The idea that atoms are the smallest particles of matter first arose during the Ancient Greece. However, only in late XVIII century, thanks to the work of such scientists as A. Lavoisier, M. V. Lomonosov and some others, it was proved that atoms really exist. However, in those days, no one wondered what their internal structure was. Scientists still regarded atoms as the indivisible "bricks" that make up all matter.
Attempts to explain the structure of the atom
Who proposed the nuclear model first of all scientists? The first attempt to create a model of these particles belonged to J. Thomson. However, it cannot be called successful in the full sense of the word. After all, Thomson believed that the atom is a spherical and electrically neutral system. The scientist assumed that positive charge distributed evenly over the volume of this ball, and inside it is a negatively charged nucleus. All attempts by the scientist to explain the internal structure of the atom were unsuccessful. Ernest Rutherford is the one who proposed the nuclear model of the structure of the atom a few years after Thomson put forward his theory.
Research History
With the help of the study of electrolysis in 1833, Faraday was able to establish that the current in the electrolyte solution is a stream of charged particles, or ions. Based on these studies, he was able to determine the minimum charge of an ion. Also an important role in the development of this direction in physics was played by the domestic chemist D. I. Mendeleev. It was he who first raised in scientific circles the question that all atoms can have the same nature. We see that before Rutherford's nuclear model of the structure of the atom was first proposed, a large number of equally important experiments were carried out by a variety of scientists. They advanced the atomistic theory of the structure of matter forward.
First experiences
Rutherford is a truly brilliant scientist, because his discoveries turned the idea of \u200b\u200bthe structure of matter upside down. In 1911, he was able to set up an experiment with which researchers were able to look into the mysterious depths of the atom, to get an idea of what its internal structure is. The first experiments were carried out by the scientist with the support of other researchers, but the main role in the discovery still belonged to Rutherford.
Experiment
Using natural sources radioactive radiation Rutherford was able to build a cannon that emitted a stream of alpha particles. It was a box made of lead, inside of which was a radioactive substance. The cannon had a slit through which all the alpha particles hit the lead screen. They could fly out only through the slot. Several more screens stood in the way of this beam of radioactive particles.
They separated particles that deviated from the previously set direction. A strictly focused target hit the target. Rutherford used a thin sheet of gold foil as a target. After the particles hit this sheet, they continued their movement and eventually hit the fluorescent screen, which was installed behind this target. When alpha particles hit this screen, flashes were recorded, by which the scientist could judge how many particles deviate from the original direction when they collide with the foil and what is the magnitude of this deviation.
Differences from previous experiences
Schoolchildren and students who are interested in those who proposed the nuclear model of the structure of the atom should know that similar experiments were carried out in physics before Rutherford. Their main idea was to collect as much information as possible about the structure of the atom from the deviations of particles from the original trajectory. All these studies led to the accumulation of a certain amount of information in science, provoked thinking about internal structure the smallest particles.
Already at the beginning of the 20th century, scientists knew that the atom contains electrons that have a negative charge. But among the majority of researchers, the prevailing opinion was that the atom from the inside is more like a grid filled with negatively charged particles. Such experiments made it possible to obtain a lot of information - for example, to determine the geometric dimensions of atoms.
genius guess
Rutherford noticed that none of his predecessors had ever tried to determine whether alpha particles could deviate at very large angles from their trajectory. The old model, sometimes called “raisin pudding” among scientists (because according to this model, the electrons in the atom are distributed like raisins in the pudding), simply did not allow the existence of dense structural components inside the atom. None of the scientists even bothered to consider this option. The researcher asked his student to re-equip the installation in such a way that large deviations of particles from the trajectory were also recorded - only in order to exclude such a possibility. Imagine the surprise of both the scientist and his student when it turned out that some particles fly apart at an angle of 180 o.
What's inside an atom?
We learned who proposed the nuclear model of the structure of the atom and what was the experience of this scientist. At that time, Rutherford's experiment was a real breakthrough. He was forced to conclude that inside the atom, most of the mass is enclosed in a very dense substance. The scheme of the nuclear model of the structure of the atom is extremely simple: inside is a positively charged nucleus.
