Element of dispersion and particle experience. Rutherford's Alpha Particle Scattering Experiment (briefly)

Introduction

Atoms, originally thought to be indivisible, are complex systems. They have a massive nucleus of protons and neutrons, around which electrons move in empty space. Atoms are very small - their dimensions are about 10 –10 –10 –9 m, and the dimensions of the nucleus are still about 100,000 times smaller (10 –15 –10 –14 m). Therefore, atoms can only be “seen” indirectly, in an image with very high magnification (for example, using a field-emission projector). But even in this case, the atoms cannot be seen in detail. Our knowledge of their internal structure is based on a huge amount of experimental data, which indirectly but convincingly supports the above. Ideas about the structure of the atom changed radically in the 20th century. influenced by new theoretical ideas and experimental data. There are still unresolved questions in the description of the internal structure of the atomic nucleus, which are the subject of intensive research. The following sections outline the history of the development of ideas about the structure of the atom as a whole; A separate article is devoted to the structure of the nucleus (ATOMIC NUCLEUS STRUCTURE), since these ideas developed largely independently. The energy required to study the outer shells of an atom is relatively small, on the order of thermal or chemical energy. For this reason, electrons were experimentally discovered long before the discovery of the nucleus. The nucleus, despite its small size, is very strongly bound, so it can be destroyed and studied only with the help of forces millions of times more intense than the forces acting between atoms. Rapid progress in understanding the internal structure of the nucleus began only with the advent of particle accelerators. It is this huge difference in size and binding energy that allows us to consider the structure of the atom as a whole separately from the structure of the nucleus. To get an idea of the size of an atom and the empty space it occupies, consider the atoms that make up a drop of water with a diameter of 1 mm. If you mentally enlarge this drop to the size of the Earth, then the hydrogen and oxygen atoms included in the water molecule will have a diameter of 1–2 m. The bulk of the mass of each atom is concentrated in its core, the diameter of which was only 0.01 mm .

Main part

I. Evolution of ideas about the structure of atoms

The discovery of the complex structure of the atom is the most important stage in the development of modern physics. In the process of creating a quantitative theory of atomic structure, which made it possible to explain atomic systems, new ideas were formed about the properties of microparticles, which are described by quantum mechanics.

The idea of atoms as indivisible smallest particles of substances, as noted above, arose in ancient times (Democritus, Epicurus, Lucretius). In the Middle Ages, the doctrine of atoms, being materialistic, did not receive recognition. By the beginning of the 18th century. atomic theory is gaining increasing popularity. By this time, the works of the French chemist A. Lavoisier (1743–1794), the great Russian scientist M.V. Lomonosov and the English chemist and physicist D. Dalton (1766–1844) proved the reality of the existence of atoms. However, at this time the question of the internal structure of atoms did not even arise, since atoms were considered indivisible.

A major role in the development of atomic theory was played by the outstanding Russian chemist D.I. Mendeleev, who in 1869 developed the periodic system of elements, in which for the first time the question of the unified nature of atoms was raised on a scientific basis. In the second half of the 19th century. It has been experimentally proven that the electron is one of the main parts of any substance. These conclusions, as well as numerous experimental data, led to the fact that at the beginning of the 20th century. The question of the structure of the atom seriously arose.

The existence of a natural connection between all chemical elements, clearly expressed in Mendeleev’s periodic system, suggests that the structure of all atoms is based on a common property: they are all closely related to each other.

However, until the end of the 19th century. In chemistry, the metaphysical conviction prevailed that the atom is the smallest particle of simple matter, the final limit of the divisibility of matter. During all chemical transformations, only molecules are destroyed and created again, while atoms remain unchanged and cannot be split into smaller parts.

For a long time, various assumptions about the structure of the atom were not confirmed by any experimental data. Only at the end of the 19th century. discoveries were made that showed the complexity of the structure of the atom and the possibility of transforming some atoms into others under certain conditions. Based on these discoveries, the doctrine of the structure of the atom began to develop rapidly.

The first indirect evidence of the complex structure of atoms was obtained from the study of cathode rays generated during an electrical discharge in highly rarefied gases. The study of the properties of these rays led to the conclusion that they are a stream of tiny particles carrying a negative electrical charge and flying at a speed close to the speed of light. Using special techniques, it was possible to determine the mass of cathode particles and the magnitude of their charge, and to find out that they do not depend either on the nature of the gas remaining in the tube, or on the substance from which the electrodes are made, or on other experimental conditions. Moreover, cathode particles are known only in a charged state and cannot be stripped of their charges and converted into electrically neutral particles: electric charge is the essence of their nature. These particles, called electrons, were discovered in 1897 by the English physicist J. Thomson.

