In 1903, the English scientist Thomson proposed a model of the atom, which was jokingly called the “raisin bun.” According to his version, an atom is a sphere with a uniform positive charge, in which negatively charged electrons are interspersed like raisins.
However, further studies of the atom showed that this theory is untenable. And a few years later, another English physicist, Rutherford, conducted a series of experiments. Based on the results, he built a hypothesis about the structure of the atom, which is still internationally accepted.
Rutherford's experiment: proposal of his own model of the atom
In his experiments, Rutherford passed a beam of alpha particles through thin gold foil. Gold was chosen for its ductility, which made it possible to create a very thin foil, almost one layer of molecules thick. Behind the foil there was a special screen, which was illuminated when bombarded by alpha particles falling on it. According to Thomson's theory, alpha particles should have passed through the foil unhindered, deflecting very slightly to the sides. However, it turned out that some of the particles behaved this way, and a very small part bounced back, as if hitting something.
That is, it was established that inside the atom there is something solid and small, from which the alpha particles bounced. It was then that Rutherford proposed a planetary model of the structure of the atom. Rutherford's planetary model of the atom explained the results of both his experiments and those of his colleagues. To this day it has not been proposed best model, although some aspects of this theory still do not agree with practice in some very narrow areas of science. But basically, the planetary model of the atom is the most useful of all. What does this model consist of?
Planetary model of the structure of the atom
As the name suggests, the atom is compared to a planet. In this case, the planet is the nucleus of an atom. And electrons rotate around the nucleus at a fairly large distance, just as satellites rotate around the planet. Only the speed of rotation of electrons is hundreds of thousands of times higher than the speed of rotation of the fastest satellite. Therefore, during its rotation, the electron creates a kind of cloud above the surface of the nucleus. And the existing charges of electrons repel the same charges formed by other electrons around other nuclei. Therefore, the atoms do not “stick together”, but are located at some distance from each other.
And when we talk about the collision of particles, we mean that they come close enough to each other long distance and are repelled by the fields of their charges. There is no direct contact. Particles in matter are generally located very far from each other. If somehow the particles of a body could be collapsed together, it would shrink billions of times. The earth would become smaller than an apple. So the main volume of any substance, strange as it may sound, is occupied by a void in which charged particles are located, held at a distance by electronic interaction forces.
The idea that atoms are the smallest particles of matter first arose during Ancient Greece. However, only at the end of the 18th century, thanks to the work of scientists such as A. Lavoisier, M.V. Lomonosov and some others, it was proven that atoms really exist. However, in those days no one wondered what their internal structure was. Scientists still regarded atoms as the indivisible “building blocks” that make up all matter.
Attempts to explain the structure of the atom
Who was the first scientist to propose the nuclear model? 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. At the same time, the scientist assumed that the positive charge was distributed evenly throughout the volume of this ball, and inside it there was a negatively charged nucleus. All the scientist’s attempts 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.
History of research
By researching electrolysis in 1833, Faraday was able to establish that the current in an electrolyte solution is a flow of charged particles, or ions. Based on these studies, he was able to determine the minimum charge of the 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 the question in scientific circles that all atoms could have the same nature. We see that before the Rutherford nuclear model of the structure of the atom was first proposed, a variety of scientists carried out a large number of no less important experiments. They promoted atomic theory structure of matter forward.
First experiments
Rutherford is truly a brilliant scientist, because his discoveries revolutionized the understanding of the structure of matter. In 1911, he was able to set up an experiment, with the help of which researchers were able to look into the mysterious depths of the atom and get an idea of what its internal structure is. The first experiments were carried out by the scientist with the support of other researchers, however the main role in the opening it still belonged to Rutherford.
Experiment
Using natural springs radioactive radiation, Rutherford was able to build a gun that emitted a stream of alpha particles. It was a box made of lead, inside of which there was a radioactive substance. There was a slot in the gun that allowed all the alpha particles to hit the lead screen. They could only fly out through the slot. In the path of this beam of radioactive particles there were several more screens.
They separated particles that deviated from a previously specified direction. A strictly focused target was hit. Rutherford used a thin sheet of gold foil as a target. Once the particles hit this sheet, they continued their movement and eventually hit a fluorescent screen that was installed behind this target. When alpha particles hit this screen, flashes were recorded, from which the scientist could judge how many particles deviated from the original direction when colliding with the foil and what the magnitude of this deviation was.
