Ancient Theories of Atoms

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1 Ancient Theories of Atoms The word atom was coined somewhere around 450 B.C. (it is an unfortunate side note about this period of history that it is usually quite difficult to give precise dates; one wishes that the ancient Greeks would have been a little bit better at keeping track of their activities) by a philosopher (which, at the time, was the same thing as a scientist) named Democritus. What is Known about Democritus Democritus was a student of Leucippus, who is said to also have subscribed to the atomic theory, though so very little is known about the actual works of both of these men that it becomes very difficult to tell where the theories of one ends and the other begins. While Leucippus is credited by many to have been the true creator of atomic theory, he is more often than not overshadowed by Democritus, who did much more to systematize this view. Though there was still not much to it. Just as the sand on the seashore, so went the logic of Democritus, when viewed from afar looks to be a single body a beach so also may all matter be made up of tiny little granules of matter the smallest of which he called atoms, which in Greek means, uncuttable. By virtue of the name itself, an atom in Democritus theory was the smallest thing in existence. It is difficult to say just how scientific atomic theory was in the case of Democritus was it a real theory, developed using something similar to today s scientific method, based on factual observation and deductive logic? Or was it a lucky shot in the dark? The jury is still out on this one, though one must give Democritus credit for his undeniably witty observation, Nothing exists except atoms and empty space; everything else is opinion. Opposition to Democritus Nevertheless, the view of Democritus was greatly overshadowed by the theories of Aristotle, which viewed the entire known universe as being made up of five distinct elements (earth, fire, air, water, and ether) which mixed and matched to form anything of substance. In his theory there was no need for everything to be made of tiny little atoms, so the view of Democritus was largely ignored. One may be quick to judge Aristotle and the rest of the scientific community of the time for so quickly dismissing the views of Democritus (which are now known to be, to a certain extent, more correct), but this would be a bit near-sighted. The truth is that in the world of the ancient Greeks, there truly was much more evidence for the views of Aristotle than that of Democritus. There was much more reason to believe that everything tangible was made up of at least four of these elements (the fifth element, ether, was not quite so scientifically based, but was founded on much more pseudo-religious grounds concerning the perfection of the heavens and

2 corruption of the Earth, but that is another story altogether), than there was to believe in these tiny things that could not possibly be seen. The Legacy of Aristotle and Democritus Because of the logic inherent in Aristotle s views, as well as the fact that as the foremost scholar of his day many of his views went almost entirely unchallenged, the atomic view of Democritus faded away and Aristotle s view increased in popularity to the point where it was nearly heretical to question it. And so things would remain for more than two thousand years. Even as other principles of Aristotle (the geocentric view of the universe, the law of gravity, the concept of light) finally began to be questioned by the likes of Galileo, Copernicus, Kepler and Newton, his theory on matter remained. It was not until the nineteenth century(!) in fact, that Scientists finally began to give the ideas of Democritus a second look.

3 Dalton's Atomic Theory Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton ( ). Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought. Dalton's Atomic Theory 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms. Based on this, he drew atoms as small spheres, which atoms of different elements having different colours and sizes. Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element that are known as "isotopes", but isotopes of an element have the same chemical properties. Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistry.

4 Joseph John Thomson J. J. Thomson In 1897, the British physicist Joseph John (J. J.) Thomson ( ) discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His model is known as the Raisin bun model the atom itself being spherical with positive (protons) and negative (electron) particles embedded in it. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved.

5 Ernest Rutherford By 1911 the components of the atom had been discovered. The atom consisted of subatomic particles called protons and electrons. However, it was not clear how these protons and electrons were arranged within the atom. J.J. Thomson suggested the "raisin bun" model. In this model the electrons and protons are uniformly mixed throughout the atom: Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards. These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Raisin Bun model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus (at the centre of the atom). He thus developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus in circular pathways, like planets orbiting around the sun.

6 James Chadwick In 1932, James Chadwick discovered the neutron. Neutrons help stabilize the protons in the atom's nucleus. Because the nucleus is so tightly packed together, the positively charged protons would tend to repel each other normally. Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms and they are about the same size as protons. However, neutrons do not have any electrical charge; they are electrically neutral. The Neutron In 1932, English Physicist James Chadwick, after a decade-long struggle to track down this tricky particle (all the methods available at the time were used only to detect charged particles), performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for gamma rays (a form of radiation consisting of high-energy photons). The test, to simplify as much as possible, went like this: A sample of Beryllium was bombarded with alpha particles (another type of naturally occurring radiation which are technically just ionized helium nuclei), which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot-Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface (paraffin was the preferred example), would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation). This was the premise, and from here, Chadwick simply had to play detective and put all the pieces of the puzzle together. For instance, he could tell that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect (when photons, such as gamma rays, strike certain surfaces, they discharge electrons, which can be simply measured), but rather discharged protons, which meant that the particles had to be more massive than previously expected. In the end, Chadwick finally solved the puzzle and officially discovered the neutron in 1932, thus vindicating Rutherford s original theory (not that Rutherford needed any more accomplishments in his already prolific scientific career). For his efforts, Chadwick received the Nobel Prize in The New Atomic Model With the discovery of the neutron, the atomic model seemed more complete than ever. The overall charges remained the same, and now there no longer seemed to be a discrepancy between the atomic mass and the atomic number. Of course, it is now known that neutrons play a much more important role within an atom than was originally thought. It would become clear to physicists in the decades after this neutral particle s original discovery that it within the atomic nucleus, both the proton and the neutron seemed to possess equal importance in determining atomic stability, which was refined even further in the 1960 s in the science known as Quantum Chromodynamics, which espoused a quark-based theory of the nucleus.

7 Neutrons would also play an important role in the processes involved in creating nuclear explosions and nuclear energy, for it is by bombardment with high-energy neutrons that scientists first learned how to split an atom. According to Chadwick s model, the nucleus of the atom contains both protons and neutrons. The electrons orbit around the nucleus randomly, like planets around the Sun.

8 Niels Bohr In 1913 Bohr published a theory about the structure of the atom based on an earlier theory of Rutherford's. Rutherford had shown that the atom consisted of a positively charged nucleus, with negatively charged electrons in orbit around it. Bohr expanded upon this theory by proposing that electrons travel only in certain successively larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom's chemical properties. Bohr proposed that the number of electrons in the orbits of atoms could be predicted, and that each orbit is a concentric circle outside an inner orbit. The first orbit holds up to 2 electrons The second orbit holds up to 8 electrons The third orbit holds up to 8 electrons The fourth orbit can hold many more electrons, but their configuration becomes more difficult to explain. Bohr also described the way atoms emit radiation by suggesting that when an electron jumps from an outer orbit to an inner one, that it emits light. Later other physicists expanded his theory into quantum mechanics. This theory explains the structure and actions of complex atoms.

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