The study of the structure of the atom practically began in 1897–1898, after the nature of cathode rays as a stream of electrons was finally established and the charge and mass of the electron were determined. Thomson proposed the first model of the atom, presenting the atom as a clump of matter with a positive electrical charge, in which so many electrons are interspersed that it turns it into an electrically neutral formation. In this model, it was assumed that, under the influence of external influences, electrons could oscillate, i.e., move at an accelerated rate. It would seem that this made it possible to answer questions about the emission of light by atoms of matter and gamma rays by atoms of radioactive substances.

Thomson's model of the atom did not assume positively charged particles inside an atom. But how then can we explain the emission of positively charged alpha particles by radioactive substances? Thomson's atomic model did not answer some other questions.

In 1911, the English physicist E. Rutherford, while studying the movement of alpha particles in gases and other substances, discovered a positively charged part of the atom. Further more thorough studies showed that when a beam of parallel rays passes through layers of gas or a thin metal plate, no longer parallel rays emerge, but somewhat diverging ones: alpha particles are scattered, i.e., they deviate from the original path. The deflection angles are small, but there are always a small number of particles (about one in several thousand) that are deflected very strongly. Some particles are thrown back as if they had encountered an impenetrable barrier. These are not electrons - their mass is much less than the mass of alpha particles. Deflection can occur when colliding with positive particles whose mass is of the same order as the mass of alpha particles. Based on these considerations, Rutherford proposed the following diagram of the structure of the atom.

At the center of the atom there is a positively charged nucleus, around which electrons rotate in different orbits. The centrifugal force arising during their rotation is balanced by the attraction between the nucleus and the electrons, as a result of which they remain at certain distances from the nucleus. Since the mass of an electron is negligible, almost the entire mass of an atom is concentrated in its nucleus. The share of the nucleus and electrons, the number of which is relatively small, accounts for only an insignificant part of the total space occupied by the atomic system.

The diagram of the structure of the atom proposed by Rutherford, or, as they usually say, the planetary model of the atom, easily explains the phenomena of deflection of alpha particles. Indeed, the size of the nucleus and electrons is extremely small compared to the size of the entire atom, which is determined by the orbits of the electrons farthest from the nucleus, so most alpha particles fly through atoms without noticeable deflection. Only in cases where the alpha particle comes very close to the nucleus does electrical repulsion cause it to deviate sharply from its original path. Thus, the study of the scattering of alpha particles laid the foundation for the nuclear theory of the atom.

II. Bohr's postulates

The planetary model of the atom made it possible to explain the results of experiments on the scattering of alpha particles of matter, but fundamental difficulties arose in justifying the stability of atoms. The first attempt to construct a qualitatively new – quantum – theory of the atom was made in 1913 by Niels Bohr. He set the goal of linking into a single whole the empirical laws of line spectra, the Rutherford nuclear model of the atom, and the quantum nature of the emission and absorption of light. Bohr based his theory on Rutherford's nuclear model. He suggested that electrons move around the nucleus in circular orbits. Circular motion, even at constant speed, has acceleration. This accelerated movement of charge is equivalent to alternating current, which creates an alternating electromagnetic field in space. Energy is consumed to create this field. The field energy can be created due to the energy of the Coulomb interaction of the electron with the nucleus. As a result, the electron must move in a spiral and fall onto the nucleus. However, experience shows that atoms are very stable formations. It follows from this that the results of classical electrodynamics, based on Maxwell’s equations, are not applicable to intra-atomic processes. It is necessary to find new patterns. Bohr based his theory of the atom on the following postulates.

Bohr's first postulate(postulate of stationary states): in an atom there are stationary (not changing with time) states in which it does not emit energy. Stationary states of an atom correspond to stationary orbits along which electrons move. The movement of electrons in stationary orbits is not accompanied by the emission of electromagnetic waves. This postulate is in conflict with the classical theory. In the stationary state of an atom, an electron, moving in a circular orbit, must have discrete quantum values of angular momentum.

Bohr's second postulate(frequency rule): when an electron moves from one stationary orbit to another, one photon with energy is emitted (absorbed)

equal to the difference between the energies of the corresponding stationary states (En and Em are, respectively, the energies of the stationary states of the atom before and after radiation/absorption). The transition of an electron from a stationary orbit number m to a stationary orbit number n corresponds to the transition of an atom from a state with energy Em into a state with energy En (Fig. 1).

