Atom
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[edit] EARLY ATOMIC THEORIES
The first recorded speculations that MATTER consisted of atoms are found in the works of the Greek philosophers LEUCIPPUS and DEMOCRITUS. The essence of their views is that all phenomena are to be understood in terms of the motions, through empty space, of a large number of tiny and indivisible bodies. (The name "atom" comes from the Greek words atomos, for "indivisible.") According to Democritus, these bodies differ from one another in shape and size, and the observed variety of substances derives from these differences in the atoms composing them.Greek atomic theory was not an attempt to account for specific details of physical phenomena. It was instead a philosophical response to the question of how change can occur in nature. Little effort was made to make atomic theory quantitative--that is, to develop it as a scientific hypothesis for the study of matter. Greek atomism, however, did introduce the valuable concept that the nature of everyday things was to be understood in terms of an invisible substructure of objects with unfamiliar properties. Democritus stated this especially clearly in one of the few sayings of his that has been preserved: "color exists by convention, sweet by convention, bitter by convention, in reality nothing exists but atoms and the void."Although adopted and extended by such later ancient thinkers as EPICURUS and Lucretius, Greek atomic theory had strong competition from other views of the nature of matter. One such view was the four-element theory of EMPEDOCLES. These alternative views, championed by Aristotle among others, were also motivated more by a desire to answer philosophical questions than by a wish to explain scientific phenomena.
[edit] ORIGINS OF MODERN ATOMISM
When interest in science revived in Europe in the 16th and 17th centuries, enough was known about Greek atomism to form the basis for further thought. Among those who revived the atomic theory were Pierre GASSENDI, Robert BOYLE, and especially Isaac NEWTON. The latter part of Newton's book Optiks is a series of detailed speculations on the atomic nature of matter and light, indicating how some of matter's properties are to be understood in terms of atoms.In the 19th century, two independent lines of reasoning strengthened the belief of most scientists, by then, in the atomic theory. Both approaches also began to reveal some quantitative properties of atoms. One approach, pioneered by John DALTON, involved chemical phenomena. The other, involving the behavior of gases, was carried out by physicists such as Rudolph CLAUSIUS and James Clerk MAXWELL.Dalton's main step forward was his introduction of atomic weightS. Dalton studied the elements then known and analyzed the data of their reactions with one another. He discovered the law of multiple proportions, which states that when several distinct reactions take place among the same elements, the quantities that enter the reactions are always in the proportions of simple integers--that is, 1 to 1, 2 to 1, 2 to 3, and so on. From this came the concept that such reacting quantities contain equal numbers of atoms and are therefore proportional to the masses of individual atoms. Dalton gave the lightest known element, hydrogen, an atomic weight of 1, and developed comparative atomic weights for the other known elements accordingly.The study of gases in terms of atomic theory was begun by Daniel BERNOULLI in the 18th century. Bernoulli showed that the pressure exerted by a gas came about as the result of collisions of the atoms of the gas with the walls of its container. In 1811, Amadeo AVOGADRO suggested that equal volumes of different gases, under the same conditions of pressure and temperature, contain equal numbers of atoms. The number of atoms in a Mass of gas equal to one gram atomic weight--a quantity of an element, in grams, that has the same numerical value as the element's atomic weight (see MOLE, unit of substance)--is now known to be approximately 6.022 x 10 (23rd). This huge value is an indication of the disparity in size between atoms and everyday objects.Avogadro himself never estimated the magnitude of this value, although it is now known as the AVOGADRO number. Estimates of its value were first given in the mid-19th century by Clausius and Maxwell. An accurate measurement was not carried out until the early 20th century, using the diffraction of X-rays by crystals. From the value of the Avogadro number it is possible to infer the Mass of the individual atoms, which for hydrogen turns out to be 1.6 x 10 (-24th) grams.
[edit] DISCOVERY OF THE ELECTRON AND OF RADIATION
By the end of the 19th century almost all scientists had become convinced of the truth of the atomic theory. By that time, ironically, evidence was just beginning to accumulate that atoms are not in fact the indivisible particles suggested by their name. One source of such evidence came from studies using gas discharge tubes, which are similar to neon lights. In such tubes, a gas at low pressure is subjected to intense electrical forces. Under these conditions, various colored glows (now known as glow DISCHARGE) are observed to traverse the tube. One blue glow at one end of the tube, around the electrode known as the CATHODE, was observed for a wide variety of gases. The glow was shown by Joseph John THOMSON in 1897 to involve a stream of negatively charged particles with individual masses much smaller than that of any atom. These particles were called electronS, and they were soon recognized to be a constituent of all atoms. That is, atoms are not indivisible but contain parts.In the late 19th and the early 20th century it was also found that some kinds of atoms are not stable. Instead they transform spontaneously into other kinds of atoms. For example, uranium atoms slowly change into lighter thorium atoms, which themselves change into still lighter atoms, eventually ending up as stable atoms of lead. ?These transformations, first observed by Antoine Henri BECQUEREL, came to be known as radioactivity, because the atomic changes were accompanied by the emission of several types of radiation.Atoms are ordinarily electrically neutral. Therefore the negative charge of the electrons in an atom must be balanced by a corresponding positive charge. Because the electrons have so little Mass, the positive constituents of an atom must also carry most of the atom's Mass. The obvious question arose as to how these varied parts are arranged within an atom. The question was answered in 1911 through the work of Ernest RUTHERFORD and his collaborators. In their experiments they passed alpha particles--a type of radiation emitted in some radioactive decays--through thin gold foils. They observed that in some instances the alpha particles emerged in the opposite direction from their initial path. This suggested a collision with a heavy object within the atoms of the gold. Because electrons are not massive enough to produce such large deflections, the positive charges must be involved. Analyzing the data, Rutherford showed that the positive charge in an atom must be concentrated in a very small volume with a radius less than 10(-14th) meter, or one ten-thousandth the size of the whole atom. This part of the atom was soon called the nucleus. Later measurements showed that the size of a nucleus is approximately given by multiplying the cube root of the atomic weight by 10(-15th) meter.
[edit] RUTHERFORD MODEL
Rutherford proposed an atomic model in which the atom was held together by electrical attraction between the nucleus and the electrons. In this model the electrons traveled in relatively distant orbits around the nucleus. The model eventually proved successful in explaining most of the phenomena of chemistry and everyday physics. Subsequent studies of the atom divided into investigations of the electronic parts of the atom, which came to be known as atomic physics, and investigations of the nucleus itself, which came to be known as nuclear physics. This division was natural, because of the immense difference in size between the nucleus and the electron orbits and the much greater energy needed to produce nuclear as compared to electronic changes.