Differences from previous experiments
Schoolchildren and students who are interested in who proposed the nuclear model of the structure of the atom should know: similar experiments were carried out in physics before Rutherford. Their main idea was to collect as much as possible from the deviations of particles from the initial trajectory more information about the structure of the atom. All these studies led to the accumulation of a certain amount of information in science and provoked thinking about internal structure smallest particles.
Already at the beginning of the 20th century, scientists knew that an atom contains electrons with a negative charge. But among most researchers, the prevailing opinion was that the inside of an atom 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 geometric dimensions atoms.
Brilliant 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 previous model, sometimes called “raisin pudding” among scientists (because according to this model, the electrons in an atom are distributed like raisins in a pudding), simply did not allow for the existence of dense components of the structure within 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 recorded - only to exclude this possibility. Imagine the surprise of both the scientist and his student when it turned out that some particles scatter at an angle of 180 degrees.
What's inside an atom?
We found out who proposed the nuclear model of the structure of the atom and what the experience of this scientist was. At that time, Rutherford's experiment was a real breakthrough. He was forced to conclude that inside an atom, most of the mass was contained in very dense matter. The diagram of the nuclear model of the structure of an atom is extremely simple: inside there is a positively charged nucleus.
Other particles called electrons orbit 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 today.
How to prepare for the lesson?
Those schoolchildren who are interested in who proposed the nuclear model of the structure of the atom can show off additional knowledge in the lesson. For example, you can talk about how Rutherford, long after his experiments, liked to give an analogy for his discovery. Guns for rebels are being smuggled into a southern African country, contained in bales of cotton. How can customs officers determine exactly where dangerous supplies are located if the entire train is filled with these bales? The customs officer may start shooting at the bales, and where the bullets will ricochet is where the weapon is located. Rutherford emphasized that this is exactly how his discovery was made.
For schoolchildren who are preparing to answer on this topic in class, 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 point of the experiment?
3. Difference between the nuclear model and other models.
The significance of Rutherford's theory
The radical conclusions that Rutherford drew from his experiments led many of his contemporaries to doubt the truth 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 of how microparticles move, he proposed a nuclear planetary model of the structure of the atom. Overall, the atom has a neutral charge. Electrons move around the nucleus, just as planets revolve around the Sun. This movement occurs due to Coulomb forces. At the moment, Rutherford’s model has undergone significant modification, but the scientist’s discovery does not lose its relevance today.
One of the first models of atomic structure was proposed J. Thomson in 1904, the Atom was imagined as a “sea of positive electricity” with electrons oscillating in it. The total negative charge of the electrons of an electrically neutral atom was equal to its total positive charge.
Rutherford's experience
To test Thomson's hypothesis and more accurately determine the structure of the atom E. Rutherford organized a series of experiments on scattering α -particles with thin metal plates - foil. In 1910, Rutherford students Hans Geiger And Ernest Marsden conducted bombing experiments α -particles of thin metal plates. They found that most α -particles pass through the foil without changing their trajectory. And this was not surprising if we accept the correctness of Thomson's model of the atom.
Source α - radiation was placed in a lead cube with a channel drilled in it, so that it was possible to obtain a flux α -particles flying in a certain direction. Alpha particles are doubly ionized helium atoms ( Not 2+). They have a +2 positive charge and a mass almost 7350 times the mass of an electron. Getting on the screen coated with zinc sulfide, α -particles caused it to glow, and with a magnifying glass one could see and count the individual flashes that appeared on the screen when each one hit it α -particles. Foil was placed between the radiation source and the screen. From the flashes on the screen one could judge the scattering α -particles, i.e. about their deviation from the original direction when passing through a layer of metal.
It turned out that the majority α -particles pass through the foil without changing their direction, although the thickness of the foil corresponded to hundreds of thousands of atomic diameters. But some α -particles were still deflected at small angles, and occasionally α -particles abruptly changed the direction of their movement and even (about 1 in 100,000) were thrown back, as if they had encountered a massive obstacle. Cases of such a sharp deviation α -particles could be observed by moving the screen with a magnifying glass along an arc.
From the results of this experiment the following conclusions could be drawn:
- There is some "obstacle" in the atom, which was called the nucleus.
- The nucleus has a positive charge (otherwise positively charged α -particles would not be reflected back).
- The nucleus has very small dimensions compared to the size of the atom itself (only a small part α -particles changed direction of movement).