Fig.1. To an explanation of Bohr's postulates

рEn>Em photon emission occurs (the transition of an atom from a state with higher energy to a state with lower energy, i.e., the transition of an electron from an orbit more distant from the nucleus to a closer one), at En<Еm – его поглощение (переход атома в состояние с большей энергией, т. е, переход электрона на более удаленную от ядра орбиту). Набор возможных дискретных частот quantum transitions and determines the line spectrum of an atom. Bohr's theory brilliantly explained the experimentally observed line spectrum of hydrogen. The successes of the theory of the hydrogen atom were achieved at the cost of abandoning the fundamental principles of classical mechanics, which has remained unconditionally valid for more than 200 years. Therefore, direct experimental proof of the validity of Bohr’s postulates, especially the first one – about the existence of stationary states – was of great importance. The second postulate can be considered as a consequence of the law of conservation of energy and the hypothesis about the existence of photons. German physicists D. Frank and G. Hertz, studying the collision of electrons with gas atoms using the retarding potential method (1913), experimentally confirmed the existence of stationary states and the discreteness of atomic energy values. Despite the undoubted success of Bohr's concept in relation to the hydrogen atom, for which it turned out to be possible to construct a quantitative theory of the spectrum, it was not possible to create a similar theory for the helium atom next to hydrogen based on Bohr's ideas. Regarding the helium atom and more complex atoms, Bohr's theory allowed us to draw only qualitative (albeit very important) conclusions. The idea of certain orbits along which an electron moves in a Bohr atom turned out to be very conditional. In fact, the movement of electrons in an atom has little in common with the movement of planets in orbit. Currently, with the help of quantum mechanics, it is possible to answer many questions regarding the structure and properties of atoms of any element.

III. Structure of the atomic nucleus

Structure of the atomic nucleus

Nucleon level

About 20 years after Rutherford “discovered” its nucleus in the depths of an atom, the neutron was discovered - a particle in all its properties the same as the nucleus of a hydrogen atom - a proton, but only without an electric charge. The neutron turned out to be extremely convenient for probing the inside of nuclei. Since it is electrically neutral, the electric field of the nucleus does not repel it - accordingly, even slow neutrons can easily approach the nucleus at distances at which nuclear forces begin to manifest themselves. After the discovery of the neutron, the physics of the microworld moved forward by leaps and bounds.

Soon after the discovery of the neutron, two theoretical physicists - the German Werner Heisenberg and the Soviet Dmitry Ivanenko - hypothesized that the atomic nucleus consists of neutrons and protons. The modern understanding of the structure of the nucleus is based on it.

Protons and neutrons are combined by the word nucleon. Protons are elementary particles that are the nuclei of atoms of the lightest chemical element - hydrogen. The number of protons in the nucleus is equal to the atomic number of the element in the periodic table and is designated Z (the number of neutrons - N). A proton has a positive electric charge, equal in absolute value to the elementary electric charge. It is approximately 1836 times heavier than an electron. A proton consists of two up-quarks with charge Q = + 2/3 and one d-quark with Q = – 1/3, connected by a gluon field. It has final dimensions of the order of 10-15 m, although it cannot be imagined as a solid ball, it rather resembles a cloud with a blurred boundary, consisting of created and annihilated virtual particles.

The electric charge of a neutron is 0, its mass is approximately 940 MeV. A neutron consists of one u-quark and two d-quarks. This particle is stable only in the composition of stable atomic nuclei; a free neutron decays into an electron, a proton and an electron antineutrino. The half-life of a neutron (the time it takes for half the original number of neutrons to decay) is approximately 12 minutes. In matter, neutrons exist in free form for even less time due to their strong absorption by nuclei. Like the proton, the neutron participates in all types of interactions, including electromagnetic ones: with general neutrality, due to its complex internal structure, electric currents exist in it.

In the nucleus, nucleons are bound by a special kind of force - nuclear. One of their characteristic features is short-acting: at distances of the order of 10-15 m or less they exceed any other forces, as a result of which the nucleons do not fly apart under the influence of electrostatic repulsion of like-charged protons. At large distances, nuclear forces very quickly decrease to zero.

The mechanism of action of nuclear forces is based on the same principle as electromagnetic forces - on the exchange of interacting objects with virtual particles.

Virtual particles in quantum theory are particles that have the same quantum numbers (spin, electric and baryon charges, etc.) as the corresponding real particles, but for which the usual relationship between energy, momentum and mass does not hold.

