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Development of atomic theory Created by : Angella Santa Monica VIII-a (03) • Francis Bacon (1561 - 1626) seems to be thinking of matter in atomic terms when he says "Rapid motion of constituent particles is both a necessary and sufficient condition for something to be hot." • Galileo (1564 - 1642) thought the appearance of new materials in a chemical change was due to rearrangement of parts too small to be seen. • Robert Boyle ( 1627 - 1691) discarded the Aristotelian idea of the elements earth, air, fire and water as constituents of matter. Instead he used a ‘corpuscular or mechanical hypothesis’, explaining many physical phenomena in terms of solid bodies moving, colliding, bouncing, and having their shapes or sizes changed. • Isaac Newton (1642 - 1727) in his book Opticks wrote: "Have not the small Particles of Bodies certain Powers, Virtues, or Forces, by which they act at a distance, not only upon the Rays of Light for reflecting, refracting and inflecting them, but also upon one another for producing a great Part of the Phenomena of Nature?" • Antoine Lavoisier (1743-1794) - Formulated the Law of Conservation of Matter: "Matter is neither gained nor lost during a chemical reaction." He did this by weighing materials before and after reactions. For example, the weights of the mercury and oxygen formed by decomposition of mercuric oxide were compared with the initial weight of the mercuric oxide. • Joseph Louis Proust (1754-1826) - Formulated the Law of Constant Porportions: "In a compound, the contsitutne elements are always present in a definite proportion by weight." Like Lavoisier, Proust also conducted quantitative experiments. He showed that regardless of how copper carbonate was prepared in the laboratory, or how it was isolated from nature, it always contained the same proportions of copper, oxygen and carbon 5:4:1 parts by weight. • John Dalton (1766-1844)- Formulated the Law of Multiple Proportions : "In the formation of two or more compounds from the same elements, the weights of one element that combine with a fixed weight of a second element are in a ratio of small whole numbers (integers) such as 2 to 1, 3 to 1, 3 to 2, or 4 to 3." He had made a quantitative study of different compounds made from the same elements, such as carbon monoxide and carbon dioxide. He found that the weight ratio of carbon to oxygen in carbon monoxide was 3:4, and the weight ratio of carbon to oxygen in carbon dioxide was 3:8. Dalton’s theory • Here is a summary of Dalton's theory. • 1. Elements are composed of tiny, separate, indivisible and indestructible particles. These particles, called atoms, maintain their identity when the element undergoes physical or chemical change. • 2. All atoms of the same element are identical and different from the atoms of every other element. • 3. Atoms combine in simple whole number ratios to form compounds. • 4. Atoms of the same elements can combine in different ratios to form more than one compound. • Berzelius, contributed significantly to the development of atomic theory. About 1807 he performed a great number of analyses of chemical compounds, and showed so many examples of the law of definite proportions that it could no longer be doubted. He also set about determining atomic weights and his first table, published in 1828, compared favourably with today's accepted values. • Whereas Dalton represented atoms of elements by circles containing a letter or symbol, Berzelius chose to omit the circle and just use an initial letter of the Latin name of the element (or two letters if more than one element began with the same letter). This led to the system we now use for writing formulae of elements and compounds and writing chemical equations. Now, 200 years later, how would you modify Dalton's theory? • Think of sub-atomic particles - protons, neutrons, electrons... • spontaneous fission of radioactive atoms, nuclear fission and fusion, • production of radioactive isotopes in an atomic pile... So the first part of the theory is no longer accepted. • Atoms of isotopes of an element are not identical. So the second part of the theory is no longer accepted. From Dalton to the Periodic Table • Modern atomic theory begins with the work of John Dalton, published in 1808. He held that all the atoms of an element are of exactly the same size and weight (see atomic weight) and are in these two respects unlike the atoms of any other element. He stated that atoms of the elements unite chemically in simple numerical ratios to form compounds. The best evidence for his theory was the experimentally verified law of simple multiple proportions, which gives a relation between the weights of two elements that combine to form different compounds. • Evidence for Dalton's theory also came from Michael Faraday's law of electrolysis. A major development was the periodic table, devised simultaneously by Dmitri Mendeleev and J. L. Meyer, which arranged atoms of different elements in order of increasing atomic weight so that elements with similar chemical properties fell into groups. By the end of the 19th cent. it was generally accepted that matter is composed of atoms that combine to form molecules. Discovery of the Atom's Structure • • • In 1911, Ernest Rutherford developed the first coherent explanation of the structure of an atom. Using alpha particles emitted by radioactive atoms, he showed that the atom consists of a central, positively charged core, the nucleus, and negatively charged particles called electrons that orbit the nucleus. There was one serious obstacle to acceptance of the nuclear atom, however. According to classical theory, as the electrons orbit about the nucleus, they are continuously being accelerated (see acceleration), and all accelerated charges radiate electromagnetic energy. Thus, they should lose their energy and spiral into the nucleus. This difficulty was solved by Niels Bohr (1913), who applied the quantum theory developed by Max Planck and Albert Einstein to the problem of atomic structure. Bohr proposed that electrons could circle a nucleus without radiating energy only in orbits for which their orbital angular momentum was an integral multiple of Planck's constant h divided by 2π. The discrete spectral lines (see spectrum) emitted by each element were produced by electrons dropping from allowed orbits of higher energy to those of lower energy, the frequency of the photon of light emitted being proportional to the energy difference between the orbits. Around the same time, experiments on x-ray spectra (see X ray) by H. G. J. Moseley showed that each nucleus was characterized by an atomic number, equal to the number of unit positive charges associated with it. By rearranging the periodic table according to atomic number rather than atomic weight, a more systematic arrangement was obtained. The development of quantum mechanics during the 1920s resulted in a satisfactory explanation for all phenomena related to the role of electrons in atoms and all aspects of their associated spectra. With the discovery of the neutron in 1932 the modern picture of the atom was complete. Kind images of revolution of atom