Nuclear planetary model of the structure of the atom definition. Planetary model of the atom

The planetary model of the atom was proposed by E. Rutherford in 1910. The first studies of the structure of the atom were made by him with the help of alpha particles. Based on the results obtained in experiments on their scattering, Rutherford suggested that all the positive charge of the atom is concentrated in a tiny nucleus at its center. On the other hand, negatively charged electrons are distributed throughout the rest of its volume.

A little background

The first brilliant guess about the existence of atoms was made by the ancient Greek scientist Democritus. Since then, the idea of ​​the existence of atoms, the combinations of which give all the substances around us, has not left the imagination of the people of science. From time to time, various representatives of it turned to it, but until the beginning of the 19th century, their constructions were just hypotheses, not supported by experimental data.

Finally, in 1804, more than a hundred years before the planetary model of the atom appeared, the English scientist John Dalton provided evidence for its existence and introduced the concept of atomic weight, which was its first quantitative characteristic. Like his predecessors, he conceived of atoms as the smallest pieces of matter, like solid balls, which cannot be divided into even smaller particles.

Discovery of the electron and the first model of the atom

Almost a century had passed when, finally, at the end of the 19th century, also the Englishman J. J. Thomson, discovered the first subatomic particle, the negatively charged electron. Since atoms are electrically neutral, Thomson thought they must be composed of a positively charged nucleus with electrons scattered throughout its volume. Based on various experimental results, in 1898 he proposed his model of the atom, sometimes called "plums in a pudding", because the atom in it was represented as a sphere filled with some positively charged liquid, into which electrons were embedded, as "plums into the pudding. The radius of such a spherical model was about 10 -8 cm. The total positive charge of the liquid is symmetrically and uniformly balanced by the negative charges of the electrons, as shown in the figure below.

This model satisfactorily explained the fact that when a substance is heated, it begins to emit light. Although this was the first attempt to understand what an atom was, it failed to satisfy the results of the experiments carried out later by Rutherford and others. Thomson agreed in 1911 that his model simply could not answer how and why the scattering of α-rays observed in experiments occurs. Therefore, it was abandoned, and it was replaced by a more perfect planetary model of the atom.

How is the atom arranged anyway?

Ernest Rutherford gave an explanation of the phenomenon of radioactivity, which brought him Nobel Prize, but his most significant contribution to science came later, when he established that the atom consists of a dense nucleus surrounded by orbits of electrons, just as the Sun is surrounded by the orbits of planets.

According to the planetary model of an atom, most of its mass is concentrated in a tiny (compared to the size of the entire atom) nucleus. Electrons move around the nucleus, traveling at incredible speeds, but most of the volume of atoms is empty space.

The size of the nucleus is so small that its diameter is 100,000 times smaller than that of an atom. The diameter of the nucleus was estimated by Rutherford as 10 -13 cm, in contrast to the size of the atom - 10-8 cm. Outside the nucleus, electrons revolve around it with high speeds, resulting in centrifugal forces that balance the electrostatic forces of attraction between protons and electrons.

Rutherford's experiments

planetary model atom arose in 1911, after the famous experiment with gold foil, which made it possible to obtain some fundamental information about its structure. Rutherford's path to discovery atomic nucleus is an good example the role of creativity in science. His search began as early as 1899, when he discovered that certain elements emit positively charged particles that can penetrate anything. He called these particles alpha (α) particles (now we know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered if alpha particles could be used to find out the structure of an atom. Rutherford decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it could produce sheets as thin as 0.00004 cm. Behind the sheet of gold foil, he placed a screen that glowed when alpha particles hit it. It was used to detect alpha particles after they had passed through the foil. A small slit in the screen allowed the alpha particle beam to reach the foil after exiting the source. Some of them must pass through the foil and continue moving in the same direction, while the other part must bounce off the foil and be reflected at sharp angles. You can see the scheme of the experiment in the figure below.

What happened in Rutherford's experiment?

Based on J. J. Thomson's model of the atom, Rutherford assumed that the solid regions of positive charge filling the entire volume of gold atoms would deviate or bend the trajectories of all alpha particles as they passed through the foil.

However, the vast majority of the alpha particles passed right through the gold foil as if it wasn't there. They seemed to be passing through empty space. Only a few of them deviate from the straight path, as it was supposed at the beginning. Below is a plot of the number of particles scattered in the respective direction versus the scattering angle.