Other particles, called electrons, revolve around this nucleus. The rest is several orders of magnitude less dense. The arrangement of electrons inside an atom is not chaotic - the particles are arranged in order of increasing energy. The researcher called the internal parts of atoms nuclei. The names that the scientist introduced are still used in science.
How to prepare for the lesson?
Those schoolchildren who are interested in those who suggested the nuclear model of the structure of the atom can show off additional knowledge in the lesson. For example, you can tell how Rutherford, long after his experiments, liked to give an analogy for his discovery. The South African country is smuggled with weapons for the rebels, which are enclosed in bales of cotton. How can customs officers determine exactly where dangerous supplies are if the entire train is full of these bales? The customs officer can start shooting at the bales, and where the bullets will ricochet, and there is a weapon. Rutherford stressed that this is how his discovery was made.
Students who are preparing to answer on this topic in the lesson, it is advisable to prepare answers to the following questions:
1. Who proposed the nuclear model of the structure of the atom?
2. What was the meaning of the experiment?
3. Difference of the nuclear model from other models.
Significance of Rutherford's theory
The radical conclusions that Rutherford drew from his experiments made many of his contemporaries doubt the validity of this model. Even Rutherford himself was no exception - he published the results of his research only two years after the discovery. Taking as a basis the classical ideas about how microparticles move, he proposed a nuclear planetary model of the structure of the atom. In general, the atom has a neutral charge. Electrons move around the nucleus, just like the planets revolve around the sun. This movement occurs due to the Coulomb forces. At the moment, Rutherford's model has undergone significant refinement, but the discovery of the scientist does not lose its relevance today.
Historical models1 of the atom reflect the levels of knowledge corresponding to a certain period in the development of science.
The first stage in the development of atomic models was characterized by the absence of experimental data on its structure.
Explaining the phenomena of the microcosm, scientists looked for analogies in the macrocosm, relying on the laws of classical mechanics.
J. Dalton, the creator of chemical atomism (1803), assumed that atoms of the same chemical element are the same spherical smallest, and therefore, indivisible particles.
The French physicist Jean Baptiste Perrin (1901) proposed a model that actually anticipated the "planetary" model. According to this model, a positively charged nucleus is located in the center of the atom, around which negatively charged electrons move in certain orbits, like planets around the Sun. The Perrin model did not attract the attention of scientists, since it gave only a qualitative, but not a quantitative, characteristic of the atom (in Fig. 7, this is shown by the discrepancy between the charge of the atomic nucleus and the number of electrons).
In 1902, the English physicist William Thomson (Kelvin) developed the idea of an atom as a positively charged spherical particle, inside which negatively charged electrons oscillate (radiate and absorb energy). Kelvin drew attention to the fact that the number of electrons is equal to the positive charge of the sphere, therefore, in general, the atom has no electric charge (Fig. 7).
A year later, the German physicist Philipp Lenard proposed a model according to which the atom is a hollow sphere, inside which there are electric dipoles (dynamides). The volume occupied by these dipoles is much less than the volume of the sphere, and the main part of the atom is empty.
According to the ideas of the Japanese physicist Gontaro (Hantaro) Nagaoka (1904), a positively charged nucleus is located in the center of the atom, and electrons move in space around the nucleus in flat rings resembling the rings of the planet Saturn (this model was called the "Saturnian" atom). Most scientists have not paid attention to the ideas of Nagaoka, although they to some extent have something in common with the modern idea of the atomic orbital.
None of the considered models (Fig. 7) explained how the properties of chemical elements are related to the structure of their atoms.
Rice. 7. Some historical models of the atom
In 1907, J. J. Thomson proposed a static model of the structure of the atom, representing the atom as a spherical particle charged with positive electricity, in which negatively charged electrons are uniformly distributed ( model"pudding", Fig. 7).
Mathematical calculations have shown that the electrons in an atom must be located on concentrically arranged rings. Thomson made a very important conclusion: the reason for the periodic change in the properties of chemical elements is associated with the features of the electronic structure of their atoms. Thanks to this, Thomson's model of the atom was highly appreciated by his contemporaries. However, it did not explain certain phenomena, for example, the scattering of α-particles when they pass through a metal plate.
