Atomic weights, determination

Elements with 1 predominant isotope can also, potentially, permit very precise atomic weight determinations since variations in isotopic composition or errors in its determination have a correspondingly small effect on the mass-spectrometrically determined value of the atomic weight. Nine elements have 1 isotope that is more than 99% abundant (H, He, N, O, Ar, V, La, Ta... 

Although the Curies noted that one equivalent gram of radium released one hundred calorics of heat per hour, they were uninterested in the practical implications of this, as they were both devoted to pure scientific discovery. During their work with pitchblende in 1898, the Curies discovered two new radioactive elements, which they named polonium (in honor of Marie s homeland) and radium. By 1902 they had isolated a pure radium salt and made the first atomic weight determination. 

Atomic Weight Determinations. Dictionary of American History, Rev. Edn., Charles Scribner s Sons, New York, 1976, pp. 218-19. 

Extensive references to this work are given by Soddy. It is noted that a major contributor to these studies was the Harvard University chemist T.W. Richards (NLC1914), an expert on atomic weight determinations. 

As new experiments were performed, the results sometimes seemed to contradict Berzelius s atomic weight determinations. Different chemists began to make different assumptions about how the weights should be calculated. There was confusion also about the terms atom and molecule themselves. At mid-century these two words were still used interchangeably. Even the discovery of simplicity sometimes proved to be confusing. In 1808 the French chemist and physicist Joseph Louis Gay-Lussac discovered a simple chemical law and chemists did not know how to explain it. 

Theodore William Richards, 1868-1928. Director of the Wolcott Gibbs Memorial Laboratory at Harvard University. The precision of his atomic weight determinations has never been surpassed. He discovered the anomalous atomic weights of lead from radioactive minerals. 

The extreme values of the individual determinations still differed by more than a unit, and it is doubtful whether metallic tantalum can be obtained sufficiently pure for atomic weight determination. 

In order to obtain specimens of arsenic trichloride sufficiently pure for atomic weight determinations, Krepelka fractionally distilled three times in nitrogen the product from the reaction of pure metallic arsenic and dry chlorine. The distillate was then fraetioned in vacuo, the middle fraction being filled into bulbs.6... 

Asbestos, platinized, 1 160 3 129 Atomic weight, determination of average, of rare earth elements in a mixture, 2 58 Azides, alkali and alkaline earth, 1 79 2 136, 139... 

All of these determinations of atomic weight, including those of Baxter and Gover on normal lead, were made by the same method, the same care as to purity of materials, and the same refinements of technique. Even Honigschmid, now in Vienna, had worked on atomic weight determinations with Richards, and so was familiar with the best procedure. The results obtained are therefore strictly comparable. 

Elements 43, Masurium 61, Illinium 84, Polonium or Radium F 89, Actinium 91, Uranium Xs do not appear in the atomic weight tables. Although their existence has been indicated by means of X-rays or radioactive properties, they have not been isolated in amounts to allow of atomic weight determination. 

Chemical analyses and even atomic weight determinations are nowhere near adequate for the purpose and differential density measurements seemed to offer the only hope of success. Density measurements moreover are preferred on account of rapidity of determination, simplicity of calculations, and freedom from molecular excitation. 

Throughout the nineteenth century the atoms of the ninety-two known elements were regarded as the ultimate indivisible particles of the material universe. Also, all the atoms of any one element were supposed to be exactly alike, and the atomic weights determined by the chemists were believed to be the actual relative weights of the atoms. The epoch making discoveries of J. J. Thomson and Aston have shown that the atoms are not the ultimate particles, but that... 

In preparing a specimen of the pure metal for atomic weight determinations, Winkler found the following solution useful ... 

The preceding method is open to the criticism that silver orthophosphate contains only 7-4 per cent, of phosphorus. Phosphorus tribromide is somewhat better in this respect, containing 11-5 per cent, of phosphorus but on the other hand the preparation and quantitative decomposition of this compound in a manner suitable for atomic weight determinations present great difficulties, the nature of which is apparent from the following narrative.1... 

Richards and his coadjutors have proved one of the fundamental errors in the work of Stas, for years regarded as a model of accuracy, to have been the employment of excessive quantities of substances. His object was to avoid errors in weighing, but his method necessitated a concentration of the solutions such as induced occlusion of extraneous material in his precipitates. This experimental defect and other sources of error have been avoided by the American school, with Richards as its leader, and a new era in the field of atomic-weight determination has been initiated. 

Bohuslav Brauner had approached the rare-earth crisis from a fradifional chemical poinf of view. Bofh the search for higher valencies and complicated atomic weight determinations had been central to his research during the last quarter of fhe nineteenfh century, but neither of fhese methodologies allowed to prove the validity of the homologous accommodation. Brauner s adherence to the Mendeleev method had led him to a... 

Atomic-Weight Determinations by the Gas-Density Method. If a sufficiently careful measurement of the density of a gas is made, under conditions such that the gas obeys the perfect-gas law, a good value for the molecular weight of the gas can be obtained, which can be used to find the atomic weight of one of the elements in the gas. The way to determine this ideal value of the density of a gas is to determine the density of the gas at smaller and smaller pressures, and to extrapolate to zero pressure-all gases approach the perfect-gas law in their behavior as the pressure becomes very low. This method of atomic-weight determination has been used extensively in recent vears by the Spanish cheijaist E. Moles. 

The determination of atomic weights has been preeminent in the study of the reliability of analytical methods and in the definition and development of reference materials. In the history of chemistry no significant problem has been under more continuous investigation than that of atomic weights. Determinations of atomic weights by chemical methods require highly refined experimental techniques, and only a few... 

---