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atom [Gr.,=uncuttable (indivisible)], basic unit of matter ; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom The atom consists of a central, positively charged core, the nucleus , and negatively charged particles called electrons that are found in orbits around the nucleus. The Nucleus Almost the entire mass of the atom is concentrated in the nucleus, which occupies only a tiny fraction of the atom's volume. The nucleus of an atom consists of neutrons and protons, the neutron being an uncharged particle and the proton a positively charged one. Their masses are almost equal. Atoms containing the same number of protons but different numbers of neutrons represent different forms, or isotopes , of the same element. The Electrons Surrounding the nucleus of an atom are its electrons; for a neutral atom, the number of electrons is equal to the atomic number. The outermost electrons of an atom determine its chemical and electrical properties. An atom may combine chemically with another atom in various ways, either by giving up or receiving electrons, thus setting up an electrical attraction between the atoms (see ion ), or by sharing one or more pairs of electrons (see chemical bond ). Because metals have few outermost electrons and tend to give them up easily, they are good conductors of electricity or heat (see conduction ). The electrons are often described as revolving about the nucleus as the planets revolve about the sun. This picture, however, is misleading. The quantum theory has shown that all particles in motion also have certain wave properties. For a particle the size of an electron, these properties are of considerable importance. As a result the electrons in an atom cannot be pictured as localized in space, but rather should be viewed as smeared out over the entire orbit so that they form a cloud of charge. The electron clouds around the nucleus represent regions in which the electrons are most likely to be found. The shapes of these clouds can be very complex, in marked contrast to the simple elliptical orbits of planets. Surprisingly, the sizes of all atoms are comparable, in spite of the large differences in the number of electrons they contain. Atomic Weight and Number The atomic number of an atom is simply the number of protons in its nucleus. The atomic weight of an atom is given in most cases by the mass number of the atom, equal to the total number of protons and neutrons combined. An atom may be conveniently symbolized by its chemical symbol with the atomic number and mass number written as subscript and superscript, respectively. For example, the symbol for uranium is U (atomic number 92); the isotopes of uranium with atomic weights 235 and 238 are indicated by 23592 U and 23892 U. Development of Atomic Theory Early Atomic Theory The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant motion, was proposed in the 5th cent. BC by the Greek philosophers Leucippus and Democritus and was adopted by the Roman Lucretius. However, Aristotle did not accept the theory, and it was ignored for many centuries. Interest in the atomic theory was revived during the 18th cent. following work on the nature and behavior of gases (see gas laws ). 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 2Ï€. 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. Contemporary Studies of the Atom With many of the problems of individual atomic structure and behavior now solved, attention has turned to both smaller and larger scales. On a smaller scale the atomic nucleus is being studied in order to determine the details of its structure and to develop sources of energy from nuclear fission and fusion (see nuclear energy ), for the atom is not at all indivisible, as the ancient philosophers thought, but can undergo a number of possible changes. On a larger scale new discoveries about the behavior of large groups of atoms have been made (see solid-state physics ). The question of the basic nature of matter has been carried beyond the atom and now centers on the nature of and relations between the hundreds of elementary particles that have been discovered in addition to the proton, neutron, and electron. Some of these particles have been used to make new types of exotic "atoms" such as positronium (see antiparticle ) and muonium (see muon ). Bibliography See G. Gamow, The Atom and Its Nucleus (1961); H. A. Boorse and L. Motz, ed., The World of the Atom (2 vol., 1966); B. H. Bransden and C. J. Joachain, Physics of Atoms and Molecules (1986).
