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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Planetary model of the atom. History and essence of scientific discovery
Directory / The most important scientific discoveries In the first atomic theory dalton it was assumed that the world consists of a certain number of atoms - elementary building blocks - with characteristic properties, eternal and unchanging. These ideas changed drastically after the discovery of the electron. All atoms must contain electrons. But how are the electrons arranged in them? Physicists could only philosophize based on their knowledge of classical physics, and gradually all points of view converged on one model proposed by J. J. Thomson. According to this model, an atom is made up of a positively charged substance with electrons interspersed (perhaps in a lot of motion) so that the atom resembles a pudding with raisins. Thomson's model of the atom could not be directly tested, but all sorts of analogies testified in its favor. In 1903, the German physicist Philipp Lenard proposed a model of an "empty" atom, inside which some undiscovered neutral particles "fly", composed of mutually balanced positive and negative charges. Lenard even gave a name to his non-existent particles - dynamides... However, the only one whose right to exist was proved by strict, simple and beautiful experiments was Rutherford's model. Ernest Rutherford (1871-1937) was born near the city of Nelson (New Zealand) in the family of a migrant from Scotland. After graduating from school in Havelock, where the family lived at that time, he received a scholarship to continue his education at Nelson Provincial College, where he entered in 1887. Two years later, Ernest passed the exam at Canterbury College, a branch of the University of New Zealand at Crichester. In college, Rutherford was greatly influenced by his teachers: physics and chemistry teacher E.W. Bickerton and mathematician J.H.H. Cook. After being awarded a Bachelor of Arts degree in 1892, Rutherford remained at Canterbury College and continued his studies on a scholarship in mathematics. The following year, he became a master of arts, having passed the exams in mathematics and physics with the best of all. In 1894, his first published work, Magnetization of Iron by High-Frequency Discharges, appeared in the New Zealand Philosophical Institute Proceedings. In 1895, a scholarship for scientific education was vacant, the first candidate for this scholarship refused for family reasons, the second candidate was Rutherford. Arriving in England, Rutherford received an invitation from J. J. Thomson to work in Cambridge in the Cavendish laboratory. In 1898, Rutherford accepted a professorship at McGill University in Montreal, where he began a series of important experiments concerning the radioactive emission of the element uranium. In Canada, he made fundamental discoveries: he discovered the emanation of thorium and unraveled the nature of the so-called "induced radioactivity"; together with Soddy, he discovered radioactive decay and its law. Here he wrote the book "Radioactivity". In their classic work, Rutherford and Soddy touched on the fundamental question of the energy of radioactive transformations. Calculating the energy of k-particles emitted by radium, they come to the conclusion that "the energy of radioactive transformations is at least 20 times, and maybe even a million times higher than the energy of any molecular transformation." Rutherford and Soddy concluded that "the energy hidden in the atom is many times greater than the energy released in ordinary chemical transformation." This enormous energy, in their opinion, should be taken into account "when explaining the phenomena of space physics." In particular, the constancy of solar energy can be explained by the fact that "processes of subatomic transformation are taking place on the Sun." The enormous scope of Rutherford's scientific work in Montreal - he published 66 articles both personally and jointly with other scientists, not counting the book "Radioactivity" - brought Rutherford fame as a first-class researcher. He receives an invitation to take the chair in Manchester. On May 24, 1907, Rutherford returned to Europe. A new period of his life began. In 1908, Rutherford was awarded the Nobel Prize in Chemistry "for his research on the decay of elements in the chemistry of radioactive substances." The following year, Rutherford challenged Ernest Marsden to find out if alpha particles could bounce off gold foil. Rutherford was absolutely convinced that massive alpha particles should experience only minor deflections when passing through gold foil. Most of them actually passed through the foil, only weakly deviating. But some alpha particles - about one in 20, as Marsden noted - were bent at angles greater than 000 degrees. Marsden was even afraid to tell Rutherford about this and carefully made