Based on his ideas about the atom, Thomson derived a formula for calculating the average deviation of α-particles, and this calculation showed that the probability of scattering of such particles at large angles is close to zero. However, it has been experimentally proved that approximately one in eight thousand alpha particles falling on gold foil is deflected through an angle greater than 90°. This contradicted Thomson's model, which assumed deviations only at small angles.
Ernest Rutherford, summarizing experimental data, in 1911 proposed a "planetary" (sometimes called "nuclear") model of the structure of the atom, according to which 99.9% of the atom's mass and its positive charge are concentrated in a very small nucleus, and negatively charged electrons, the number which is equal to the charge of the nucleus, revolve around it, like planets solar system 1 (Fig. 7).
Rutherford, together with his students, set up experiments that made it possible to investigate the structure of the atom (Fig. 8). A stream of positively charged particles (α-particles) was directed to the surface of a thin metal (gold) foil 2 from a source of radioactive radiation 1. On their way, a fluorescent screen 3 was installed, which made it possible to observe the direction of the further movement of α-particles.
Rice. 8. Rutherford's experience
It was found that most of the α-particles passed through the foil, practically without changing their direction. Only individual particles (an average of one in ten thousand) were deflected and flew almost in the opposite direction. It was concluded that most of the atom's mass is concentrated in the positively charged nucleus, which is why the α-particles are so strongly deflected (Fig. 9).
Rice. 9. Scattering of α-particles by an atomic nucleus
Electrons moving in an atom, in accordance with the laws of electromagnetism, must radiate energy and, losing it, be attracted to the oppositely charged nucleus and, therefore, "fall" on it. This should lead to the disappearance of the atom, but since this did not happen, it was concluded that this model was inadequate.
At the beginning of the 20th century, the German physicist Max Planck and theoretical physicist Albert Einstein created the quantum theory of light. According to this theory, radiant energy, such as light, is emitted and absorbed not continuously, but in separate portions (quanta). Moreover, the value of the energy quantum is not the same for different radiations and is proportional to the frequency of oscillations of the electromagnetic wave: E = hν, where h – Planck's constant equal to 6.6266 10 -34 J s, ν is the radiation frequency. This energy is carried by particles of light - photons.
In an attempt to artificially combine the laws of classical mechanics and quantum theory, the Danish physicist Niels Bohr in 1913 supplemented Rutherford's model of the atom with two postulates about a stepwise (discrete) change in the energy of electrons in an atom. Bohr believed that an electron in a hydrogen atom can only be located on well-defined stationary orbits, whose radii are related to each other as squares natural numbers (1 2: 2 2: 3 2: ... :p 2). Electrons move around atomic nucleus in stationary orbits. The atom is in a stable state, without absorbing or emitting energy - this is Bohr's first postulate. According to the second postulate, energy emission occurs only when an electron moves to an orbit closer to the atomic nucleus. When an electron moves to a more distant orbit, energy is absorbed by the atom. This model was improved in 1916 by the German theoretical physicist Arnold Sommerfeld, who pointed out the motion of electrons along elliptical orbits.
The planetary model, due to its visibility and Bohr's postulates, for a long time used to explain atomic and molecular phenomena. However, it turned out that the motion of an electron in an atom, the stability and properties of an atom, in contrast to the motion of the planets and the stability of the solar system, cannot be described by the laws of classical mechanics. This mechanics is based on Newton's laws, and the subject of its study is the movement of macroscopic bodies, performed at speeds that are small compared to the speed of light. To describe the structure of the atom, it is necessary to apply the concepts of quantum (wave) mechanics about the dual corpuscular-wave nature of microparticles, which were formulated in the 1920s by theoretical physicists: the Frenchman Louis de Broglie, the Germans Werner Heisenberg and Erwin Schrödinger, the Englishman Paul Dirac and others.
In 1924, Louis de Broglie put forward the hypothesis that the electron has wave properties (the first principle of quantum mechanics) and proposed a formula for calculating its wavelength. The stability of an atom is explained by the fact that the electrons in it do not move in orbits, but in certain regions of space around the nucleus, called atomic orbitals. The electron occupies almost the entire volume of the atom and cannot "fall on the nucleus" located in its center.
In 1926, Schrödinger, continuing the development of L. de Broglie's ideas about the wave properties of an electron, empirically selected Mathematical equation, similar to the string vibration equation, which can be used to calculate the binding energies of an electron in an atom at different energy levels. This equation has become the basic equation of quantum mechanics.