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The ancient Greek philosophers Leucippus and Democritus believed that atoms existed, but they had no idea as to their nature. Centuries later, in 1803, the English chemist John Dalton, guided by the experimental fact that chemical elements cannot be decomposed chemically, was led to formulate his atomic theory. Dalton's atomic theory was based on the assumption that atoms are tiny indivisible entities, with each chemical element consisting of its own characteristic atoms.âœ¶ âœ¶See Atoms article for further discussion of Dalton's atomic theory. The atom is now known to consist of three primary particles: protons, neutrons, and electrons, which make up the atoms of all matter. A series of experimental facts established the validity of the model. Radioactivity played an important part. Marie Curie suggested, in 1899, that when atoms disintegrate, they contradict Dalton's idea that atoms are indivisible. There must then be something smaller than the atom (subatomic particles) of which atoms were composed. Long before that, Michael Faraday's electrolysis experiments and laws suggested that, just as an atom is the fundamental particle of an element, a fundamental particle for electricity must exist. The "particle" of electricity was given the name electron. Experiments with cathode-ray tubes, conducted by the British physicist Joseph John Thomson, proved the existence of the electron and obtained the charge-to-mass ratio for it. The experiments suggested that electrons are present in all kinds of matter and that they presumably exist in all atoms of all elements. Efforts were then turned to measuring the charge on the electron, and these were eventually successful by the American physicist Robert Andrews Millikan through the famous oil drop experiment. The study of the so-called canal rays by the German physicist Eugen Goldstein, observed in a special cathode-ray tube with a perforated cathode, let to the recognition in 1902 that these rays were positively charged particles (protons ). Finally, years later in 1932 the British physicist James Chadwick discovered another particle in the nucleus that had no charge, and for this reason was named neutron. As a physical chemist, George Stoney made significant contributions to our understanding of molecular motion. However, this Irish scientist is better known for assigning a name to negative atomic charges, electrons, while addressing the Royal Society of Dublin in 1891. â€”Valerie Borek Joseph John Thomson had supposed that an atom was a uniform sphere of positively charged matter within which electrons were circulating (the "plum-pudding" model). Then, around the year 1910, Ernest Ruthorford (who had discovered earlier that alpha rays consisted of positively charged particles having the mass of helium atoms) was led to the following model for the atom: Protons and neutrons exist in a very small nucleus, which means that the tiny nucleus contains all the positive charge and most of the mass of the atom, while negatively charged electrons surround the nucleus and occupy most of the volume of the atom. In formulating his model, Rutherford was assisted by Hans Geiger and Ernest Marsden, who found that when alpha particles hit a thin gold foil, almost all passed straight through, but very few (only 1 in 20,000) were deflected at large angles, with some coming straight back. Rutherford remarked later that it was as if you fired a 15-inch artillery shell at a sheet of paper and it bounced back and hit you. The deflected particles suggested that the atom has a very tiny nucleus that is extremely dense and positive in charge. Also working with Rutherford was Henry G. J. Moseley who, in 1913, performed an important experiment. When various metals were bombarded with electrons in a cathode-ray tube, they emitted X rays, the wavelengths of which were related to the nuclear charge of the metal atoms. This led to the law of chemical periodicity, which provided refinement of the periodic table introduced by Mendeleev in 1869. According to this law, all atoms of an element have the same number of protons in the nucleus. It is called the atomic number and is given the symbol Z. Hydrogen is the simplest element and has Z = 1. Through Rutherford's work it was known that that electrons are arranged in the space surrounding the atomic nucleus. A planetary model of the atom, with the electrons moving in circular orbits around the nucleus seemed an acceptable model. However, such a "dynamic model" violated the laws of classical electrodynamics, according to which a charged particle, such as an electron, moving in the positive electric field of the nucleus, should lose energy by radiation and eventually spiral into the nucleus. To solve this contradiction, in 1913, the Danish physicist Neils Bohr (then studying under Rutherford) postulated that the electron orbiting the nucleus could move only in certain orbits, having in each a certain "quantized" energy. It turns out that the colors in fireworks would help prove him right. The colorful lights of fireworks are emitted by "excited" atoms; that is, by atoms that have absorbed extra energy. Light consists of electromagnetic waves, each (monochromatic) color with a characteristic wavelength Î» and frequency v. Frequency is related to energy E through the famous Planck equation, E = hÎ½, where h is Planck's constant (6.6256 x 10âˆ’34 J s). Note that white light, such as sunlight, is a mixture of light of all colors, so it does not have a characteristic wavelength. For this reason we say that white light has a "continuous spectrum." On the other hand, excited atoms emit a "line spectrum" consisting of a set of monochromatic visible radiations. Each element has a characteristic line spectrum that can be used to identify the element. Note that line emission spectra can also be obtained by heating a salt of a metal with a flame. For instance, common salt (sodium chloride) provides a strong yellow light to the flame coming from excited sodium, while copper salts emit a blue-green light and lithium salts a red light. The colors of fireworks are due to this phenomenon. Scientists in the late nineteenth century tried to quantify the line spectra of the elements. In 1885 the Swedish school teacher Johann Balmer discovered a series of lines in the visible spectrum of hydrogen, the wavelengths of which could be related with a simple equation: in which Î» is wavelength, k is constant, a = 2, and b = 3, 4, 5, â€¦ This group of lines was called the Balmer series. For the red line b = 3, for the green line b = 4, and for the blue line b = 5. Similar series were further discovered: in the infrared region, the Paschen series (with a = 3 and b = 4, 5 â€¦ in the above equation), and much later in the ultraviolet region, the Lyman series (with a = 1 and b = 2, 3 â€¦). In 1896 the Swedish spectroscopist Johannes Rydberg developed a general equation that allowed the calculation of the wavelength of the red, green, and blue lines in the atomic spectrum of hydrogen: where nL is the number of the lower energy level to which an electron falls and nH is the number of the higher energy level from which it falls. R is called the Rydberg constant (1.0974 x 10âˆ’7 mâˆ’1). R was later shown to be 2Ï€ 2me 4Z2/h 3c, where m is the mass of the electron, e is its charge, Z is the atomic number, h is Planck's constant, and c is the speed of light. As noted earlier, Bohr had suggested the quantization of Ruthford's model of the atom. Although he was not aware of the work of Balmer and Paschen when he wrote the first version of his 1913 article, he had incorporated Planck's constant h into his model, which turned out to be an important decision. Bohr assumed that the absorption or emission of radiation can occur only by "jumps" of the electron from one stationary orbit to another. (See Figure 1.) The energy differences between two such allowed orbits then provided the characteristic frequencies of the emitted light. Î”E = E n1 âˆ’ E n2 = hÎ½ Planck's constant h was named by Bohr the "quantum of action." Bohr's theory was in close agreement with many experimental facts regarding one-electron atoms (the hydrogen
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Answers:Off the top of my head I would say look at work by Proust and Lavoisier. What comes to mind are the the "law of conservation of mass" and the "law of constant proportion."