Surprisingly, a tiny percentage of the particles bounced back from the foil, like a basketball bouncing off a backboard. Rutherford realized that these deviations were the result of a direct collision between alpha particles and the positively charged components of the atom.

The nucleus takes center stage

Based on the negligible percentage of alpha particles reflected from the foil, we can conclude that all the positive charge and almost all the mass of the atom are concentrated in one small area, and the rest of the atom is mostly empty space. Rutherford called the area of ​​concentrated positive charge the nucleus. He predicted and soon discovered that it contained positively charged particles, which he named protons. Rutherford predicted the existence of neutral atomic particles called neutrons, but he failed to detect them. However, his student James Chadwick discovered them a few years later. The figure below shows the structure of the nucleus of a uranium atom.

Atoms consist of positively charged heavy nuclei surrounded by negatively charged extremely light particles-electrons rotating around them, and at such speeds that mechanical centrifugal forces simply balance their electrostatic attraction to the nucleus, and in this connection the stability of the atom is allegedly ensured.

The disadvantages of this model

Rutherford's main idea was related to the idea of ​​a small atomic nucleus. The assumption about the orbits of the electrons was pure conjecture. He did not know exactly where and how electrons revolve around the nucleus. Therefore, Rutherford's planetary model does not explain the distribution of electrons in orbits.

In addition, the stability of the Rutherford atom was possible only with the continuous movement of electrons in orbits without loss of kinetic energy. But electrodynamic calculations have shown that the movement of electrons along any curvilinear trajectories, accompanied by a change in the direction of the velocity vector and the appearance of a corresponding acceleration, is inevitably accompanied by the emission of electromagnetic energy. In this case, according to the law of conservation of energy, the kinetic energy of the electron must be very quickly spent on radiation, and it must fall on the nucleus, as shown schematically in the figure below.

But this does not happen, since atoms are stable formations. A typical scientific contradiction arose between the model of the phenomenon and the experimental data.

From Rutherford to Niels Bohr

The next major step forward in atomic history came in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. She determined more clearly the places where electrons could be. Although later scientists would develop more sophisticated atomic designs, Bohr's planetary model of the atom was basically correct, and much of it is still accepted today. It had many useful applications, for example, with its help they explain the properties of various chemical elements, the nature of their radiation spectrum and the structure of the atom. The planetary model and the Bohr model were the most important milestones that marked the emergence of a new direction in physics - the physics of the microworld. Bohr received the 1922 Nobel Prize in Physics for his contributions to our understanding of the structure of the atom.

What new did Bohr bring to the model of the atom?

While still a young man, Bohr worked in Rutherford's laboratory in England. Since the concept of electrons was poorly developed in Rutherford's model, Bohr focused on them. As a result, the planetary model of the atom was significantly improved. Bohr's postulates, which he formulated in his article "On the Structure of Atoms and Molecules", published in 1913, read:

1. Electrons can move around the nucleus only at fixed distances from it, determined by the amount of energy they have. He called these fixed levels energy levels or electron shells. Bohr envisioned them as concentric spheres, with a nucleus at the center of each. In this case, electrons with lower energy will be found at lower levels, closer to the nucleus. Those who have more energy will be found on more high levels, away from the core.

2. If an electron absorbs some (quite certain for a given level) amount of energy, then it will jump to the next, higher energy level. Conversely, if he loses the same amount of energy, he will return back to his original level. However, an electron cannot exist on two energy levels.

This idea is illustrated by a figure.

Energy portions for electrons

The Bohr model of the atom is actually a combination of two various ideas: the Rutherford atomic model with electrons revolving around the nucleus (in fact, this is the Bohr-Rutherford planetary model of the atom), and the ideas of the German scientist Max Planck on the quantization of the energy of matter, published in 1901. A quantum (in plural- quanta) is the minimum amount of energy that can be absorbed or emitted by a substance. It is a kind of discretization step for the amount of energy.

If energy is compared to water and you want to add it to matter in the form of a glass, you cannot just pour water in a continuous stream. Instead, you can add it in small amounts, like a teaspoonful. Bohr believed that if electrons can only absorb or lose fixed amounts of energy, then they should only vary their energy by these fixed amounts. Thus, they can only occupy fixed energy levels around the nucleus that correspond to quantized increments of their energy.

