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The JJ Thomson atomic theory: a historical introduction

Undoubtedly, it is an extremely sophisticated and intellectually unjustified task to name a discovery that would demonstrate the power and majesty of scientific progress with the same brightness and persuasiveness, as the invention of the atomic theory. Naturally, we can remember a considerably great number of diverse inventions and scientific theories that have caused global changes in the style of life of all humankind, such as the invention of electronic computers, which initiated the process of global integration of humanity, the creation of the first models of the human psyche, first spaceflights, etc. In truth, this list can be extended almost indefinitely, guided by the individual preferences of the author. Nevertheless, the invention of the atomic theory that has rewarded the humankind with practically unlimited energy resources, as well as cursed it with a threat of a global nuclear war, perfectly demonstrates the power of science. Obviously, this discovery would be impossible without diverse previously performed inventions and scientific theories, which have laid the foundations of modern physics, such as we know it today. One of these significant inventions was the JJ Thomson atomic theory. Doubtlessly, the importance of this work cannot be overrated. Therefore, those students who nourish a desire to understand all the principles of various disciplines in the sphere of modern physics, such as the quantum theory or the string theory, have to pay attention to the JJ Thomson atomic theory in order to realize the convoluted path of scientific thought.

In fact, the history of the JJ Thomson atomic theory can be traced back to 1897 when this talented British scientist discovered a new particle, which has been called an electron. This discovery was a result of the series of experiments with cathode rays. During his experiments, he placed two metal plates on both sides of the tube, one of which was positively charged, whereas the other was negatively charged. Therefore, the beam was passing through the electric field produced by these charged plates. As a result, he observed a phenomenon of a deflection of cathode rays. The beam was deflected toward the positively charged plate. Obviously, when the polarity was reversed the direction in which the deflection of the beam occurred also has been changed. By performing this elegant experiment, Thomson proved that the cathode rays could be deflected by an electric field, thereby, the hypothetical particles, which form the cathode rays, have negative electrical charges. In fact, this was a revolutionary discovery, considering the fact that at that time a great amount of physicists thought that cathode rays are immaterial. Moreover, the conventional model of an atom, which was generally accepted by the scientific community, could not be used for explanation of this phenomenon. In other words, Thomson discovered new particles that just could not exist according to the conventional atomic theory of that time. This experiment served as a powerful impulse for the formation of new atomic theory that could propose a new atomic model taking into account these new facts about the atomic structure.

The main properties of the electrons discovered by Thomson

During the series of different experiments with cathode rays, Thomson discovered a great number of unique properties that are characteristic for the electrons. In fact, all the experiments necessary to demonstrate these properties were accomplished using standard vacuum tubes and emitters that expose cathode rays (at that time these rays were also known as Lenard rays). Here is a brief list of these properties along with a concise description of experiments, which were performed in order to register them experimentally.

  • Firstly, Thomson proved that the negatively charged particles, which constitute a beam of cathode rays, are moving in a straight line. In order to prove this hypothesis, he placed a Maltese cross on the path of the cathode rays. As a result, he observed a shadow of the cross on the opposite side of the tube. This demonstrative experiment also proved that metal objects are opaque for the cathode rays.
  • Secondly, he proved that the particles that constitute the beam have kinetic energy and momentum. Thomson placed a wheel with plates made of mica on the path of the beam. When the tube was activated, the wheel began rotating towards the anode. In fact, this experiment was also a considerable proof that cathode rays are material, which served as an extra proof for the JJ Thomson atomic theory.
  • Thirdly, Thomson demonstrated that the electrons produce energy when striking objects. He placed a platinum plank on the path of the rays. After the tube was activated, the plank became red hot. It was an obvious proof that negatively charged particles not only transfer energy but also produce it as a result of a strike.
  • In addition, during the experiment with the moving beam, which changed its direction according to the location of the anode, Thomson demonstrated that electrons are moving with an extremely high velocity. Of course, due to imprecise laboratory equipment, he could not measure the velocity of the electron. However, the obtained results were enough to formulate the JJ Thomson atomic theory according to which the velocity of electrons was regarded as extremely high.
  • During the further experiments with vacuum tubes, he also discovered a phenomenon of a fluorescence. When electrons hit the potentially fluorescent objects, such as various crystals, they induce prolonged fluorescence of the substance. Thereby, by discovering this physical phenomenon Thomson showed that electrons transfer a considerably large amount of kinetic energy.

The main postulates of the JJ Thomson atomic theory

In 1904 Thomson suggested a new atomic theory, which explained these properties of an atom. Naturally, nowadays it is widely known as the JJ Thomson atomic theory. He proposed a new atomic model that was called the plum pudding model. Thomson’s theory was founded on two important experimental facts: atoms are neutrally charged, whereas electrons are negatively charged. Unfortunately, more recent experiments, which have been performed by the Thomson’s student - Ernest Rutherford, proved that this atomic model was incorrect. Nevertheless, it has to be regarded as a revolutionary theory that significantly boosted the development of atomic physics. According to the JJ Thomson atomic theory, the atom is regarded as a neutrally charged sphere. The negatively charged electrons, which occupy the central region of the atomic space, are paired with positively charged particles. Therefore, the electrons are located in the region with a positive charge. Their orbits are stabilized due to the fact that the electric charge of the atomic particles prevents them from moving outside the positively charged atomic cloud. In other words, the Thomson’s atomic model can be visualized as a pudding in which a dough plays a role of the positively charged atomic cloud, whereas the chaotically located raisins represent negatively charged electrons.

The weaknesses of the JJ Thomson atomic theory: the Thomson problem


Obviously, as well as all other scientific theories, the JJ Thomson atomic theory was not free from several weaknesses. For example, the measures of the electron’s kinetic energy obtained in the spectroscopic experiments were not equal to the results of theoretic computations performed in accordance with the JJ Thomson atomic theory. Thomson and his students finished several series of experiments aimed at determination of the potential energy of electrons. Nevertheless, the theoretical calculations still contained a considerable amount of weak points. In fact, these disadvantages of the JJ Thomson atomic theory have induced a whole group of significant theoretical problems that caused a great impact on the evolution of quantum physics. The most significant of these weak points was the so-called Thomson problem. The main objective of this theoretical demand is to determine the minimum of the electrostatic potential energy of the configuration of N electrons, which are constrained to the surface of the unit sphere, taking into account that these electrons repel each other according to Coulomb's law. This theoretical objective was proposed by Thomson himself in 1904, right after he announced his famous plum pudding model. The primal target of this assignment was to explain the fact of coexisting of negatively charged electrons and neutrally charged atoms in accordance with the main postulates of the JJ Thomson atomic theory. However, it also contributed to appearance and further development of a great bunch of related physical problems, such as the theoretical assignments connected with the problem of definition of the geometry of the minimum energy configuration. In fact, the solutions to this problem are found only for a few special cases (for example, if N = 1, the solution for the Thomson problem is trivial because in this case the electron may occur at any point of the hypothetical atomic surface). Thereby, we can summarize that despite its incorrectness the JJ Thomson atomic theory has played a significant role in the evolution of the atomic physics.

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