide was also formed.

The value adopted by the Chemical Society on the recommendation of the Sub-Committee on Atomic Weights (1929) is Te = 127.5.

Tellurium is a mixture of three isotopes, the mass-numbers of which are 128, 130 and 126. The intensities of the first two are about equal and double that of the third, so that it would seem probable that the mean atomic weight is at least as high as 128. The discrepancy between this conclusion and the actual values obtained remains to be explained. Tellurium is unique in that all its mass-numbers form members of isobaric pairs, these being shared by xenon, the element of next higher even atomic number. The atomic number of tellurium is 52.