Atomic weights precision

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Because of their central importance in chemistry, atomic weights have been continually refined and improved since the first tabulations by Dalton (1803 -5). By 1808 Dalton had included 20 elements in his list and these results were substantially extended and improved by Berzelius during the following decades. An illustration of the dramatic and continuing improvement in accuracy and precision during the past 100 y is given in Table 1.3. In 1874 no atomic weight was quoted to better than one part in 200, but by 1903 33 elements had values quoted to one part in 10 and 2 of these (silver and... 

Accurate atomic weight values do not automatically follow from precise measurements of relative atomic masses, however, since the relative abundance of the various isotopes must also be determined. That this can be a limiting factor is readily seen from Table 1.3 the value for praseodymium (which has only 1 stable naturally occurring isotope) has two more significant figures than the value for the neighbouring element cerium which has 4 such isotopes. In the twelve years since the first edition of this book was published the atomic weight values of no fewer than 55 elements have been improved, sometimes spectacularly, e.g. Ni from 58.69( 1) to 58.6934(2). 

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... 

In summary, as a consequence of the factors considered in this and the preceding section, the atomic weights of only the 20 mononuclidic elements can be regarded as constants of nature . For all other elements variability in atomic weight is potentially possible and in several instances is known to occur to an extent which affects the reliability of quantitative results of even modest precision. 

Boron has 2 stable naturally occurring isotopes and the variability of their concentration (particularly the difference between borates from California (low in °B) and Turkey (high in °B) prevents the atomic weight of boron being quoted more precisely than 10.811(7) (p. 17). Each isotope has a nuclear spin (Table 6.1) and this has proved particularly valuable in nmr spectroscopy, especially for The great... 

Some of the important properties of the elements are given in Table 18.1. The imprecision of the atomic weights of Kr and Xe reflects the natural occurrence of several isotopes of these elements. For He, however, and to a lesser extent Ar, a single isotope predominates ( He, 99.999 863% " Ar, 99.600%) and much greater precision is possible. The natural preponderance of " Ar is indeed responsible for the well-known inversion of atomic weight order of Ar and K in the periodic table, and the position of Ar in front of K was only finally accepted when it was shown that the atomic weight of He placed it in front of Li. The second isotope of helium, He, has only been available in significant amounts since... 

Some of the important properties of Group 5 elements are summarized in Table 22.1. Having odd atomic numbers, they have few naturally occurring isotopes Nb only 1 and V and Ta 2 each, though the second ones are present only in very low abundance 0.250%, Ta 0.012%). As a consequence (p. 17) their atomic weights have been determined with considerable precision. On the other hand, because of difficulties in removing all impurities, reported values of their bulk properties have often required revision. 

As can be seen from Table 23.1, which summarizes some of the important properties of Group 6, each of these elements has several naturally occurring isotopes which imposes limits on the precision with which their atomic weights have been determined, especially for Mo and W. 

Table 27.1 lists some of the important atomic and physical properties of these three elements. The prevalence of naturally occurring isotopes in this triad limits the precision of their quoted atomic weights, though the value for Ni was improved by more than two orders of magnitude in 1989... 

A selection of some important properties of the elements is given in Table 29.1. Because the elements each have several naturally occurring isotopes their atomic weights cannot be quoted with great precision. 

Historians differ regarding the precise assignment of elements to these values. In particular they disagree with respect to the identity of the element depicted as 7 or 14. According to some it is twice the atomic weight of lithium, while others maintain that it is beryllium using an older value for its atomic weight. 

If chemistry was characterized in the nineteenth century by the precise measurement of the products of chemical combustion and combination, as well as by the precise calculation of elementary combining proportions or atomic weights, physics, too, came increasingly to be identified not just with experimentalism but with precise measurement and the "last decimal place." As Maxwell put it shortly before his death in 1879,... 

Lee DC, Halliday AN (1995) Precise determinations of the isotopic compositions and atomic weights of molybdenum, tellurium, tin and tungsten using ICP magnetic-sector multiple collector mass-spectrometry. Int J Mass Spectr Ion Proc 146 35-46... 

We have known for many years that large isotopic fractionations of heavy elements like Pb develop in the source regions of TIMS machines. Nonetheless, most of us held fast to the conventional wisdom that no significant mass-dependent isotopic fractionations were likely to occur in natural or laboratory systems for elements that are either heavy or engaged in bonds with a dominant ionic character. With the relatively recent appearance of new instrumentation like MC-ICP-MS and heroic methods development in TIMS analyses, it became possible to make very precise measurements of the isotopic ratios of some of these non-traditional elements, particularly if they comprise three or more isotopes. It was eminently reasonable to reexamine these systems in this new light. Perhaps atomic weights could be refined, or maybe there were some unexpected isotopic variations to discover. There were. 

Trace elements include heavy metals, some of which have recently received particular attention. Many definitions of heavy metals have been put forward. The simplest and most precise describes heavy metals as all metal compounds of atomic weight over 20. Other definitions are based on the specific weight, and give the lower limits for heavy metals as 4.5, 5, or even 6 g per cm. Due to toxicity of some heavy metals and the possibility of environmental contamination, the potential for high risk is linked to Hg, Cd, As, Pb, as well as Cu, Zn, Sn, Cr, Ni. 

Instead of using the integers 37 and 35 as atomic masses, take the more precise atomic weights of the isotopes from Table 3-3 ... 

This method is certainly valid when the property in question is the molecular weight, which is precisely the sum of the contributions from each atom according to the atomic weights and number of occurrences. For some properties, this method can be reasonably effective. But this method does not work very well if the property is not additive, or if the groups have strong interactions in a way that is either synergistic or antagonistic. 

Oxygen is present as three stable isotopes, abundance of which varies slightly with its natural source, so that the precision of its atomic weight is hmited to 15.9994 0.0003. is the most abundant (99.760% 15.994915), the two... 

Jean-Charles Galissard de Marignac, 1817-1894. Swiss chemist who discovered ytterbia and gadolima and made many important contributions to the chemistry of the rare earths. Professor of chemistry at the University of Geneva. He made precise determinations of the atomic weights of many elements, and by separating tantahc and columbic (mobic) acids, proved that tantalum and columbium (niobium) are not identical. 

Marignac s life work, which, like that of Stas, consisted in making many precise determinations of atomic weights in order to test William Prout s hypothesis (71), won Berzelius sincerest praise, for he wrote ... 

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. 

It is unfortunate that Keir did not push on with this idea of an element as any body that we have not yet been able to decompose. As we shall see later, this is precisely the definition that comes to be utilized by many later chemists, and constitutes the operational basis for the new nomenclature and ultimately the expression of composition in terms of atomic weights. I know of no earlier advocacy of this idea so clearly stated as this by James Keir. 

No one saw more clearly than Berzelius did that achieving such a chemical calculus depended upon establishing with great precision the atomic weights of a few key elements as standards from which others could be calculated. And no one undertook the task more urgently and successfully. 

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