The discovery of the wave properties of the electron showed that the dissemination of knowledge about the macrocosm to the objects of the microcosm is unlawful. In 1927, Heisenberg established that it is impossible to determine the exact position in space of an electron with a certain speed, therefore, ideas about the motion of an electron in an atom are of a probabilistic nature (the second principle of quantum mechanics).
The quantum mechanical model of the atom (1926) describes the state of the atom through mathematical functions and does not have a geometric expression (Fig. 10). Such a model does not consider the dynamic nature of the structure of the atom and the question of the size of an electron as a particle. It is believed that electrons occupy certain energy levels and emit or absorb energy during transitions to other levels. On fig. 10 energy levels are shown schematically as concentric rings located at different distances from the atomic nucleus. The arrows show the transitions of electrons between energy levels and the emission of photons accompanying these transitions. The scheme is shown qualitatively and does not reflect the real distances between energy levels, which can differ from each other by dozens of times.
In 1931, the American scientist Gilbert White first proposed a graphical representation of atomic orbitals and an "orbital" model of the atom (Fig. 10). Models of atomic orbitals are used to reflect the concept of "electron density" and to demonstrate the distribution of negative charge around a nucleus in an atom or a system of atomic nuclei in a molecule.
Rice. 10. Historical and modern models atom
In 1963, the American artist, sculptor and engineer Kenneth Snelson proposed a "ring-faced model" of the electron shells of an atom (Fig. 10), which explains the quantitative distribution of electrons in an atom over stable electron shells. Each electron is modeled by a ring magnet (or a closed loop with electric shock having a magnetic moment). Ring magnets are attracted to each other and form symmetrical shapes from rings - ringhedra. The presence of two poles in magnets imposes a limitation on possible options assemblies of rings. Models of stable electron shells are the most symmetrical figures of the rings, composed taking into account the presence of their magnetic properties.
The presence of a spin in an electron (see Section 5) is one of the main reasons for the formation of stable electron shells in an atom. Electrons form pairs with opposite spins. The ring-faced model of an electron pair, or a filled atomic orbital, is two rings located in parallel planes on opposite sides of the atomic nucleus. When more than one pair of electrons is located near the nucleus of an atom, the rings-electrons are forced to mutually orient themselves, forming an electron shell. In this case, closely spaced rings have different directions of magnetic lines of force, which is denoted different color rings representing electrons.
Model experiment shows that the most stable of all possible ring-faced models is the model of 8 rings. Geometrically, the model is formed in such a way as if an atom in the form of a sphere was divided into 8 parts (divided three times in half) and one ring-electron was placed in each part. In ring-faced models, rings of two colors are used: red and blue, which reflect the positive and negative value of the electron spin.
The "wave-faced model" (Fig. 10) is similar to the "ring-faced" model, with the difference that each electron of an atom is represented by a "wave" ring, which contains an integer number of waves (as proposed by L. de Broglie).
The interaction of the electrons of the electron shell on this model of the atom is shown by the coincidence of the points of contact of the blue and red "wave" rings with the nodes of the standing waves.
Models of the atom have the right to exist and the limits of application. Any model of an atom is an approximation reflecting in a simplified form certain part knowledge about the atom. But none of the models fully reflects the properties of the atom or its constituent particles.
Many models today are only of historical interest. When building models of microworld objects, scientists relied on what can be directly observed. This is how the models of Perrin and Rutherford (an analogy with the structure of the solar system), Nagaoka (a kind of planet Saturn), Thomson ("raisin pudding") appeared. Some ideas were discarded (Lenard's dynamic model), others were revisited after some time, but at a new, higher theoretical level: the Perrin and Kelvin models were developed in the Rutherford and Thomson models. Ideas about the structure of the atom are constantly being improved. How accurate is the modern - "quantum-mechanical" model - time will tell. That is why a question mark is drawn at the top of the spiral, symbolizing the path of cognition (Fig. 7).
