Atomic weight dalton

And now Berzelius set himself a still greater task. While working on a new textbook, he came across the work of Richter on the proportions in which substances combine. This started him on an investigation of atomic weights. Dalton s relative... 

TABLE 1.1 Dalton s Relative Atomic Weights (Dalton, 1827)... 

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... 

A great deal of difficulty was encountered at first, because Dalton s fifth postulate gave an incorrect ratio of numbers of atoms in many cases. Such a large number of incorrect results were obtained that it soon became apparent that the fifth postulate was not correct. It was not until some 50 years later than an experimental method was devised to determine the atomic ratios in compounds, at which time the scale of relative atomic weights was determined in almost the present form. These relative weights are called the 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. 

Table 1.1 Dalton s system of relative atomic weights, 1810 Relative to 1810 values recalculated H = 1.000 relative to O = 16.000...

With the acceptance of Dalton s Atomic Theory, it became necessary to determine the atomic weights of the various elements, i.e., not the absolute atomic weights, but the relative weights of the various atoms with reference to one of them as unit." ... 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Although Dalton s theory was found to be unrealistically simple, he did compel chemists to adopt a standard scale of atomic weights. Because the combining weight of oxygen is approximately 16 times that of hydrogen, the preceding chart can be revised, as shown in Table 1-2. 

Gay-Lussac announced the results of his experiments in 1808. The reaction was mixed. Although Berzelius accepted the results and used them in his atomic-weight research, Dalton maintained that they couldn t possibly be correct. The truth is, he said, that gases do not unite in equal or exact measures in any one instance when they... 

Cover Alchemist s Laboratory, David Lees/CORBIS Dalton s List of Atomic Weight and Symbols, Science Photo Library. 

Dalton s application of the Greek atomic theory to the facts established by the analyses of compounds enabled him to attach to each element a number which he called the atomic weight of the element, and to summarise all the facts concerning the compositions of compounds in the statement, that the elements combine in the ratios of their atomic weights, or in the ratios of whole multiples of their atomic weights. All the investigations which have been made into the compositions of compounds, since Dalton s time, have confirmed the generalisation which followed from Dalton s application of the atomic theory. 

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. 

Isotopes answer a puzzle that troubled chemists ever since Dalton proposed his atomic theory. Dalton said that the key property of an atom is not its size or shape but its weight. Each element is characterized by an atomic weight defined relative to that of hydrogen. The fact that these relative atomic weights were usually... 

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