- The core has large mass, compared to mass α -particles
Rutherford explained the results of the experiment by proposing "planetary" model of the atom which likened him solar system. According to the planetary model, at the center of the atom there is a very small nucleus, the size of which is approximately 100,000 times smaller sizes the atom itself. This nucleus contains almost the entire mass of the atom and carries a positive charge. Electrons move around the nucleus, the number of which is determined by the charge of the nucleus. The external trajectory of the electrons determines the external dimensions of the atom. The diameter of an atom is on the order of 10 -8 cm, and the diameter of the nucleus is on the order of 10 -13 ÷10 -12 cm.
The greater the charge of an atomic nucleus, the stronger the repulsion from it α -particle, the more often cases of strong deviations will occur α -particles passing through the metal layer, from the initial direction of movement. Therefore, scattering experiments α -particles make it possible not only to detect the existence of an atomic nucleus, but also to determine its charge. Already from Rutherford's experiments it followed that the charge of the nucleus (expressed in units of electron charge) is numerically equal to the serial number of the element in the periodic table. This has been confirmed G. Moseley, who established in 1913 a simple connection between the wavelengths of certain lines in the X-ray spectrum of an element and its atomic number, and D. Chadwick, who in 1920 determined with great accuracy the charges of atomic nuclei of a number of elements by scattering α -particles
Was installed physical meaning serial number of an element in the periodic table: the serial number turned out to be the most important constant of an element, expressing the positive charge of the nucleus of its atom. From the electrical neutrality of an atom it follows that the number of electrons rotating around the nucleus is equal to the atomic number of the element.
This discovery provided a new rationale for the arrangement of elements in the periodic table. At the same time, it also eliminated the apparent contradiction in the Mendeleev system - the position of some elements with higher atomic mass ahead of elements with lower atomic mass (tellurium and iodine, argon and potassium, cobalt and nickel). It turned out that there is no contradiction here, since the place of an element in the system is determined by the charge of the atomic nucleus. It was experimentally established that the nuclear charge of a tellurium atom is 52, and that of an iodine atom is 53; therefore tellurium, despite the large atomic mass, must stand before iodine. In the same way, the charges of the nuclei of argon and potassium, nickel and cobalt fully correspond to the sequence of arrangement of these elements in the system.
So, the charge of the atomic nucleus is the basic quantity on which the properties of the element and its position in the periodic table depend. That's why periodic law of mendeleev can currently be formulated as follows:
The properties of elements and the simple and complex substances they form are periodically dependent on the charge of the nucleus of the atoms of the elements
Determining the serial numbers of elements based on the charges of the nuclei of their atoms made it possible to establish the total number of places in the periodic table between hydrogen, which has serial number 1, and uranium (atomic number 92), which at that time was considered the last member of the periodic system of elements. When the theory of atomic structure was created, places 43, 61, 72, 75, 85 and 87 remained unoccupied, which indicated the possibility of the existence of as yet undiscovered elements. Indeed, in 1922 the element hafnium was discovered, which took place 72; then in 1925 - rhenium, which took place 75. The elements that should occupy the remaining four empty places in the table turned out to be radioactive and were not found in nature, but they were obtained artificially. The new elements were named technetium (serial number 43), promethium (61), astatine (85) and francium (87). Currently, all the cells of the periodic table between hydrogen and uranium are filled. However, she herself periodic table is not complete.
Atomic spectra
The planetary model was a major step in the theory of atomic structure. However, in some respects it contradicted firmly established facts. Let's consider two such contradictions.
First, Rutherford's theory could not explain the stability of the atom. An electron rotating around a positively charged nucleus must, like an oscillating electric charge, emit electromagnetic energy in the form of light waves. But by emitting light, the electron loses part of its energy, which leads to an imbalance between the centrifugal force associated with the rotation of the electron and the force of electrostatic attraction of the electron to the nucleus. To restore equilibrium, the electron must move closer to the nucleus. Thus, the electron, continuously emitting electromagnetic energy and moving in a spiral, will approach the nucleus. Having exhausted all its energy, it must “fall” onto the nucleus, and the atom will cease to exist. This conclusion contradicts the real properties of atoms, which are stable formations and can exist without destruction for an extremely long time.
