Selenium atomic weight

In the various editions of his textbook, and in the publications dealing specifically with his predictions, Mendeleev repeatedly gives the example of calculating the atomic weight of the element selenium, a value that was known at the time and which could thus be used to test the reliability of his method. 

The realization that the better ordering criterion is atomic number rather than atomic weight invites us to consider triads of atomic numbers. This reveals a most remarkable fact, namely that —50% of all conceivable triads on a conventional periodic table are in fact exact. For example, the elements sulfur, selenium, and tellurium have atomic numbers of 16, 34, and 52, respectively, thus showing that the atomic number of the middle of these three... 

Selenium is characterized by an atomic weight of 78.96, an atomic number of 34, a melting point of 271°C, a boiling point of 685°C, and a density of 4.26 to 4.79. Chemical properties, uses, and environmental persistence of selenium were documented by a number of researchers whose works constitute the major source material for this section Rosenfeld and Beath (1964) Bowen (1966) Lakin (1973) Stadtman (1974, 1977) Frost and Ingvoldstad (1975) Chau et al. (1976) Harr (1978) Wilber (1980,1983) Zieve and Peterson (1981) Robberecht and Von Grieken (1982) Cappon and Smith (1982) Nriagu and Wong (1983) Eisler (1985) USPHS (1996). 

Consider the proper placement of tellurium and iodine in the periodic table, as shown in Figure 1-3. Te has the heavier atomic weight. The chemical properties of tellurium are like those of selenium because both are semi-metallic elements that form compounds like those of sulfur. Iodine resembles bromine because these elements are nonmetallic halogens that form compounds like those of chlorine. Therefore, the order in the table cannot be based solely on atomic weight. 

Jons Jacob Berzelius, 1779-1848. Professor of chemistry and medicine at the Stockholm Medical School. He determined the atomic weights of most of the elements then known, discovered selenium and the earth ceria, and isolated silicon, thorium, and zirconium. Among his students may be mentioned Wohler, Heinrich and Gustav Rose, Mosander, Sefstrom, and... 

Berzelius determined the atomic weights of nearly all the elements then known, and was the first chemist to determine them accurately. (19). He referred his atomic weights to oxygen, which, however, he allowed to equal 100, instead of 16 as in our present system. In his little laboratory that looked like a kitchen and in which the sandbath on the stove was never allowed to cool, Berzelius discovered the important elements selenium, silicon, thorium, cerium, and zirconium (18). 

The first determination of the atomic weight of selenium was due to Berzelius,3 who in 1818 synthesised the tetrachloride, with the following result 4... 

In 1847 Sacc determined the atomic weight by several methods, of which only two proved of value, namely (i) pure selenium dioxide was reduced with ammonium hydrogen sulphite and the resulting selenium weighed (ii) barium selenite was calcined with sulphuric acid and the resulting barium sulphate weighed. The results were as follows 5... 

In general, the metal selenides correspond in composition with the sulphides, are much less stable, oxidise quickly in air, and when prepared in the wet way cannot be dried without some decomposition. They are generally deeper in colour than the corresponding sulphides, a fact which is probably associated with the higher atomic weight of selenium. 

Selenium and tellurium arc generally considered rare elements and are not found in abundance each, however, has a claim to special interest. The peculiar electrical properties of selenium appeal especially to the physical chemist, but the compounds of this element are of growing importance, especially in their application to the glass industry. Both selenium and tellurium offer attractive fields for research. In the case of tellurium, owing to what had been considered an erroneous value for the atomic weight, the very considerable amount of work done in the past has been too much confined to one direction interest in the chemistry of tellurium appears now to be widening, however. 

Thus, for example, the boiling-points of the hydrides of sulphur, selenium, and tellurium steadily fall with the atomic weights of the electro-negative elements ... 

A photometric titration of selenium and S20eF2 established the oxidation state of the yellow species as + conductometric measurements showed that two fiuorosulfate ions are produced per four selenium atoms and the molecular weight of Se was established by cryoscopy. The absorption spectrum of the yellow 804 solution in HSOgF is shown in Fig. 4. 

The enzyme glutathione peroxidase contains 0.34% selenium by weight. (The atomic weight of Se is 78.96.) The MW of the enzyme determined from gel filtration is about 88,000. What is the likely quaternary structure of this enzyme ... 

The distinctions between the system of equivalents ordinarily employed in this country, and the 2 volume notation adopted in this work, may be thus expressed. In the new system, the atomic weight of carbon is 12, of oxygen is 16, of sulphur is 32 and similarly with selenium and tellurium all the other atomic weights are unaltered. The atomic expressions for all bodies not containing carbon, oxygen, or sulphur, are unchanged thus, H, N, HCl, KI, NH, c. When in an atomic formula the atoms of carbon, oxygen, and sulphur, form even numbers, these numbers are halved thus, alcohol becomes G H O, ethyl... 

Selenium (atomic number 34, atomic weight 78.96) is a nonmetal and has several chemical forms and valences. Selenium is in group VI of the periodic table, and therefore it has a bioinorganic chemistry that is related to sulfur. 

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