So from the Bohr model grows a quantum approach to explaining what the structure of the atom is. The planetary model and the Bohr model were a kind of steps from classical physics to quantum physics, which is the main tool in the physics of the microcosm, including atomic physics.

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 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 the entire 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. With this scattering, unexpectedly a 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 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 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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One of the first models of the structure of the atom was proposed J. Thomson in 1904, the Atom was presented as a "sea of ​​positive electricity" with electrons oscillating in it. The total negative charge of the electrons of an electrically neutral atom was equated 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 thin metal plates - foil. In 1910 Rutherford students Hans Geiger And Ernest Marsden carried out 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.

A source α - radiation was placed in a lead cube with a channel drilled in it, so that it was possible to obtain a flow α -particles flying in a certain direction. Alpha particles are doubly ionized helium atoms ( Not 2+). They have a positive charge of +2 and a mass almost 7350 times the mass of an electron. Hitting a screen coated with zinc sulfide, α -particles caused it to glow, and with a magnifying glass one could see and count individual flashes that appear on the screen when each α -particles. A foil was placed between the radiation source and the screen. From the flashes on the screen it was possible to judge the scattering α -particles, i.e. about their deviation from the original direction when passing through the metal layer.

It turned out that the majority α -particles pass through the foil without changing its direction, although the thickness of the foil corresponded to hundreds of thousands of atomic diameters. But some share α -particles still deviated by 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 in an arc.

From the results of this experiment, the following conclusions could be drawn:

1. There is some "obstacle" in the atom, which has been called the nucleus.
2. The nucleus has a positive charge (otherwise positively charged α particles would not be reflected back).
3. The nucleus is very small compared to the size of the atom itself (only a small part α -particles changed direction).
4. The nucleus has more mass than the mass α -particles.

Rutherford explained the results of the experiment by proposing "planetary" model of the atom likened it to the solar system. According to the planetary model, in 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 outer trajectory of the electrons determines the outer dimensions of the atom. The diameter of an atom is about 10 -8 cm, and the diameter of the nucleus is about 10 -13 ÷10 -12 cm.

The greater the charge of the atomic nucleus, the stronger will be repelled from it α -particle, the more often there will be cases of strong deviations α -particles passing through the metal layer, from the original 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. It already followed from Rutherford's experiments that the charge of the nucleus (expressed in units of the electron charge) is numerically equal to the ordinal number of the element in the periodic system. It's been confirmed G. Moseley, who in 1913 established a simple relationship between the wavelengths of certain lines of the X-ray spectrum of an element and its serial 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 the serial number of the element in the periodic system: the serial number turned out to be the most important constant of the element, expressing the positive charge of the nucleus of its atom. From the electrical neutrality of the atom, it follows that the number of electrons revolving around the nucleus is equal to the ordinal number of the element.

This discovery gave a new justification for the arrangement of elements in the periodic system. At the same time, it eliminated the apparent contradiction in Mendeleev's system - the position of some elements with a larger atomic mass ahead of elements with a 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 charge of the nucleus of the tellurium atom is 52, and that of the iodine atom is 53; therefore tellurium, despite the large atomic mass, must stand up to iodine. Similarly, 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 main quantity on which the properties of the element and its position in the periodic system depend. That's why periodic law Mendeleev can currently be formulated as follows:

Properties of elements and the simple and complex substances are in a periodic dependence on the charge of the nucleus of the atoms of the elements

Determining the serial numbers of elements by the charges of the nuclei of their atoms made it possible to establish total number places in the periodic system between hydrogen, which has serial number 1, and uranium (atomic number 92), which was considered at that time the last member of the periodic system of elements. When the theory of the structure of the atom was created, places 43, 61, 72, 75, 85 and 87 remained unoccupied, which indicated the possibility of the existence of yet undiscovered elements. And indeed, in 1922, the element hafnium was discovered, which took the place of 72; then in 1925 - rhenium, which took place 75. The elements that should occupy the remaining four free 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 (number 43), promethium (61), astatine (85), and francium (87). At present, all cells of the periodic system between hydrogen and uranium are filled. However, she periodic system is not completed.

Atomic spectra

The planetary model was a major step in the theory of the structure of the atom. However, in some respects it contradicted well-established facts. Let's consider two such contradictions.