Secondly, Rutherford's model led to incorrect conclusions about the nature of atomic spectra. When light emitted by a hot solid or liquid body is passed through a glass or quartz prism, a so-called continuous spectrum is observed on a screen placed behind the prism, the visible part of which is a colored stripe containing all the colors of the rainbow. This phenomenon is explained by the fact that the radiation of a hot solid or liquid body consists of electromagnetic waves of various frequencies. Waves of different frequencies are refracted differently by the prism and fall on different places screen. Set of frequencies electromagnetic radiation emitted by a substance is called the emission spectrum. On the other hand, substances absorb radiation of certain frequencies. The combination of the latter is called the absorption spectrum of the substance.
To obtain a spectrum, you can use a diffraction grating instead of a prism. The latter is a glass plate, on the surface of which thin parallel strokes are applied at a very close distance from each other (up to 1500 strokes per 1 mm). Passing through such a grating, light decomposes and forms a spectrum similar to that obtained using a prism. Diffraction is inherent in any wave motion and serves as one of the main proofs of the wave nature of light.
When heated, a substance emits rays (radiation). If radiation has one wavelength, then it is called monochromatic. In most cases, radiation is characterized by several wavelengths. When the radiation is decomposed into monochromatic components, a radiation spectrum is obtained, where its individual components are expressed as spectral lines.
Spectra obtained by emission from free or weakly bound atoms (for example, in gases or vapors) are called atomic spectra.
Radiation emitted by solids or liquids always gives a continuous spectrum. Radiation emitted by hot gases and vapors, as opposed to radiation solids and liquids, contains only certain wavelengths. Therefore, instead of a continuous stripe on the screen, you get a series of individual colored lines separated by dark spaces. The number and location of these lines depend on the nature of the hot gas or steam. Thus, potassium vapor produces a spectrum consisting of three lines - two red and one violet; in the spectrum of calcium vapor there are several red, yellow and green lines, etc.
Radiation emitted by solids or liquids always gives a continuous spectrum. Radiation emitted by hot gases and vapors, unlike radiation from solids and liquids, contains only certain wavelengths. Therefore, instead of a continuous stripe on the screen, you get a series of individual colored lines separated by dark spaces. The number and location of these lines depend on the nature of the hot gas or steam. Thus, potassium vapor gives a spectrum consisting of three lines - two red and one violet; in the spectrum of calcium vapor there are several red, yellow and green lines, etc.
Such spectra are called line spectra. It was found that light emitted by gas atoms has a line spectrum, in which spectral lines can be combined into series.
In each series, the arrangement of lines corresponds to a certain pattern. The frequencies of individual lines can be described Balmer's formula:
The fact that the atoms of each element give a completely definite spectrum, inherent only to this element, and the intensity of the corresponding spectral lines is higher, the higher more content element in a taken sample, is widely used to determine the quality and quantitative composition substances and materials. This research method is called spectral analysis.
The planetary model of the structure of the atom turned out to be unable to explain the line spectrum of emission of hydrogen atoms, much less the combination of spectral lines in a series. An electron rotating around a nucleus must approach the nucleus, continuously changing its speed. The frequency of the light it emits is determined by the frequency of its rotation and therefore must change continuously. This means that the emission spectrum of an atom must be continuous, continuous. According to this model, the frequency of radiation of an atom must be equal to the mechanical frequency of vibrations or be a multiple of it, which does not agree with Balmer’s formula. Thus, Rutherford's theory could not explain either the existence of stable atoms or the presence of their line spectra.
Quantum theory of light
In 1900 M. Planck showed that the ability of a heated body to emit radiation can be correctly described quantitatively only by assuming that radiant energy is emitted and absorbed by bodies not continuously, but discretely, i.e. in separate portions - quanta. At the same time, the energy E each such portion is related to the radiation frequency by a relationship called Planck's equations:
Planck himself for a long time believed that the emission and absorption of light by quanta is a property of emitting bodies, and not the radiation itself, which is capable of having any energy and therefore could be absorbed continuously. However, in 1905 Einstein, analyzing the phenomenon of the photoelectric effect, came to the conclusion that electromagnetic (radiant) energy exists only in the form of quanta and that, therefore, radiation is a stream of indivisible material “particles” (photons), the energy of which is determined by Planck's equation.
Photoelectric effect is the emission of electrons by a metal under the influence of light incident on it. This phenomenon was studied in detail in 1888-1890. A. G. Stoletov. If you place the installation in a vacuum and apply it to a record M negative potential, then no current will be observed in the circuit, since in the space between the plate and the grid there are no charged particles capable of carrying electric current. But when the plate is illuminated by a light source, the galvanometer detects the emergence of a current (called photocurrent), the carriers of which are electrons emitted from the metal by light.
