Dalton s theory

On the basis of Dalton s theory, the atom can be defined as the smallest particle of an element that can enter into a chemical reaction. 

Walko JZ. Turning Dalton s theory into practice. Innovation 1989 18 24. 

While some chemists were busy breaking down various substances available to them in an attempt to discover new elements, others were probing into the structures of the elements themselves. In 1804, John Dalton, an English chemist, proposed a theory to explain some of the known properties of elements. Dalton s theory stated that each element, indeed, all matter, consisted of huge... 

In order to account for certain facts not known in Dalton s time, it has been necessary to make some changes in Dalton s theory. For example, it is now known that the atoms of a given element do not all necessarily have to have the same weight. 

Such modifications of the atomic theory as those we have briefly discussed above, although profoundly modifying, and, indeed, controverting the philosophical significance of Dalton s theory as originally formulated, leave its chemical significance practically... 

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. 

The English chemist John Dalton became one of the most famous scientists of the eighteenth century. Although he was known to the public for one idea, that chemical compounds were formed when the atoms of one element joined with the atoms of another, there was much more than this to Dalton s theory. He revolutionized chemistry by emphasizing that atoms have relative weights and that these relative weights can be measured. 

Perhaps it is worth emphasizing again that the importance of Dalton s theory didn t lie in the assumption that matter is composed of indestructible atoms. That was a very old idea. On the contrary, his idea was significant because it was a theory that explained how chemical compounds are formed and because the idea of atoms with different relative weights made it possible to turn chemistry into a quantitative science. As long as chemists held on to the old idea that elements could combine with one another in a variety of different proportions, they could describe chemical reactions only in a qualitative manner. It was Dalton who changed all this. 

Dalton s theory hardly won immediate acceptance, and debate about it continued for decades. Chemists generally accepted the idea that elements combine in fixed proportions, and they found the idea of relative weights extremely useful. However, many doubted the reality of Dalton s atoms. Atoms, after all, were too small to be seen, and... 

Fig. 1.8. Dalton s atoms and the electronic states in an atom. A, a chart in Dalton s A New System of Chemical Philosophy, published in 1808. In modern symbols, these atoms are 1, H 2, N 3, C 4, O 5, P 6, S 7, Mg 8, Ca 9, Na 10, K 11, Sr 12, Ba 13, Fe 14, Zn 15, Cu 16, Pb 17, Ag 18, Pt 19, Au 20, Hg. The major modem modification to Dalton s theory is that the atoms are divisible. The contour maps in B represent typical electronic states in atoms. The outermost contour on each map represents a density of 10 A The successive contours rcpre.sent an increase of a factor of 2. The regions with dashed-curve contours have opposite phases in the wavefunction from those with solid-curve contours.

People have been thinking about tiny objects for a long time. The ancient Greek philosopher Democritus (ca. 460-370 b.c.e.) believed that properties of matter depended on the shapes of small, indivisible bits of matter called atoms. Although this idea failed to catch on at the time— no one could see these atoms because they were so small—in 1803, the British chemist John Dalton (1766-1844) proposed a similar theory. Dalton s theory was an important advance and helped scientists understand chemical reactions—for example, the reaction of two atoms of hydrogen (H) and one atom of oxygen (O) to form H O—but atoms themselves remained cloaked in mystery. 

Ultimately, Dalton s theory completely transformed chemical composition, from a chaos of unorganized empirical knowledge into a highly rational science centered on the concept of atomic weight. The particular values... 

Dalton s theory of the atom is generally considered to be what made the atom a scientifically fruitful concept in chemistry. To be sme, by Dalton s time the atom had already had a two-millenium history as a philosophical idea, and corpuscular thought had long been viable in natmal philosophy (that is, in what we would today call physics). 

Thns, 2008 was perhaps an arbitrary year to celebrate 200 years of Dalton s theory, bnt as good a year as any. The Symposium Series volume appears in 2010, which is 200 years after the pnblication of Part 11 of Dalton s New System. Readers interested in learning more abont Dalton s life and work are directed to Arnold Thackray s 1972 volume which remains authoritative even after nearly fom decades (6). 