Answers:No part of Dalton's MODEL was wrong, it was just incomplete. A model is simply an explanation of what has been observed. Dalton"s model was excellent based on what he was able to do at the time. Then Rutherford, J.J. Thomson, Millikin, and others built on Dalton' model. Personally, I think Dalton was one of the great minds of the 19th century.
Answers:John Dalton is best known for his contributions to atomic theory but he also identified the phenomenon of colour blindness, an affliction that he shared with his brother. He was a prodigy and became a teacher at his local school when he was just 12. In his New System of Chemical Philosophy (1808), he proposed the idea of tiny particles or atoms which could not be created or destroyed. He also developed the techniques of atomic weights and formulae. Many of his ideas were widely taken up, although many scientists rejected his notion that matter was made up of hard, indivisible atoms. Gradually, like most scientific innovations, atomism became generally accepted. Dalton s atomic theory was very close to an ancient one proposed by Al Razi The Greek philosopher Democritus had given us early ideas on atoms At one time Dalton taught James Joule and his ideas on atoms were key to the statistics of Kelvin and Maxwell Dalton s atomic model is one of the fundamentals of physics and chemistry. This theory of atomic composition was hypothesized and partially confirmed by the English chemist and Physicist John Dalton. Dalton came with his Atomic theory as a result of his research into gases. He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements. Through deductive reasoning and experimentation, he made an interesting discovery. His findings led him to hypothesize that elements combine at the atomic level in fixed ratios. This ratio would naturally differ in compounds due to the unique atomic weights of the elements being combined. This was a revolutionary idea but further experimentation by himself and others confirmed his theory. The findings became the basis of of Dalton s Atomic Laws or Model. These laws focus on five basic theorems. First, Pure Elements consist of particles called atoms. Second,atoms of an element are all the same for that element. That means gold is gold and oxygen is oxygen down to the last atom. Third, atoms of different elements can be told apart by their atomic weights. Fourth, atoms of elements unite to form chemical compounds. Finally, atoms can neither be created or destroyed in chemical reaction. The grouping only changes. The last of Dalton s Atom Model were at the time considered true for all reactions involving atoms. This was later corrected with the discovery of nuclear fission and fusion. So we now know that this only holds true for chemical reactions. Like other scientific theories, Dalton s atomic model has been perfected over time with the research and discoveries of other scientists. We now know that the atom can be divided into even smaller particles and we have even discovered the actual internal atom structure, even able to view it through modern technology. We now know that atomic weight is a product of the structure of the atoms as well. This atomic theory made possible modern chemistry and physics. Up until Dalton s time the atom was only considered to a philosophical construct passed down by the ancient Greeks. Dalton s ground breaking work made theory reality. This understanding the atom helped to fuel many other discoveries such as the fundamental forces and Einstein s theory of relativity. It is especially is important when one goes into Quantum physics a discipline that looks at physics at the atomic and subatomic levels. If you enjoyed this article there are others that you will enjoy on Universe today. There is an interesting article about the number or atoms in the universe. There is also an interesting article about dark matter. There are also great resources on the web if you want to learn more about Dalton s model. There is an article on the Central Queensland University that goes into detail about Dalton s Atomic model. There is also a great article on the Clackham Community College website that simply explains the theory. You can also check out Astronomy Cast. Episode 138 Quantum Mechanics talks in detail about Quantum physics.
Answers:John Dalton believed that atoms were homogenous (that electrons were scattered around like 'currants in a bun'). Ernest Rutherford showed that most of an atom is empty space and the mass of an atom is mainly carried in the nucleus. He showed this as most particles passed through the foil (as most of the atom is empty space) but some are deflected because they hit the positively charged nucleus.