It turned out that when the lighting intensity changes, only the number of electrons emitted by the metal changes, i.e. photocurrent strength. But the maximum kinetic energy of each electron emitted from the metal does not depend on the intensity of illumination, but changes only when the frequency of the light incident on the metal changes. It is with an increase in wavelength (i.e., with a decrease in frequency) that the energy of the electrons emitted by the metal decreases, and then, at a wavelength specific to each metal, the photoelectric effect disappears and does not appear even at very high light intensity. Thus, when illuminated with red or orange light, sodium does not exhibit a photoelectric effect and begins to emit electrons only at a wavelength less than 590 nm (yellow light); in lithium, the photoelectric effect is detected at even shorter wavelengths, starting from 516 nm (green light); and the ejection of electrons from platinum under the influence of visible light does not occur at all and begins only when platinum is irradiated with ultraviolet rays.
These properties of the photoelectric effect are completely inexplicable from the standpoint of the classical wave theory of light, according to which the effect should be determined (for a given metal) only amount of energy, absorbed by the metal surface per unit time, but should not depend on the type of radiation incident on the metal. However, these same properties receive a simple and convincing explanation if we assume that the radiation consists of individual portions, photons, with a very specific energy.
In fact, an electron in a metal is bound to the metal atoms, so that a certain energy must be expended to tear it out. If the photon has the required amount of energy (and the energy of the photon is determined by the frequency of the radiation), then the electron will be ejected and the photoelectric effect will be observed. In the process of interacting with a metal, the photon completely gives up its energy to the electron, because the photon cannot be split into parts. The energy of the photon will be partially spent on breaking the bond between the electron and the metal, and partially on imparting kinetic energy of motion to the electron. Therefore, the maximum kinetic energy of an electron knocked out of a metal cannot be greater than the difference between the photon energy and the binding energy of the electron with the metal atoms. Consequently, with an increase in the number of photons incident on the metal surface per unit time (i.e., with an increase in illumination intensity), only the number of electrons ejected from the metal will increase, which will lead to an increase in the photocurrent, but the energy of each electron will not increase. If the photon energy is less than the minimum energy required to eject an electron, the photoelectric effect will not be observed for any number of photons incident on the metal, i.e. at any lighting intensity.
Quantum theory of light, developed Einstein, was able to explain not only the properties of the photoelectric effect, but also the patterns of the chemical action of light, the temperature dependence of the heat capacity of solids and a number of other phenomena. It turned out to be extremely useful in the development of ideas about the structure of atoms and molecules.
From the quantum theory of light it follows that the photon is incapable of fragmentation: it interacts as a whole with the electron of the metal, knocking it out of the plate; as a whole, it interacts with the light-sensitive substance of the photographic film, causing it to darken at a certain point, etc. In this sense, the photon behaves like a particle, i.e. exhibits corpuscular properties. However, the photon also has wave properties: this is manifested in the wave nature of the propagation of light, in the photon’s ability to interfere and diffraction. A photon differs from a particle in the classical sense of the term in that its exact position in space, like the exact position of any wave, cannot be specified. But it also differs from the “classical” wave in its inability to divide into parts. Combining corpuscular and wave properties, the photon is, strictly speaking, neither a particle nor a wave - it is characterized by corpuscular-wave duality.
The first information about the complex atomic structure were obtained by studying the processes of passage electric current through liquids. In the thirties of the XIX century. The experiments of the outstanding physicist M. Faraday suggested that electricity exists in the form of separate unit charges.
The discovery of the spontaneous decay of atoms of some elements, called radioactivity, became direct evidence of the complexity of the structure of the atom. In 1902, English scientists Ernest Rutherford and Frederick Soddy proved that during radioactive decay, a uranium atom turns into two atoms - a thorium atom and a helium atom. This meant that atoms were not immutable, indestructible particles.
Rutherford's atomic model
By studying the passage of a narrow beam of alpha particles through thin layers of matter, Rutherford discovered that most alpha particles pass through a metal foil consisting of many thousands of layers of atoms without deviating from the original direction, without experiencing scattering, as if there were no objects in their path no obstacles. However, some particles were deflected at large angles, experiencing the action of large forces.