IV. Rutherford's experiments

In a magnetic field, a flux of radioactive radiation breaks down into 3 components: alpha rays, beta rays and gamma rays.

The phenomenon of radioactivity indicated the complex structure of the atom

Conclusion

In conclusion, we come to the conclusion that the Rutherford-Bohr concept is already more than particles of absolute truth, although the further development of physics has revealed many errors in this concept. An even larger part of absolutely correct knowledge is contained in the quantum mechanical theory of the atom.

The discovery of the complex structure of the atom was a major event in physics, since the ideas of classical physics about atoms as solid and indivisible structural units of matter were refuted

Lasers

Based on the quantum theory of radiation, quantum generators of radio waves and quantum generators of visible light - lasers - were built. Lasers produce coherent radiation of very high power. Laser radiation is very widely used in various fields of science and technology, for example, for communications in space, for recording and storing information (laser disks) and welding, in medicine.

Spectra

Bohr's theory made it possible to explain the existence of line spectra.
Formula (1) gives a qualitative idea of why atomic emission and absorption spectra are lined. In fact, an atom can emit waves only of those frequencies that correspond to differences in energy values E 1 , E 2 , . . . , E n ,. . That is why the emission spectrum of atoms consists of separately located sharp bright lines. At the same time, an atom can absorb not any photon, but only one with energy hν which is exactly equal to the difference E n − E k some two allowed energy values E n And E k. Moving to a higher energy state E n, atoms absorb exactly the same photons that they are capable of emitting during the reverse transition to the original state E k. Simply put, atoms take from the continuous spectrum those lines that they themselves emit; This is why the dark lines of the absorption spectrum of a cold atomic gas are located exactly in those places where the bright lines of the emission spectrum of the same gas in a heated state are located.

Continuous spectrum hydrogen emission spectrum hydrogen absorption spectrum

The word "atom" translated from Greek means "indivisible." For a long time, until the beginning of the 20th century, an atom meant the smallest indivisible particles of matter. By the beginning of the 20th century. Science has accumulated many facts that indicate the complex structure of atoms.

Great progress in the study of the structure of atoms was achieved in the experiments of the English scientist Ernest Rutherford on the scattering of alpha particles when passing through thin layers of matter. In these experiments, a narrow beam of α particles emitted by a radioactive substance was directed at thin gold foil. A screen was placed behind the foil, capable of glowing under the impacts of fast particles. It was found that most α-particles deviate from straight-line propagation after passing through the foil, that is, they are scattered, and some α-particles are generally thrown back. Rutherford explained the scattering of α-particles by the fact that the positive charge is not uniformly distributed in a ball with a radius of 10 -10 m, as previously assumed, but is concentrated in the central part of the atom - the atomic nucleus. When passing near the nucleus, an a-particle having a positive charge is repelled from it, and when it hits the nucleus, it is thrown back in the opposite direction. This is how particles that have the same charge behave, therefore, there is a central positively charged part of the atom, in which a significant mass of the atom is concentrated. Calculations showed that to explain the experiments, it is necessary to take the radius of the atomic nucleus to be approximately 10 -15 m.

Rutherford suggested that the atom was structured like a planetary system. The essence of Rutherford's model of the structure of the atom is as follows: in the center of the atom there is a positively charged nucleus, in which all the mass is concentrated; electrons rotate around the nucleus in circular orbits at large distances (like planets around the Sun). The charge of the nucleus coincides with the number of the chemical element in the periodic table.

h is Planck's constant.

1. The word “atom” translated from Greek means “indivisible.” For a long time, until the beginning of the 20th century, an atom meant the smallest indivisible particles of matter. By the beginning of the 20th century. Science has accumulated many facts that indicate the complex structure of atoms.

Great advances in the study of the structure of atoms were achieved in the experiments of the English scientist Ernest Rutherford on the scattering of alpha particles when passing through thin layers of matter. In these experiments, a narrow beam of alpha particles emitted by a radioactive substance was directed at thin gold foil. A screen was placed behind the foil, capable of glowing under the impacts of fast particles. It was found that the majority of α-particles deviate from straight-line propagation after passing through the foil, i.e., they are scattered, and some α-particles are generally thrown back. Rutherford explained the scattering of alpha particles by the fact that the positive charge is not uniformly distributed in a ball with a radius of 10^~10 m, as previously assumed, but is concentrated in the central part of the atom - the atomic nucleus. When passing near the nucleus, an a-particle having a positive charge is repelled from it, and when it hits the nucleus, it is thrown back in the opposite direction. This is how particles that have the same charge behave, therefore, there is a central positively charged part of the atom, in which a significant mass of the atom is concentrated. Calculations showed that to explain the experiments, it is necessary to take the radius of the atomic nucleus to be approximately 10^~15 m.