First, Rutherford's theory could not explain the stability of the atom. An electron revolving around a positively charged nucleus must, like an oscillating electric charge, emit electromagnetic energy in the form of light waves. But, emitting light, the electron loses some of its energy, which leads to an imbalance between the centrifugal force associated with the rotation of the electron, and the force of the electrostatic attraction of the electron to the nucleus. To restore equilibrium, the electron must move closer to the nucleus. Thus, the electron, continuously radiating 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 real properties atoms, which are stable formations, and can exist without being destroyed 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 band 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 unequally refracted by a prism and fall on different places screen. Frequency constellation electromagnetic radiation, emitted by the substance, and is called the emission spectrum. On the other hand, substances absorb radiation of certain frequencies. The totality of the latter is called the absorption spectrum of a substance.

To obtain a spectrum, instead of a prism, you can use a diffraction grating. 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 the radiation has one wavelength, then it is called monochromatic. In most cases, the 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 by spectral lines.

The spectra obtained by radiation 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, in contrast to radiation solids and liquids, contains only certain wavelengths. Therefore, instead of a continuous strip on the screen, a series of separate colored lines separated by dark gaps is obtained. The number and location of these lines depend on the nature of the hot gas or vapor. So, potassium vapor gives - a spectrum consisting of three lines - two red and one violet; there are several red, yellow and green lines in the spectrum of calcium vapors, etc.

Radiation emitted by solids or liquids always gives a continuous spectrum. The radiation emitted by hot gases and vapors, in contrast to the radiation of solids and liquids, contains only certain wavelengths. Therefore, instead of a continuous strip on the screen, a series of separate colored lines separated by dark gaps is obtained. The number and location of these lines depend on the nature of the hot gas or vapor. So, potassium vapor gives a spectrum consisting of three lines - two red and one violet; there are several red, yellow and green lines in the spectrum of calcium vapors, etc.

Such spectra are called line spectra. It was found that the light emitted by the atoms of gases has a line spectrum, in which the spectral lines can be combined in 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 the higher, the more content element in a sample is widely used to determine the qualitative and quantitative composition of 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 emission spectrum of hydrogen atoms, and even more so the combination of spectral lines in a series. An electron revolving around the nucleus must approach the nucleus, continuously changing the speed of its movement. The frequency of the light emitted by it is determined by the frequency of its rotation and, therefore, must be continuously changing. This means that the radiation spectrum of an atom must be continuous, continuous. According to this model, the radiation frequency of an atom must be equal to the mechanical vibration frequency or be a multiple of it, which is inconsistent with the Balmer 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. Plank showed that the ability of a heated body to emit radiation can be correctly quantitatively described 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 frequency of radiation by a relation called Planck's equations:

Planck himself long time believed that the emission and absorption of light by quanta is a property of radiating bodies, and not of 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 Planck's equation.

photoelectric effect The emission of electrons by a metal under the action of light incident on it is called. This phenomenon was studied in detail in 1888-1890. A. G. Stoletov. If you place the setup in a vacuum and apply to the plate M negative potential, then no current will be observed in the circuit, since there are no charged particles in the space between the plate and the grid that can carry electricity. But when the plate is illuminated with a light source, the galvanometer detects the occurrence of a current (called a photocurrent), the carriers of which are the electrons pulled out by light from the metal.

It turned out that when the light 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 the wavelength (i.e. with a decrease in frequency) that the energy of the electrons emitted by the metal decreases, and then, at a wavelength determined for each metal, the photoelectric effect disappears and does not appear even at very high light intensity. So, when illuminated with red or orange light, sodium does not show 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 lower wavelengths, starting from 516 nm ( green light); and pulling out electrons from platinum under the action 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 by the 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 separate portions, photons, with a well-defined energy.

In fact, an electron in a metal is bound to the atoms of the metal, so that a certain amount of energy must be expended to pull it out. If the photon has the required amount of energy (and the energy of the photon is determined by the frequency of radiation), then the electron will be ejected, and the photoelectric effect will be observed. In the process of interaction with the 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 partly spent on breaking the bond between the electron and the metal, and partly on imparting the 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 an electron with 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 the 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 light intensity.

quantum theory of light, developed Einstein, was able to explain not only the properties of the photoelectric effect, but also the laws of the chemical action of light, temperature dependence 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.