sure at first that there was no mistake in his experiments. Rutherford almost did not believe in this result of observations. Many years later, Rutherford recalled: “It was perhaps the most incredible event that I have ever experienced in my life. It was as implausible as if you fired a 15-inch projectile at a piece of tissue paper and it returned back and hit you." But I had to believe in the implausible, and in 1911 Rutherford came to the conclusion that the results of experiments on the scattering of alpha particles by gold foil can only be explained by assuming that alpha particles pass at a very small distance from other positively charged particles with sizes much smaller than atomic sizes. The gold atom must consist of a small positively charged nucleus and surrounding electrons. This was the birth of the idea of the atomic nucleus and a new branch of physics - nuclear physics. This idea was by 1911 not entirely new. It was put forward earlier by Johnston Stoney, the Japanese physicist Nagaoka and some other scientists. But all these hypotheses were purely speculative, while Rutherford's idea was based on experiment. The results of the experiments that led Rutherford to the idea of the planetary structure of the atom, the scientist outlined in a large article "The Scattering of Alpha and Beta Particles in Substance and the Structure of the Atom", published in May 1911 in the English "Philosophical Journal". Physicists all over the world could now evaluate another, this time convincingly experimentally confirmed, model of the structure of the atom ... Rutherford was indefatigable. And then he undertook a new study: he began to determine the number of alpha particles deflected by the foil at different angles depending on the electric charge of the nuclei of the atoms of the substance from which the foil was made. The patience of the researchers was rewarded. Analyzing the results of these experiments, Rutherford derived a formula relating the number of alpha particles deflected through a certain angle to the nuclear charge of the target foil substance. Now it was possible to determine the nature of the target material from experiments on the scattering of alpha particles. The first nuclear method of chemical analysis appeared in the hands of researchers! The scientists compared the behavior of targets made of various materials and found that the greater the nuclear charge, the more alpha particles deviate from a straight path. And here, for the first time, physical experiments lifted the veil of secrecy over the periodic law of elements. From Rutherford's experiments it followed that if Mendeleev arranged the elements in a row as the charge of their nuclei increased, then no permutations would be required! Physicists have clarified the formulation of the periodic law, the chemical properties of elements are in a periodic dependence not on the atomic mass of the elements, but on the electric charge of their nuclei. It is in accordance with the magnitude of the charge of the nuclei that the elements line up in the order in which Mendeleev placed them, relying on his encyclopedic knowledge of the chemical properties of elements ... What keeps an electron from falling onto a massive nucleus? Of course, the rapid rotation around it. But in the process of rotation with acceleration in the field of the nucleus, the electron must radiate part of its energy in all directions and, gradually decelerating, nevertheless fall onto the nucleus. This thought haunted the authors of the planetary model of the atom. The next obstacle on the way of the new physical model, it seemed, was to destroy the entire picture of the atomic structure, constructed with such difficulty and proven by clear experiments... Rutherford was sure that a solution would be found, but he could not imagine that it would happen so soon. The defect in the planetary model of the atom will be corrected by the Danish physicist Niels Bohr. Almost at the same time that the scientists of the world received an issue of the "Philosophical Journal" with Rutherford's article on the structure of the atom, the twenty-five-year-old Niels Bohr successfully defended his dissertation on the electronic theory of metals at the University of Copenhagen. The Danish physicist Niels Henrik David Bohr (1885–1962) was born in Copenhagen, the second of three children of Christian Bohr and Ellen (nee Adler) Bohr. His father was a renowned professor of physiology at the University of Copenhagen. He studied at the Gammelholm Grammar School in Copenhagen and graduated in 1903. Bohr and his brother Harald, who became a famous mathematician, were avid football players during their school days. Later, Nils was fond of skiing and sailing. If at school Niels Bohr was generally considered a student of ordinary abilities, then at the University of Copenhagen his talent very soon made him talk about himself. Niels was recognized as an unusually