Rutherford's planetary model of the structure of the atom could not explain a number of well-known facts: an electron with a charge must fall onto the nucleus due to Coulomb forces of attraction, and an atom is a stable system; When moving in a circular orbit, approaching the nucleus, an electron in an atom must emit electromagnetic waves of all possible frequencies, i.e., the emitted light must have a continuous spectrum, but in practice the result is different: the electrons of atoms emit light that has a line spectrum. The Danish physicist Nielier Bohr was the first to try to resolve the contradictions in the planetary nuclear model of atomic structure.

Bohr based his theory on two postulates. The first postulate: an atomic system can only be in special stationary or quantum states, each of which has its own energy; in a stationary state, an atom does not emit. This means that an electron (for example, in a hydrogen atom) can be located in several well-defined orbits. Each electron orbit corresponds to a very specific energy.

The second postulate: during the transition from one stationary state to another, a quantum of electromagnetic radiation is emitted or absorbed. The energy of a photon is equal to the difference between the energies of an atom in two states: , where

h is Planck's constant.

When an electron moves from a nearby orbit to a more distant one, an atomic system absorbs a quantum of energy. When an electron moves from a more distant orbit to a closer orbit relative to the nucleus, the atomic system emits an energy quantum.

In science, for a very long time it was believed that an Atom is the smallest, INDIVISIBLE particle of matter.

