Modern atomic theory

Early Chemical Discoveries and the Atomic Theory— Modern chemistry began with eighteenth-century discoveries leading to the formulation of two basic laws of chemical combination, the law of conservation of mass and the law of constant composition (definite proportions). These discoveries led to Dalton s atomic theory— that matter is composed of indestructible particles called atoms, that the atoms of an element are identical to one another but different from atoms of all other elements, and that chemical compounds are combinations of atoms of different elements. Based on this theory, Dalton proposed still another law of chemical combination, the law of multiple proportions. 

The concept that all substances are composed of elements and atoms goes back at least 2000 years. Originally, only four elements were recognized air, earth, fire, and water. Each substance was thought to consist of very small particles, called atoms, that could not be subdivided any further. This early mental concept of the nature of matter was extremely prescient, considering there were no experimental results to indicate that matter should be so and none to verify that it was so. Modern atomic theory is much more rigorously based, and we even have the ability to see atoms with special tunneling microscopes. All of chemistry is based on how atoms react with each other. 

Chemical analysis is an indispensable servant of modern technology whilst it partly depends on that modem technology for its operation. The two have in fact developed hand in hand. From the earliest days of quantitative chemistry in the latter part of the eighteenth century, chemical analysis has provided an important basis for chemical development. For example, the combustion studies of La Voisier and the atomic theory proposed by Dalton had their bases in quantitative analytical evidence. The transistor... 

The first modern atomic theory was developed by John Dalton and first presented in 1808. Dalton used the term atom (first used by Democritus) to describe the tiny, indivisible particles of an element. Dalton also thought that atoms of an element are the same and atoms of different elements are different. In 1897, J. J. Thompson discovered the existence of the first subatomic particle, the electron, by using magnetic and electric fields. In 1909, Robert Millikan measured the charge on the electron in his oil drop experiment (electron charge = -1.6022 x 10-19 coulombs), and from that he calculated the mass of the electron. 

As you know, Dalton s atomic theory no longer applies in its original form, and Mendeleev s periodic table has undergone many changes. For example, scientists later discovered that atoms are not the most basic unit of matter because they are divisible. As well, the modern periodic table lists the elements in order of their atomic number, not their atomic mass. Of course, it also includes elements that had not been discovered in Mendeleev s time. Even so, in modified form, both of these inventions are still studied and used today in every chemistry course around the world. 

When Mendeleev invented the periodic table, he was well-acquainted with Dalton s atomic theory. He knew nothing, however, about subatomic particles, and especially the electron, which is the foundation for the modern periodic table s distinctive shape. Because the original periodic table developed out of experimental observations, chemists did not need an understanding of atomic structure to develop it. (As you will see in section 3.3, however, the periodic table easily accommodates details about atomic structure. In fact, you will learn that the modern periodic table s distinctive design is a natural consequence of atomic structure.)... 

Dalton s atomic theory chem Theory forming the basis of accepted modern atomic theory, according to which matter is made of particles called atoms, reactions must take place between atoms or groups of atoms, and atoms of the same element are all alike but differ from atoms of another element. dol tonz 3,tam-ik the-o-re ... 

Because Dalton did not know the chemical formula for compounds, he assumed the greatest simplicity. This worked fine for some compounds such as CO or NO, but introduced error for other compounds, for example, assuming water was HO. Nevertheless, Dalton s ideas laid the foundation for the modern atomic theory. Dalton s ideas briefly summarized are ... 

The mass number gives the total number of protons and neutrons in an atom of an element, but it does not convey the absolute mass of the atom. To work with the masses of elements, we use comparative masses. Initially, Dalton and the other pioneers of the atomic theory used the lightest element hydrogen and compared masses of other elements to hydrogen. The modern system uses C-12 as the standard and defines one atomic mass unit (amu) as 1/12 the mass of one C-12 atom. One amu is approximately 1.66 X 10 g. This standard means the masses of individual protons and neutrons are slightly more than 1 amu as shown in Table 4.6. 

By the end of the seventeenth century, then, scientists were not really any closer to enumerating the elements than were the Greek philosophers. Yet a hundred years later the British chemist John Dalton (1766-1844) wrote a textbook that outlined a recognizably modern atomic theory and gave a list of elements that, while still very incomplete and sometimes plain wrong, is in content and in spirit a clear precursor to today s tabulation of the hundred and more elements. Why had our understanding of the elements changed so fast ... 

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. 

Morrisson, M. S. Modern Alchemy. Occultism and the Emergence of Atomic Theory, Oxford University Press Oxford/New York, 2007. 

The periodic system developed from Bohr s atomic theory is of the greatest importance in chemical science because it demonstrates that the properties of the elements depend on their positions in the system. It is immediately apparent that chemical valency depends on the number of loosely-bound electrons in the atom. Thus, the alkali metals have one such electron while the divalent alkaline-earth metals have two, etc. Valency is therefore closely connected with electronic structure and provides the foundation for the modern theory of the chemical bond, the basis of which is to be found in the coupling or transfer of the valency electrons. 

This is the first written work on assaying, and it displays that art already full-grown, so far as concerns gold and silver, and to some extent copper and lead for if we eliminate the words dependent on the atomic theory from modern works on dry assaying, there has been but very minor progress. 2... 

The atomic theory of Lucretius contrasted with modern doctrines of atoms and evolution, London, 1884. 

Modern mineralogy is the product of research and discovery by many persons. Robert Hooke 11665) foretold the atomic theory by constructing models of alum crystals out of leaden musket balls. Nicolaus Sieiio (I f>69)... 

De Broglie s theory was extended by Schrodinger and led to wave mechanics, which is fundamental for modern atomic theory. Here again, we have a long line of development of physics, originated by Einstein,... 

John Dalton (1766-1844) introduces modern atomic theory into the science of chemistry. 

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. 

Bauschlicher, C. W., Jr., 1995b, The Application of Ab Initio Electronic Structure Calculations to Molecules Containing Transition Metal Atoms in Modern Electronic Structure Theory, Part II, Yarkony, D. R. (ed.), World Scientific, Singapore. 

Modern atomic theory depends on the application of developments in wave mechanics to the movement of electrons. The pioneers of this theory are Lois de Broglie, Heisenberg and Schrodinger. 

Modern atomic theory explains the structures and behaviors of the atom much better than the earlier atomic theories. This theory explains the probability of finding electrons around the nucleus by virtue of quantum numbers and orbitals. The quantum numbers are the integer numbers designating the energy levels of the electrons in an atom, and the orbitals are the probable regions in which the electrons might be found around the nucleus. 

Which one of the following atomic theories proposed by Dalton is wrong according to modern atomic theory ... 

In which option are the pioneers of modern atomic theory is given in the correct order ... 

The modern atomic theory depends on which information below ... 

Modern atomic theory received a shot in the arm when it was recognized that the individual atom has light absorption and emission spectra occurring at narrow lines of the spectrum at specific wavelengths, as opposed to the broad bands typical of the polyatomic molecules and compounds. Since the line spectrum of each element is characteristic of that element, atomic spectroscopy can be used for precise elementary analysis of many types of chemically simple and complex materials. These studies make use of the wave character of light, as well as light s particle character. 

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. 

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