| ...[I]n the year 1808, an English chemist John Dalton showed that the relative proportion of the chemical elements needed to form a complicated chemical compound is always a ratio of whole numbers. He interpreted this rule as indicating that all chemical compounds are built up from particles representing simple chemical elements. The failure of medieval alchemy to turn one chemical element into another supplied supporting evidence of the apparent indivisibility of these particles. So, without much hesitation they were given the old Greek name: 'atoms'. Although we know now that these 'Dalton's atoms' are not at all indivisible (they are, in fact, formed from still smaller particles), the name 'atom' stuck. ... There are ninety-two different kinds of atoms (corresponding to ninety-two different chemical elements), and each kind of atom possesses rather complicated characteristic properties. This in itself invites the suggestion that they might have rather complicated structures constructed out of more elementary ones. How are Dalton's atoms to be built up from the elementary particles? The first step towards answering this question was taken in 1911 by the celebrated British physicist Ernest Rutherford... He was studying the structure of atoms by bombarding them with alpha particles... These positively charged particles are emitted in the process of disintegration of radioactive elements. Rutherford observed the deflection (that is to say, the scattering) of these projectiles after their passage through a piece of matter. He found that whereas most of the projectiles were able to pass through with very little deviation, a few recoiled through exceptionally large angles. It was as though they had scored a bullseye on something very small and highly concentrated within the atom. In this way, he came to the conclusion that all atoms must possess a very dense, positively charged central core, or nucleus. This he envisaged as being surrounded by a rather rarefied cloud of negative electric charge. It was later discovered that the atomic nucleus is made up of a certain number of positively charged protons and electrically neutral neutrons. These are so similar to each other (apart from their charge) that they are known under the collective name: nucleons. They are held tightly together by a short-ranged powerful cohesive force known as the strong nuclear force. It gets its name because it is strong to keep protons bound within the nucleus despite the repulsive force active between their positive charges. As for the surrounding cloud, this consists of negative electrons swarming around under the restraining influence of the electrostatic attraction exerted by the positive charge of the protons in the nucleus. (You recall, of course, that like charges repel, whereas unlike charges attract.) The number of electrons forming the atomic cloud varies from one type of atom to another, and determines all the physical and chemical properties of a given type of atom. The number of electrons varies along the natural sequence of chemical elements from one (for hydrogen) up to ninety-two (for the heaviest naturally occurring element: uranium) In spite of the apparent simplicity of Rutherford's atomic model, its detailed understanding turned out to be anything but simple. For example, what was to stop all the electrons being quickly drawn into the nucleus by the electrostatic attraction? According to classical ideas, the only explanation must be that the electrons are avoiding the nucleus in much the same way as the planets in the Solar System avoid being pulled into the Sun. This they do by moving in orbits about the centre of attraction (in that case, gravitational attraction). But unfortunately, classical physics also says that when the orbiting body is electrically charged, it will progressively radiate energy away—a form of light-emission. It was calculated that, due to these steady energy losses, all the electrons forming an atomic cloud should collapse on the nucleus within a negligible fraction of a second. This seemingly sound conclusion of classical theory stands, however, in sharp contradiction to the empirical fact that atomic clouds are, on the contrary, quite stable. Instead of collapsing on the nucleus, atomic electrons continue their motion around the central body for an indefinite period of time. Thus we see that a deep-rooted conflict arises between the basic ideas of classical mechanics, and the empirical data concerning the mechanical behaviour of atoms. It was this contradiction that brought the famous Danish physicist Niels Bohr to the realization that classical mechanics, which claimed for centuries a privileged and secure position in the system of natural sciences, should from now on be considered as a restricted theory. It is applicable only to the macroscopic world of our everyday experience, but fails badly in its application to the much more delicate types of motions taking place within atoms. | — George Gamow, Russell Stannard, The New World of Mr Tompkins: George Gamow's Classic Mr Tompkins in Paperback, Chapter 11 1/2 The Remainder of the Previous Lecture through which Mr Tompkins Dozed | Indexes/13 |
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