Planetary model of the atom
Planetary model of an atom: nucleus (red) and electrons (green)
Planetary model of the atom, or Rutherford model, - historical model of the structure of the atom, which was proposed by Ernest Rutherford as a result of an experiment with alpha particle scattering. According to this model, the atom consists of a small positively charged nucleus, in which almost all the mass of the atom is concentrated, around which electrons move, just as the planets move around the sun. The planetary model of the atom corresponds to modern ideas about the structure of the atom, taking into account the fact that the movement of electrons is of a quantum nature and is not described by the laws of classical mechanics. Historically, Rutherford's planetary model succeeded Joseph John Thomson's "plum pudding model", which postulates that negatively charged electrons are placed inside a positively charged atom.
Rutherford proposed a new model for the structure of the atom in 1911 as a conclusion from an experiment on the scattering of alpha particles on gold foil, carried out under his leadership. During this scattering, an unexpectedly large number of alpha particles were scattered at large angles, which indicated that the scattering center has small size and it contains a significant electric charge. Rutherford's calculations showed that a scattering center, positively or negatively charged, must be at least 3000 times smaller size an atom, which at that time was already known and estimated to be about 10 -10 m. Since at that time the electrons were already known, and their mass and charge were determined, the scattering center, which was later called the nucleus, must have had an opposite charge to the electrons. Rutherford did not link the amount of charge to atomic number. This conclusion was made later. And Rutherford himself suggested that the charge is proportional to the atomic mass.
The disadvantage of the planetary model was its incompatibility with the laws of classical physics. If electrons move around the nucleus like planets around the Sun, then their movement is accelerated, and, therefore, according to the laws of classical electrodynamics, they should have radiated electromagnetic waves, lose energy and fall on the core. The next step in the development of the planetary model was the Bohr model, postulating other, different from the classical, laws of electron motion. Completely the contradictions of electrodynamics were able to solve quantum mechanics.
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- Eise Eisingi Planetarium
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planetary model of the atom- planetinis atomo modelis statusas T sritis fizika atitikmenys: angl. planetary atom model vok. Planetenmodell des Atoms, n rus. planetary model of the atom, f pranc. modele planétaire de l'atome, m … Fizikos terminų žodynas
Bohr model of the atom- Bohr model of a hydrogen-like atom (Z nuclear charge), where a negatively charged electron is enclosed in an atomic shell surrounding a small, positively charged atomic nucleus ... Wikipedia
Model (in science)- Model (French modèle, Italian modello, from Latin modulus measure, measure, sample, norm), 1) a sample that serves as a standard (standard) for serial or mass reproduction (M. car, M. clothing, etc. .), as well as the type, brand of any ... ...
Model- I Model (Model) Walter (24.1.1891, Gentin, East Prussia, 21.4.1945, near Duisburg), fascist German General Field Marshal (1944). In the army since 1909, participated in the 1st World War of 1914 18. From November 1940 he commanded the 3rd tank ... ... Great Soviet Encyclopedia
STRUCTURE OF THE ATOM- (see) built from elementary particles three types(see), (see) and (see), forming a stable system. The proton and neutron are a part of atomic (see), electrons form an electron shell. Forces act in the nucleus (see), thanks to which ... ... Great Polytechnic Encyclopedia
Atom- This term has other meanings, see Atom (meanings). Helium atom Atom (from other Greek ... Wikipedia
Rutherford Ernest- (1871 1937), English physicist, one of the creators of the theory of radioactivity and the structure of the atom, founder of a scientific school, foreign corresponding member of the Russian Academy of Sciences (1922) and honorary member of the USSR Academy of Sciences (1925). Born in New Zealand, after graduating from ... ... encyclopedic Dictionary
Άτομο
corpuscle- Helium atom Atom (another Greek ἄτομος indivisible) is the smallest part of a chemical element, which is the carrier of its properties. An atom consists of an atomic nucleus and an electron cloud surrounding it. The nucleus of an atom consists of positively charged protons and ... ... Wikipedia
corpuscles- Helium atom Atom (another Greek ἄτομος indivisible) is the smallest part of a chemical element, which is the carrier of its properties. An atom consists of an atomic nucleus and an electron cloud surrounding it. The nucleus of an atom consists of positively charged protons and ... ... Wikipedia
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- A set of tables. Physics. Grade 11 (15 tables), . Educational album of 15 sheets. Transformer. Electromagnetic induction in modern technology. Electronic lamps. Cathode-ray tube. Semiconductors. semiconductor diode. Transistor.…
Planetary model of the atom
Planetary model of an atom: nucleus (red) and electrons (green)
Planetary model of the atom, or Rutherford model, - historical model of the structure of the atom, which was proposed by Ernest Rutherford as a result of an experiment with alpha particle scattering. According to this model, the atom consists of a small positively charged nucleus, in which almost all the mass of the atom is concentrated, around which electrons move, just as the planets move around the sun. The planetary model of the atom corresponds to modern ideas about the structure of the atom, taking into account the fact that the movement of electrons is of a quantum nature and is not described by the laws of classical mechanics. Historically, Rutherford's planetary model succeeded Joseph John Thomson's "plum pudding model", which postulates that negatively charged electrons are placed inside a positively charged atom.