1. The first to violate these ideas was Thomson: he believed that an atom is a kind of positive substance in which electrons are interspersed “like raisins in a cupcake.” The importance of this theory is that the atom was no longer recognized as indivisible
2. Rutherford conducted an experiment on the scattering of alpha particles. Heavy elements (gold foil) were bombarded with radioactive material. Rutherford expected to see glowing circles, but he saw glowing rings.
Rutherford's explanation: The center of the atom contains all the positive charge, and the electrons have no effect on the flow of alpha particles.
3. Planetary model of the hydrogen atom according to BORU	By emitting a portion of energy (visible), an atom gives only its own set of wavelengths - a spectrum. Types of spectra: 1. Radiation (emission) spectrum: (provided by bodies in a heated state) a) Solid - give all atoms in solid, liquid or dense gases b) Lined - give atoms in a gaseous state 1. Absorption spectrum: if light is passed through a substance, then this substance will absorb exactly those waves that it emits in a heated state (dark stripes appear on the continuous spectrum) Spectral analysis is a method for determining the chemical composition of a substance from its emission or absorption spectrum. The method is based on the fact that each chemical element has its own set of wavelengths. Application of spectral analysis: in criminology, medicine, astrophysics. A spectrograph is a device for performing spectral analysis. A spectroscope differs from a spectrograph in that it can be used not only to observe spectra, but also to take a photograph of the spectrum. Ticket No. 21 1. Thermodynamic approach to the study of physical phenomena. Internal energy and ways to change it. First law of thermodynamics. Application of the first law of thermodynamics to isothermal, isochoric and adiabatic processes. 2. Models of the structure of the atomic nucleus; nuclear forces; nucleon model of the nucleus; nuclear binding energy; nuclear reactions. 1. Each body has a very specific structure; it consists of particles that move chaotically and interact with each other, therefore any body has internal energy. Internal energy is a quantity characterizing the body’s own state, i.e. the energy of the chaotic (thermal) movement of microparticles of the system (molecules, atoms, electrons, nuclei, etc.) and the energy of interaction of these particles. The internal energy of a monatomic ideal gas is determined by the formula U = 3/2 t/M RT. The internal energy of a body can change only as a result of its interaction with other bodies. There are two ways to change internal energy: heat transfer and mechanical work (for example, heating during friction or compression, cooling during expansion). Heat transfer is a change in internal energy without doing work: energy is transferred from more heated bodies to less heated ones. Heat transfer is of three types: thermal conductivity (direct exchange of energy between chaotically moving particles of interacting bodies or parts of the same body); convection (transfer of energy by flows of liquid or gas) and radiation (transfer of energy by electromagnetic waves). The measure of transferred energy during heat transfer is the quantity of heat (Q). These methods are quantitatively combined into the law of conservation of energy, which for thermal processes reads as follows: the change in the internal energy of a closed system is equal to the sum of the amount of heat transferred to the system and the work of external forces performed on the system. , where is the change in internal energy, Q is the amount of heat transferred to the system, A is the work of external forces. If the system itself does the work, then it is conventionally designated A. Then the law of conservation of energy for thermal processes, which is called the first law of thermodynamics, can be written as follows: , i.e. the amount of heat transferred to the system goes towards doing work by the system and changing its internal energy. During isobaric heating, the gas does work on external forces, where V1 and V2 are the initial and final volumes of the gas. If the process is not isobaric, the amount of work can be determined by the area of the ABCD figure enclosed between the line expressing the dependence p(V) and the initial and final volumes of gas V Let us consider the application of the first law of thermodynamics to isoprocesses occurring with an ideal gas. In an isothermal process, the temperature is constant, therefore, the internal energy does not change. Then the equation of the first law of thermodynamics will take the form: , i.e., the amount of heat transferred to the system goes to perform work during isothermal expansion, which is why the temperature does not change. In an isobaric process, the gas expands and the amount of heat transferred to the gas goes to increase its internal energy and to perform work: . During an isochoric process, the gas does not change its volume, therefore, no work is done by it, i.e. A = 0, and the equation of the first law has the form , i.e., the transferred amount of heat goes to increase the internal energy of the gas. Adiabatic is a process that occurs without heat exchange with the environment. Q = 0, therefore, when a gas expands, it does work by reducing its internal energy, therefore, the gas cools. The curve depicting the adiabatic process is called adiabatic. 2. Composition of the nucleus of an atom. Nuclear forces. Mass defect and binding energy of the atomic nucleus. Nuclear reactions. Nuclear energy. The nucleus of an atom of any substance consists of protons and neutrons. (The common name for protons and neutrons is nucleons.) The number of protons is equal to the charge of the nucleus and coincides with the element number in the periodic table. The sum of the number of protons and neutrons is equal to the mass number. For example, the nucleus of an oxygen atom consists of 8 protons and 16 - 8 = 8 neutrons. The nucleus of an atom consists of 92 protons and 235 - 92 = 143 neutrons. The forces that hold protons and neutrons in the nucleus are called nuclear forces*. This is the most powerful type of interaction. In 1932, English physicist James Chadwick discovered particles with zero electrical charge and unit mass. These particles were called neutrons. The neutron is designated n. After the discovery of the neutron, physicists D. D. Ivanenko and W. Heisenberg in 1932 put forward the proton-neutron model of the atomic nucleus. According to this model, the nucleus of an atom of any substance consists of protons and neutrons. (The common name for protons and neutrons is nucleons.) The number of protons is equal to the charge of the nucleus and coincides with the element number in the periodic table. The sum of the number of protons and neutrons is equal to the mass number. For example, the nucleus of an oxygen atom consists of 8 protons and 16 - 8 = 8 neutrons. The nucleus of an atom consists of 92 protons and 235 - 92 = 143 neutrons. Chemical substances that occupy the same place in the periodic table, but have different atomic masses, are called isotopes. Isotopic nuclei differ in the number of neutrons. For example, hydrogen has three isotopes: protium - the nucleus consists of one proton, deuterium - the nucleus consists of one proton and one neutron, tritium - the nucleus consists of one proton and two neutrons. If we compare the masses of nuclei with the masses of nucleons, it turns out that the mass of the nucleus of heavy elements is greater than the sum of the masses of protons and neutrons in the nucleus, and for light elements the mass of the nucleus is less than the sum of the masses of protons and neutrons in the nucleus. Therefore, there is a mass difference between the mass of the nucleus and the sum of the masses of protons and neutrons, called the mass defect. M = Mn - (Mp + Mn). Since there is a connection between mass and energy, then during the fission of heavy nuclei and during the synthesis of light nuclei, energy must be released that exists due to a mass defect, and this energy is called the binding energy of the atomic nucleus. The release of this energy can occur during nuclear reactions. A nuclear reaction is a process of changing the charge of a nucleus and its mass, which occurs during the interaction of a nucleus with other nuclei or elementary particles. When nuclear reactions occur, the laws of conservation of electrical charges and mass numbers are satisfied: the sum of the charges (mass numbers) of nuclei and particles entering into a nuclear reaction is equal to the sum of the charges (mass numbers) of the final products (nuclei and particles) of the reaction. A fission chain reaction is a nuclear reaction in which the particles causing the reaction are formed as products of the reaction. A necessary condition for chain development