Based on the results of experiments on observing the scattering of alpha particles in matter Rutherford proposed a planetary model of the structure of the atom. According to this model The structure of the atom is similar to the structure of the solar system. At the center of every atom there is positively charged nucleus radius ≈ 10 -10 m like planets orbit negatively charged electrons. Almost all the mass is concentrated in the atomic nucleus. Alpha particles can pass through thousands of layers of atoms without scattering because most of the space inside atoms is empty, and collisions with light electrons have little effect on the movement of a heavy alpha particle. Alpha particles are scattered during collisions with atomic nuclei.
Rutherford's atomic model could not explain all the properties of atoms.
According to the laws of classical physics, an atom from a positively charged nucleus and electrons revolving in circular orbits should emit electromagnetic waves. The emission of electromagnetic waves should lead to a decrease in the potential energy reserve in the nucleus-electron system, to a gradual decrease in the radius of the electron’s orbit and the electron’s fall onto the nucleus. However, atoms usually do not emit electromagnetic waves, electrons do not fall on atomic nuclei, that is, atoms are stable.
Quantum postulates of N. Bohr
To explain the stability of atoms Niels Bohr proposed to abandon the usual classical concepts and laws when explaining the properties of atoms.
The basic properties of atoms receive a consistent qualitative explanation based on the acceptance quantum postulates of N. Bohr.
1. The electron rotates around the nucleus only in strictly defined (stationary) circular orbits.
2. An atomic system can only be in certain stationary or quantum states, each of which corresponds to a certain energy E. An atom does not emit energy in stationary states.
Stationary state of the atom with minimal energy reserve is called underlying condition, all other states are called excited (quantum) states. An atom can remain in the ground state for an infinitely long time; the lifetime of an atom in an excited state lasts 10 -9 -10 -7 seconds.
3. Emission or absorption of energy occurs only when an atom transitions from one stationary state to another. Energy of a quantum of electromagnetic radiation during transition from a stationary state with energy E m into a state of energy E n equal to the difference between the energies of an atom in two quantum states:
∆E = E m – E n = hv,
Where v– radiation frequency, h= 2ph = 6.62 ∙ 10 -34 J ∙s.
Quantum model of atomic structure
Subsequently, some provisions of N. Bohr's theory were supplemented and rethought. The most significant change was the introduction of the concept of an electron cloud, which replaced the concept of the electron only as a particle. Later, Bohr's theory was replaced by quantum theory, which takes into account the wave properties of the electron and other elementary particles, forming an atom.
basis modern theory atomic structure is a planetary model, supplemented and improved. According to this theory, the nucleus of an atom consists of protons (positively charged particles) and neurons (particles without a charge). And around the nucleus electrons (negatively charged particles) move along uncertain trajectories.
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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 imperfections, the Thomson model made it possible to explain the phenomena of emission, absorption and scattering of light by atoms, as well as to establish the number of electrons in the atoms of light elements.
Rice. 1. Atom, according to Thomson's model. Electrons are held inside a positively charged sphere by elastic forces. Those of them that are on the surface can easily be “knocked 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 atomic structure 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 force of Coulomb attraction is equal to the centrifugal force. However, in accordance with the theory of the electromagnetic field, electrons in this case should move in a spiral, continuously emitting energy, and fall onto the nucleus. However, the atom is stable.
In addition, with continuous radiation of energy, the atom must exhibit 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 atomic structure, without completely denying previous ideas. Bohr based his theory on two postulates.
The first postulate says that an electron can rotate around the nucleus only in certain stationary orbits. While on them, it does not emit 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 reserve of the electron (E 1, E 2 ...) remains constant. The closer the orbit is to the nucleus, the less energy reserve of the electron E 1 ˂ E 2 …˂ E n . The electron energy in orbits is determined by the equation:
where m is the electron mass, h is Planck’s constant, n – 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, irradiation, etc.), then the electron can absorb a quantum of energy and move to an orbit more distant from the nucleus (Fig. 3). In this case, we speak of an excited state of the atom. During the reverse transition of the electron (to an orbit closer to the nucleus), energy is released in the form of a quantum of radiant energy - a photon. This is indicated by a specific line in the spectrum. Based on 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 the 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 state. Electron transitions to the 1st orbit form a frequency group of the Lyman series, to the 2nd – the Balmer series, and 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 Bohr's formula for hydrogen spectrum series
However, Bohr's theory could not 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 have accumulated indicating that the electron is also capable of exhibiting wave properties. Classical mechanics was unable to explain the movement of micro-objects that simultaneously possess 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 movement and interaction of microparticles with very low mass (Table 2).
table 2
Properties of elementary particles that form an atom