To explain his atomic theory Dalton made use of circles, each with a marking to indicate a specific element. These circles served to explain Dalton s theory but they were too difficult to work with to show complicated.chemical reactions. 

In 1799 Joseph Proust (1754-1826), a French chemist, observed that specific compounds always contained the same elements in the same ratio by mass. This came to be known as the law of definite proportions. The law of definite proportions provided a means for determining relative weights for numerous atoms and verified John Dalton s theory that elements are made up of atoms. Dalton (1766-1844) was an English teacher, chemist, and physicist. He used modern scientific methodology to develop long-lasting atomic theories. 

Dalton s Atomic Theory was an important milestone in the development of chemistry, but modern chemistry students will correctly note that it was incomplete, and in some cases, just plain wrong. For example, not all atoms of a given element are identical, because Dalton did not know about the existence of isotopes. Likewise, we now know that atoms are comprised of still smaller particles and that nuclear processes convert atoms of one element into atoms of other elements. By the very nature of science, when a hypothesis, law or model—no matter how dearly held—fails to make correct predictions, it must be discarded or modified. So, significant portions of Dalton s original theory have been modified. However, the importance of Dalton s theory can hardly be understated and should not be assessed by whether or not it was correct in the finest details, but in how it provided a working foundation that guided current and future scientists in their quest to understand the physical world. 

In 1808 John Dalton proposed his atomic theory, making a major contribution to the understanding of inorganic compounds. Essentially, Dalton s theory postulated that the elements are composed of small, indivisible particles called atoms and that these atoms combine in ratios of small whole numbers to form chemical compounds. 

O Compare Dalton s atomic theory with the modern atomic theory. Explain why scientists modified Dalton s theory. 

Q O In your opinion, should chemistry students learn about Dalton s theory if scientists no longer agree with it completely Justify your answer. 

Thomson had tried to convince Davy of the value of the theory. But Davy was adamant m his opposition, and caricatured Dalton s theory so skilfully that many were astonished how any man of sense or science would be taken up with such a tissue of absurdities Charles William Eliot, President of Harvard University, who began his career in the field of education as a teacher of chemistry, cautioned his students as late as 1868 that the existence of atoms is itself an hypothesis and not a probable one. All dogmatic assertion upon it is to be regarded with distrust." Berthollet, too, was so sceptical of the atomic theory that as late as 1890 he still wrote the formula for water as if it were hydrogen peroxide—to him atoms were but fabrications of the mind. Wilhelm Ostwald, who did not hesitate to champion the unorthodox theories of many young chemical dreamers, wanted as late as 1910 to do away completely with the atomic theory. 

Dalton s theory explained why the classical laws of chemical... 

Atoms are so tiny that, until recently, the masses of individual atoms could not be measured directly (Figure 3.7). However, because mass was so important in Dalton s theory, some measure of atomic masses was necessary. Therefore, a relative scale—the atomic mass scale—is used. This scale is sometimes called the atomic weight scale. On this scale, an average of the masses of all the atoms of the naturally occurring mixture of isotopes of a given element is measured relative to the mass of an atom of a standard. 

In the past 200 years a great deal of experimental evidence has accumulated to support the atomic model. This theory has proved to be both extremely useful and physically reasonable. When atoms were first suggested by the Greek philosophers Democritus and Leucippus about 400 B.c., the concept was based mostly on intuition. In fact, for the following 20 centuries, no convincing experimental evidence was available to support the existence of atoms. The first real scientific data were gathered by Lavoisier and others from quantitative measurements of chemical reactions. The results of these stoichiometric experiments led John Dalton to propose the first systematic atomic theory. Dalton s theory, although crude, has stood the test of time extremely well. 

Thus far, we have not dwelled on the three-dimensional nature of molecules because this added layer of complexity was not needed. Historically, this was the order of events, too— more or less. It wasn t until the mid-1800s, a half century after Dalton s theory of the atoms became generally accepted, that a substantial number of European scientists began to grapple with the possibility, and the consequences, of three-dimensional arrangements of atoms in molecules. 

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