39. Experiment on alpha particle scattering.

The first attempt to create a model of the atom based on accumulated experimental data (1903) belongs to J. Thomson. He believed that the atom is an electrically neutral spherical system with a radius of approximately 10–10 m. The positive charge of the atom is evenly distributed throughout the entire volume of the ball, and negatively charged electrons are located inside it (Fig. 6.1.1). To explain the line emission spectra of atoms, Thomson tried to determine the location of electrons in an atom and calculate the frequencies of their vibrations around equilibrium positions. However, these attempts were unsuccessful. A few years later, in the experiments of the great English physicist E. Rutherford, it was proven that Thomson's model was incorrect.

Figure 6.1.1.

J. Thomson's model of the atom

The first direct experiments to study the internal structure of atoms were carried out by E. Rutherford and his collaborators E. Marsden and H. Geiger in 1909–1911. Rutherford proposed using atomic probing using α-particles, which arise during the radioactive decay of radium and some other elements. The mass of alpha particles is approximately 7300 times the mass of an electron, and the positive charge is equal to twice the elementary charge. In his experiments, Rutherford used α-particles with a kinetic energy of about 5 MeV (the speed of such particles is very high - about 107 m/s, but still significantly less than the speed of light). α particles are fully ionized helium atoms. They were discovered by Rutherford in 1899 while studying the phenomenon of radioactivity. Rutherford bombarded atoms of heavy elements (gold, silver, copper, etc.) with these particles. The electrons that make up the atoms, due to their low mass, cannot noticeably change the trajectory of the α particle. Scattering, that is, a change in the direction of motion of α-particles, can only be caused by the heavy, positively charged part of the atom. The diagram of Rutherford's experiment is shown in Fig. 6.1.2.

Figure 6.1.2.

Scheme of Rutherford's experiment on α-particle scattering. K – lead container with a radioactive substance, E – screen coated with zinc sulfide, F – gold foil, M – microscope)

From a radioactive source enclosed in a lead container, alpha particles were directed onto a thin metal foil. Scattered particles fell on a screen covered with a layer of zinc sulfide crystals, capable of glowing when hit by fast charged particles. Scintillations (flashes) on the screen were observed by eye using a microscope. Observations of scattered α particles in Rutherford's experiment could be carried out at different angles φ to the original direction of the beam. It was found that most α particles pass through a thin layer of metal with little or no deflection. However, a small part of the particles are deflected at significant angles exceeding 30°. Very rare alpha particles (about one in ten thousand) were deflected at angles close to 180°.

This result was completely unexpected even for Rutherford. His ideas were in sharp contradiction with Thomson's model of the atom, according to which the positive charge is distributed throughout the entire volume of the atom. With such a distribution, the positive charge cannot create a strong electric field that can throw α particles back. The electric field of a uniform charged ball is maximum on its surface and decreases to zero as it approaches the center of the ball. If the radius of the ball in which all the positive charge of the atom is concentrated decreased by n times, then the maximum repulsive force acting on the α-particle would increase by n2 times according to Coulomb’s law. Consequently, for a sufficiently large value of n, alpha particles could experience scattering at large angles up to 180°. These considerations led Rutherford to the conclusion that the atom is almost empty, and all its positive charge is concentrated in a small volume. Rutherford called this part of the atom the atomic nucleus. This is how the nuclear model of the atom arose. Rice. 6.1.3 illustrates the scattering of an α particle in a Thomson atom and in a Rutherford atom.

Ernest Rutherford (1871-1937).

English physicist, founder of nuclear physics, member of the Royal Society of London (1903, president in 1925-1930) and most academies around the world. Born in Brightwater (New Zealand). In 1899 discovered alpha and beta rays in 1900 - a decay product of radium (emanation) and introduced the concept of half-life. Together with F. Soddy in 1902 - 1903. developed the theory of radioactive decay and established the law of radioactive transformations. In 1903 proved that alpha rays consist of positively charged particles (Nobel Prize in Chemistry, 1908).

In 1908 together with G. Geiger, he designed a device for recording individual charged particles (Geiger counter). Installed in 1911 the law of scattering of alpha particles by atoms of various elements (Rutherford's formula), which made it possible to create in 1911 a new model of the atom - planetary (Rutherford's model).