Rutherford proposed a new model for the structure of the atom in 1911 as a conclusion from an experiment on the scattering of alpha particles on gold foil, carried out under his leadership. During this scattering, an unexpectedly large number of alpha particles were scattered at large angles, which indicated that the scattering center was small and a significant electric charge was concentrated in it. Rutherford's calculations showed that a scattering center, positively or negatively charged, must be at least 3000 times smaller than the size of an atom, which at that time was already known and estimated to be about 10 -10 m. Since electrons were already known at that time, and their mass and charge are determined, then the scattering center, which was later called the nucleus, must have had the opposite charge to the electrons. Rutherford did not link the amount of charge to atomic number. This conclusion was made later. And Rutherford himself suggested that the charge is proportional to the atomic mass.
The disadvantage of the planetary model was its incompatibility with the laws of classical physics. If electrons move around the nucleus like a planet around the Sun, then their movement is accelerated, and, therefore, according to the laws of classical electrodynamics, they should radiate electromagnetic waves, lose energy and fall on the nucleus. The next step in the development of the planetary model was the Bohr model, postulating other, different from the classical, laws of electron motion. Completely the contradictions of electrodynamics were able to solve quantum mechanics.
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See what the "Planetary Model of the Atom" is in other dictionaries:
planetary model of the atom- planetinis atomo modelis statusas T sritis fizika atitikmenys: angl. planetary atom model vok. Planetenmodell des Atoms, n rus. planetary model of the atom, f pranc. modele planétaire de l'atome, m … Fizikos terminų žodynas
Bohr model of a hydrogen-like atom (Z nucleus charge), where a negatively charged electron is enclosed in an atomic shell surrounding a small, positively charged atomic nucleus ... Wikipedia
Model (French modèle, Italian modello, from Latin modulus measure, measure, sample, norm), 1) a sample that serves as a standard (standard) for serial or mass reproduction (M. of a car, M. of clothes, etc.). ), as well as the type, brand of any ... ...
I Model (Model) Walter (January 24, 1891, Gentin, East Prussia, April 21, 1945, near Duisburg), Nazi German General Field Marshal (1944). In the army since 1909, participated in the 1st World War of 1914 18. From November 1940 he commanded the 3rd tank ... ... Great Soviet Encyclopedia
STRUCTURE OF THE ATOM- (see) is built from elementary particles of three types (see), (see) and (see), forming a stable system. The proton and neutron are a part of atomic (see), electrons form an electron shell. Forces act in the nucleus (see), thanks to which ... ... Great Polytechnic Encyclopedia
This term has other meanings, see Atom (meanings). Helium atom Atom (from other Greek ... Wikipedia
- (1871 1937), English physicist, one of the creators of the theory of radioactivity and the structure of the atom, founder of a scientific school, foreign corresponding member of the Russian Academy of Sciences (1922) and honorary member of the USSR Academy of Sciences (1925). Born in New Zealand, after graduating from ... ... encyclopedic Dictionary
Helium atom An atom (another Greek ἄτομος indivisible) is the smallest part of a chemical element, which is the carrier of its properties. An atom consists of an atomic nucleus and an electron cloud surrounding it. The nucleus of an atom consists of positively charged protons and ... ... Wikipedia
Helium atom An atom (another Greek ἄτομος indivisible) is the smallest part of a chemical element, which is the carrier of its properties. An atom consists of an atomic nucleus and an electron cloud surrounding it. The nucleus of an atom consists of positively charged protons and ... ... Wikipedia
Books
- A set of tables. Physics. Grade 11 (15 tables), . Educational album of 15 sheets. Transformer. Electromagnetic induction in modern technology. Electronic lamps. Cathode-ray tube. Semiconductors. semiconductor diode. Transistor.…
The first model of the structure of the atom was proposed by J. Thomson in 1904, according to which the atom is a positively charged sphere with electrons embedded in it. Despite its imperfection, the Thomson model made it possible to explain the phenomena of emission, absorption, and scattering of light by atoms, as well as to determine the number of electrons in atoms of light elements.