capable researcher. His graduation project, in which he determined the surface tension of water from the vibration of a water jet, earned him a gold medal from the Royal Danish Academy of Sciences. In 1907 he became a bachelor. He received his master's degree from the University of Copenhagen in 1909. His doctoral dissertation on the theory of electrons in metals was considered a masterful theoretical study. In 1911, Bohr decided to go to Cambridge to work for a few months in the laboratory of J. J. Thomson, the discoverer of the electron. Niels' mother and his brother Harald approved of the idea. Perhaps his fiancee Margaret was not very happy, but she also agreed. Bohr then painfully pondered Rutherford's model and looked for convincing explanations of what obviously happens in nature despite all doubts: electrons, without falling on the nucleus and without flying away from it, constantly revolve around their nucleus. Here is what K. Manolov and V. Tyutyunnik write in the book "Biography of the Atom": "If hydrogen has only one electron, how can one explain the fact that it emits several different wavelengths of light rays?" Bor thought. He again returned to Nicholson's theory. The excellent agreement between the calculated and observed wavelength ratios of the spectra is a strong argument in favor of this theory. However, Nicholson identifies the frequency of radiation with the frequency of vibrations of a mechanical system. But systems in which frequency is a function of energy cannot emit a finite amount of homogeneous radiation, since their frequency will change during radiation. In addition, the systems calculated by Nicholson will be unstable for some mode shapes. Finally, Nicholson's theory cannot explain Balmer's and Rydberg's serial laws. - Hansen, I think there is an answer! Bor said. - With the help of the stability condition of the electron orbit in the atom that I have derived, it is possible to calculate the speed of the electron in the orbit, its radius and the total energy of the electron in any orbit. Moreover, all formulas contain the same factor, the so-called quantum number, which takes the same integer values 1, 2, 3, 4, etc. Each of these numbers corresponds to a certain radius of the orbit ... - Bohr paused a bit and continued . - Well, of course, now everything is clear. An atom can exist without radiating energy only in certain stationary states, each of which is characterized by its own energy value. If an electron moves from one orbit to another, the atom either emits or absorbs energy in the form of special portions - quanta!.. - So that's the secret! Hansen exclaimed. - So, the spectrum of an atom reflects its structure! - Now everything falls into place. It is clear why the hydrogen atom emits several types of rays. If we number the orbits, starting with the closest to the nucleus, then we can say that the electron jumps from the fourth to the first, from the third to the first, from the third to the second orbit, etc. Each jump is accompanied by the emission of light of the corresponding wavelength. I really hope that I will be able to find a quantitative dependence ... In 1913, Niels Bohr published the results of lengthy reflections and calculations, the most important of which have since become known as Bohr's postulates: there is always a large number of stable and strictly defined orbits in the atom, along which an electron can rush indefinitely, because all the forces acting on it , are balanced; An electron can move in an atom only from one stable orbit to another equally stable one. If, during such a transition, the electron moves away from the nucleus, then it is necessary to impart to it from the outside a certain amount of energy equal to the difference in the energy reserve of the electron in the upper and lower orbits. If an electron approaches the nucleus, then it "resets" the excess energy in the form of radiation ... Probably, Bohr's postulates would have taken a modest place among a number of interesting explanations of new physical facts obtained by Rutherford, if not for one important circumstance. Bohr, using the relationships he found, was able to calculate the radii of "allowed" orbits for an electron in a hydrogen atom. Knowing the difference between the energies of an electron in these orbits, it was possible to construct a curve describing the radiation spectrum of hydrogen in various excited states and to determine what wavelength the hydrogen atom should especially readily emit if excess energy is supplied to it from the outside, for example, with the help of bright mercury light. lamps. This theoretical curve completely coincided with the emission spectrum of excited hydrogen atoms, measured by the Swiss scientist J. Balmer back in 1885! The planetary model of the atom received powerful support, Rutherford and Bohr had more and more supporters. Author: Samin D.K.
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