He put forward the idea of artificial transformation of atomic nuclei (1914). In 1919 carried out the first artificial nuclear reaction, converting nitrogen into oxygen, thereby laying the foundations of joint nuclear physics, discovered the proton. In 1920 predicted the existence of the neutron and deuteron. Together with M. Oliphant, he experimentally proved it in 1933. validity of the law of the relationship between mass and energy in nuclear reactions. In 1934 carried out the fusion reaction of deuterons with the formation of tritium.

The first experiments to study the structure of the atom were undertaken by Ernest Rutherford in 1911. They became possible thanks to the discovery of the phenomenon of radioactivity, in which, as a result of the natural radioactive decay of heavy elements, heavy elements are released -particles. It turned out that these particles have a positive charge equal to the charge of two electrons; their mass is approximately 4 times greater than the mass of a hydrogen atom, i.e. they are ions of the helium atom (). The energy of the particles varies from eV for uranium to eV for thorium. The speed of the particles is m/s, so they can be used to “shoot through” thin metal foil. Information about the scattering of particles is shown in Fig. 1.

Research has shown that a small number of particles deviated significantly from the original direction of movement. In some cases the scattering angle was close to 180 degrees. Based on the data obtained, E. Rutherford made conclusions that formed the basis planetary model of the atom:

There is a nucleus in which almost the entire mass of the atom and all its positive charge are concentrated, and the dimensions of the nucleus are much smaller than the dimensions of the atom itself;

The electrons that make up an atom move around the nucleus in circular orbits.

Based on these two premises and assuming that the interaction between an incident particle and a positively charged nucleus is determined by Coulomb forces, Rutherford established that atomic nuclei have dimensions ()m, i.e. they are () times smaller than the size of atoms.

The model of the atom proposed by Rutherford resembles the solar system, i.e. in the center of the atom there is a nucleus (“Sun”), and electrons—“planets”—move in orbits around it. This is why Rutherford's model was called planetary atomic model.

This model was a step forward to the modern understanding of the structure of the atom. The underlying concept atomic nucleus, in which the entire positive charge of the atom and almost all of its mass are concentrated, has retained its meaning to this day.

However, the assumption that electrons move in circular orbits incompatible neither with the laws of classical electrodynamics, nor with the line nature of the emission spectra of atomic gases.

Let us illustrate what has been said about Rutherford’s planetary model using the example of the hydrogen atom, which consists of a massive nucleus (proton) and an electron moving around it in a circular orbit. Since the orbital radius m (first Bohr orbit) and electron speed m/s, its normal acceleration . An electron moving with acceleration in a circular orbit is a two-dimensional oscillator. Therefore, according to classical electrodynamics, it should radiate energy in the form of an electromagnetic wave. As a result, the electron will inevitably approach the nucleus in time s. However, in reality, the hydrogen atom is a stable and “long-lived” electromechanical system.

Rutherford's experiments on the scattering of alpha particles. Nuclear model of the atom.

It is known that the word “atom” translated from Greek means “indivisible”. The English physicist J. Thomson developed (in the late 19th century) the first “model of the atom,” according to which the atom is a positively charged sphere within which electrons floated. The model proposed by Thomson needed experimental verification, since the phenomena of radioactivity and the photoelectric effect could not be explained using Thomson's atomic model. Therefore, in 1911, Ernest Rutherford conducted a series of experiments to study the composition and structure of atoms. In these experiments, a narrow beam a -particles emitted by a radioactive substance were directed onto thin gold foil. Behind it was a screen capable of glowing under the impacts of fast particles. It was found that the majority are a -particles deviate from linear propagation after passing through the foil, i.e., scatter, and some a -particles are thrown back 180 0 .

Trajectories A-particles flying at different distances from the nucleus

Lasers

Emission and absorption of light by atoms

According to Bohr's postulates, an electron can be in several specific orbits. Each electron orbit corresponds to a certain energy. When an electron moves from a near to a distant orbit, an atomic system absorbs a quantum of energy. When an electron moves from a more distant orbit to a closer orbit relative to the nucleus, the atomic system emits an energy quantum.

Spectra

Continuous spectrum

Conclusions from Rutherford's alpha particle scattering experiment: 1. There is an atomic nucleus, i.e. a small body in which almost the entire mass of an atom and all the positive charge are concentrated. 2. Almost the entire mass of the atom is concentrated in the nucleus. 3. Negative particles - electrons - rotate around the nucleus in closed orbits. 4. The negative charge of all electrons is distributed throughout the entire volume of the atom. Nuclear model of an atom:

Slide 9 from the presentation "Rutherford's experiment, model of the atom". The size of the archive with the presentation is 174 KB.

Physics 9th grade

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