Rice. 1. Atom, according to the Thomson model. Electrons are held inside a positively charged sphere by elastic forces. Those of them that are on the surface can easily "knock out", leaving an ionized atom.
2.2 Rutherford model
Thomson's model was refuted by E. Rutherford (1911), who proved that the positive charge and almost the entire mass of an atom are concentrated in a small part of its volume - the nucleus around which electrons move (Fig. 2).
Rice. 2. This model of the structure of the atom is known as planetary, because the electrons revolve around the nucleus like the planets of the solar system.
According to the laws of classical electrodynamics, the motion of an electron in a circle around the nucleus will be stable if the Coulomb attraction force is equal to the centrifugal force. However, according to the theory of the electromagnetic field, the electrons in this case should move in a spiral, continuously radiating energy, and fall on the nucleus. However, the atom is stable.
In addition, with continuous radiation of energy, an atom should have a continuous, continuous spectrum. In fact, the spectrum of an atom consists of individual lines and series.
Thus, this model contradicts the laws of electrodynamics and does not explain the line nature of the atomic spectrum.
2.3. Bohr model
In 1913, N. Bohr proposed his theory of the structure of the atom, without completely denying the previous ideas. Bohr based his theory on two postulates.
The first postulate says that the electron can rotate around the nucleus only in certain stationary orbits. Being on them, it does not radiate or absorb energy (Fig. 3).
Rice. 3. Model of the structure of the Bohr atom. The change in the state of an atom when an electron moves from one orbit to another.
When moving along any stationary orbit, the energy supply of an electron (E 1, E 2 ...) remains constant. The closer the orbit is to the nucleus, the lower the electron energy reserve Е 1 ˂ Е 2 …˂ Е n . The energy of an electron in orbits is determined by the equation:
where m is the electron mass, h is Planck's constant, n is 1, 2, 3… (n=1 for the 1st orbit, n=2 for the 2nd, etc.).
The second postulate says that when moving from one orbit to another, an electron absorbs or releases a quantum (portion) of energy.
If atoms are exposed to influence (heating, radiation, etc.), then an electron can absorb an energy quantum and move to an orbit more distant from the nucleus (Fig. 3). In this case, one speaks of an excited state of the atom. During the reverse transition of an electron (to an orbit closer to the nucleus), energy is released in the form of a quantum of radiant energy - a photon. In the spectrum, this is fixed by a certain line. Based on the formula
,
where λ is the wavelength, n = quantum numbers characterizing the near and far orbits, Bohr calculated the wavelengths for all series in the spectrum of the hydrogen atom. The results obtained were consistent with the experimental data. The origin of discontinuous line spectra became clear. They are the result of the emission of energy by atoms during the transition of electrons from an excited state to a stationary one. Transitions of electrons to the 1st orbit form a group of frequencies of the Lyman series, to the 2nd - the Balmer series, to the 3rd Paschen series (Fig. 4, Table 1).
Rice. 4. Correspondence between electronic transitions and spectral lines of the hydrogen atom.
Table 1
Verification of the Bohr formula for series of the hydrogen spectrum
However, Bohr's theory failed to explain the splitting of lines in the spectra of multielectron atoms. Bohr proceeded from the fact that the electron is a particle, and used the laws characteristic of particles to describe the electron. At the same time, facts were accumulating that showed that the electron is also capable of exhibiting wave properties. Classical mechanics turned out to be unable to explain the motion of micro-objects, which simultaneously have the properties of material particles and the properties of a wave. This problem was solved by quantum mechanics - a physical theory that studies the general patterns of motion and interaction of microparticles with a very small mass (Table 2).
table 2
Properties of elementary particles that form an atom