Atomic weight definition

The periodic table of the elements is based on atomic number and reactivity. The elements are represented by their chemical symbol, which is listed in large font below their respective full name. Atomic number and standard atomic weight (definitions are provided in Sections 2.1.1 and 2.1.1.1) are listed above and below the respective chemical symbol. Atomic weights of elements given within square brackets do not occur naturally. 

Atomic Weight. As of this writing (ca 1994) the definition of atomic weights is based on carbon-12 [7440-44-0], the most abundant isotope of carbon, which has an atomic weight defined as exactiy 12 (21). 

The equivalent weight of an ion (or an element) is the ratio of its formula weight to its valence. According to an alternative definition that is also suitable for compounds, an equivalent weight represents the amount of a substance which will react with one atomic weight of hydrogen or its chemical equivalent. 

To understand how the electron has been applied to explanations of the periodic table we must start with the discovery of the periodic system itself. The Russian chemist Dimitri Mendeleev announced in 1869 that the properties of elements arranged in order of increasing atomic weight appeared to repeat after certain definite intervals. Yet even as this discovery became increasingly well established, Mendeleev remained strongly opposed to any attempt to reduce or explain the periodicity in terms of atomic structure. He resisted the notion of any form of primary matter, which was actively discussed by his contemporaries, and opposed... 

Loring published chemistry books—Studies in Valency (1913), Atomic Theory (1921), Definition of the Principle of Equivalence (1922), and The Chemical Elements (1923). During the brief existence of the Alchemical Society, he published twenty articles (eight of them lead articles) in Chemical News on such subjects as atomic weight, the radio-atoms, the evolution of chemical elements, and a five-part Introduction to the Theory of Relativity. He also published seven correspondences in the journal, and Chemical News reviewed his Studies in Valency positively. 

All elements, by definition, have a unique proton number, but some also have a unique number of neutrons (at least, in naturally occurring forms) and therefore a unique atomic weight - examples are gold (Au Z = 79, N = 118, giving A =197), bismuth (Bi Z = 83, N = 126, A = 209), and at the lighter end of the scale, fluorine (F Z = 9, N = 10, A = 19) and sodium (Na Z = 11, N= 12, A = 23). Such behavior is, however, rare in the periodic table, where the vast majority of natural stable elements can exist with two or more different neutron numbers in their nucleus. These are termed isotopes. Isotopes of the same element have the same number of protons in their nucleus (and hence orbital electrons, and hence chemical properties), but... 

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. 

Chlorinity was first defined in 1902 and, hence, was affected by subsequent refinements in measurement of the atomic weight of chlorine. To make chlorinity independent of any such future changes, a new definition of chlorinity was adopted in 1937, e g., the mass of silver required to precipitate completely the halogens in 0.3285234kg of sample seawater. ... 

Dalton s Law of Multiple Proportions meant that two elements combine in simple whole number ratios. Dalton believed that compounds found in nature would be simple combinations. Hence, knowing that hydrogen combines with oxygen to give water, Dalton s formula for water would consist of 1 H and 1 O. Its formula would be HO using modern nomenclature. Both Proust s Law of Definite Proportions and Dalton s Law of Multiple Proportions are outcomes of an atomic view of nature. In 1808 Dalton published his table of relative atomic weights along with his ideas on atomism in A New System of Chemical Philosophy. 

NE OF THE CENTRAL THEMES of this book is to show how the development of the concept of neutral salt in the eighteenth century made possible the creation of a compositional nomenclature by L.-B. Guyton de Morveau in 1782, which when adapted to the new chemistry of Lavoisier led to the creation of a definition of simple body the material element. The second major theme then describes how this new chemistry led to the final development of modern chemical composition in its atomic structure introduced by John Dalton. His atomic theory contained the symbolic operators that furnished the most convenient representation of the material composition of bodies that had become available by the end of the eighteenth century. The idea of an individual atomic weight unique to each element depended most immediately upon the concept of simple body, introduced by the authors of the M thode de nomenclature chimique in 1787. The new nomenclature was itself based on the principle that a name of a body ought to correspond to its composition. 

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. 

The isotopic composition of the elements (including the exact atomic mass and the abundance of the isotopes), the atomic weights of elements, definitions and abbreviations are summarized in Appendix II. 

Compound a substance consisting of two or more different kinds of atoms in definite proportions by weight. 

That the atomic weight of uranium lead is extremely variable has already been shown. In order to interpret this variability its sources must be studied both geologically and mineralogically. On the geologic side of the question the uranium ore can be divided in to three principal classes, which are sharply distinct. The definitely crystallized varieties of uraninite occur in coarse pegmatites, associated with feldspar, quartz, mica, beryl, and other minor accessories. The massive pitchblende is found in metalliferous veins, together with sulphide ores of copper, lead, iron, zinc, and so forth. As for camotite, that is a secondary mineral, found commonly as an incrustation on sandstone, and often, also upon fossil wood. There may be other modes of occurrence, but these are the most distinctive. 

The latter assumption is now seen to be extremely doubtful for,the varying atomic weights prove that more than one kind of lead must be considered. Thorium lead especially must be taken into account, for many uraninites contain it, and in thorianite the percentage of thoria is more than five times that of uranium oxide. The ratio of lead to its parent elements is therefore much less than Boltwood assumed, and the calculated age of thorianite is vastly reduced. Boltwood, however, doubted the derivation of lead from thorium, a fact which was not definitely known at the time his paper was written. The evidence of the atomic weights is also much later. 

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