Dalton, John atomic weights

Different names are used for the atomic weight by different authors and different abbreviations are used for it. The term dalton is used by some, in honor of John Dalton, and these authors use the abbreviation D. Other authors use the name atomic mass unit. The abbreviation u rather than amu is sometimes encountered. 

The relative molecular mass of a molecule is the sum of the atomic masses of its constituent atoms. The term has replaced molecular weight because weight is a parameter that depends on the magnitude of gravitational attraction. Since relative molecular mass is a ratio (of the mass of the molecule to one-twelfth of the mass of the carbon-12 atom) no units are required. It has, however, become accepted practice to use daltons as a unit of molecular mass, commemorating John Dalton s atomic theory of matter. Relative molecular mass is an approximate indication of size a spherical molecule of 5000 ddtons (or 5 kDa) has a diameter of approximately 2.4 nm. 

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. 

British chemist and physicist John Dalton, who drew up the first list of atomic weights. 

John Dalton (1766-1844), an Englishman, began teaching at a Quaker school when he was 12. His fascination with science included an intense interest in meteorology (he kept careful daily weather records for 46 years), which led to an interest in the gases of the air and their ultimate components, atoms. Dalton is best known for his atomic theory, in which he postulated that the fundamental differences among atoms are their masses. He was the first to prepare a table of relative atomic weights. 

The History of the Atomic Weight Scale. John Dalton (1776-1844), who in 1803 made the old atomic hypothesis into a useful scientific theory by developing... 

The problem of determining the shape of a molecule has been the subject of research for a long time." This was addressed at the beginning of the nineteenth century by John Dalton. He proposed that the combining proportions of atoms in the molecules that make up compounds could be explained by assuming that each element had a different weight (its atomic weight). By chemical analysis of simple compounds he could determine their formulae and propose the connectivities of the component atoms. Attempts to describe the shapes of molecules continued to interest... 

During the years 1803 to 1808, John Dalton put forward his atomic theory, emphasizing a quantitative characteristic of the atoms, their weights. All atoms of the same element had the same weight, but the weights of atoms of different elements were different. The determination of atomic weights, whether relative or absolute, turned out to be a harder task than the enunciation of the principle. 

The modern definition of atomic mass (called atomic weight by John Dalton) can now be... 

John Dalton, born Eaglesfield, England, 1766. Considered the founder of quantitative chemical atomic theory law of definite proportions, pioneered determination of atomic weights. Cofounder of British Association for the Advancement of Science. Died Manchester, England, 1844. 

The second son of a modest Quaker weaver in England s Lake District, John Dalton s contribution to chemistry was to reintroduce a systematic atomic theory based on the elements of Lavoisier. We say reintroduce because the concept of atoms was certainly nothing new Democritus postulated atoms in pre-Aristotelian Greek philosophy, and atoms were proposed by Descartes and Hooke. In 1738 Daniel Bernoulli correctly derived Boyle s law by assuming gases consisted of collections of particles that continuously collided with the container walls. But Dalton did not propose atoms as an abstraction or mathematical device Dalton s atoms were physical. They had characteristic masses (atomic weights) and combinations of these atoms in fixed ratios made up the range of chemical compounds. 

Rocke s in4)ortant book Chemical Atomism in the Nineteenth Century From Dalton to Catmizaro (Columbus Ohio State University Press, 1984) emphasizes the importance of atomic weight for the development of nineteenth-century chemical atomism. Obviously this factor distinguishes the chemistry of John Dalton and his heirs from the chymistry of the seventeenth century, but the operational consideration of an atom as that which resists laboratory analysis remains the same. See Rocke, Chemical Atomism, pp. i—15. 

Atomic weights, as distinct from equivalent weights, were first obtained in the early 1800s by John Dalton, who indirectly inferred them from measurements on the masses of the relevant elements combined together. But there were complications in this apparently simple method that forced Dalton to make assumptions about the chemical formulas of the compounds in question. The key to this question is the valence, or combining power, of an element. For example, a univalent atom combines with hydrogen atoms in a ratio of 1 1 divalent atoms, such as oxygen, combine in a ratio of 2 1 and so on. 

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