It follows from the quantum theory of light that a photon is unable to break up: it interacts as a whole with a metal electron, knocking it out of the plate; as a whole, it also interacts with the light-sensitive substance of the photographic film, causing it to darken at a certain point, and so on. 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 ability of the photon 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 - the inability to divide into parts. Combining corpuscular and wave properties, the photon is, strictly speaking, neither a particle nor a wave - it has a corpuscular-wave duality.

Details Category: Physics of the atom and atomic nucleus Posted on 03/10/2016 18:27 Views: 4106

Ancient Greek and ancient Indian scientists and philosophers believed that all the substances around us consist of tiny particles that do not divide.

They were sure that there was nothing in the world that would be smaller than these particles, which they called atoms . And, indeed, later the existence of atoms was proved by such famous scientists as Antoine Lavoisier, Mikhail Lomonosov, John Dalton. The atom was considered indivisible up to late XIX- the beginning of the twentieth century, when it turned out that this was not so.

The discovery of the electron. Thomson model of the atom

Joseph John Thomson

In 1897, the English physicist Joseph John Thomson, studying experimentally the behavior of cathode rays in magnetic and electric fields, found out that these rays are a stream of negatively charged particles. The speed of movement of these particles was below the speed of light. Therefore, they had mass. Where did they come from? The scientist suggested that these particles are part of the atom. He called them corpuscles . Later they were called electrons . Thus the discovery of the electron put an end to the theory of the indivisibility of the atom.

Thomson model of the atom

Thomson proposed the first electronic model atom. According to it, an atom is a sphere, inside of which there is a charged substance, the positive charge of which is evenly distributed throughout the volume. And in this substance, like raisins in a bun, electrons are interspersed. In general, the atom is electrically neutral. This model was called the "plum pudding model".

But Thomson's model turned out to be wrong, which was proven British physicist Sir Ernest Rutherford.

Rutherford's experience

Ernest Rutherford

How is an atom actually arranged? Rutherford gave an answer to this question after his experiment, carried out in 1909 together with the German physicist Hans Geiger and the New Zealand physicist Ernst Marsden.

Rutherford's experience

The purpose of the experiment was to study the atom with the help of alpha particles, a focused beam of which, flying at great speed, was directed to the thinnest gold foil. Behind the foil was a luminescent screen. When particles collided with it, flashes appeared that could be observed under a microscope.

If Thomson is right, and the atom is made up of a cloud of electrons, then the particles should easily fly through the foil without being deflected. Since the mass of the alpha particle exceeded the mass of the electron by about 8000 times, the electron could not act on it and deviate its trajectory at a large angle, just as a 10 g pebble could not change the trajectory of a moving car.

But in practice, everything turned out differently. Most of the particles actually flew through the foil, practically not deviating or deviating by a small angle. But some of the particles deviated quite significantly or even bounced back, as if there was some kind of obstacle in their path. As Rutherford himself said, it was as incredible as if a 15-inch projectile bounced off a piece of tissue paper.

What caused some alpha particles to change direction so much? The scientist suggested that the reason for this was a part of the atom, concentrated in a very small volume and having a positive charge. He named her the nucleus of an atom.

Rutherford's planetary model of the atom

Rutherford model of the atom

Rutherford came to the conclusion that the atom consists of a dense positively charged nucleus located in the center of the atom and electrons that have a negative charge. Almost all the mass of an atom is concentrated in the nucleus. In general, the atom is neutral. The positive charge of the nucleus is equal to the sum of the negative charges of all the electrons in the atom. But the electrons are not embedded in the nucleus, as in Thomson's model, but revolve around it like the planets revolve around the sun. The rotation of electrons occurs under the action of the Coulomb force acting on them from the nucleus. The speed of rotation of electrons is enormous. Above the surface of the core, they form a kind of cloud. Each atom has its own electron cloud, negatively charged. For this reason, they do not "stick together", but repel each other.

Due to its resemblance to solar system Rutherford's model was called planetary.

Why does the atom exist

However, Rutherford's model of the atom failed to explain why the atom is so stable. Indeed, according to the laws of classical physics, an electron, rotating in orbit, moves with acceleration, therefore, it radiates electromagnetic waves and loses energy. In the end, this energy must run out, and the electron must fall into the nucleus. If this were the case, the atom could only exist for 10 -8 s. But why isn't this happening?

The reason for this phenomenon was later explained by the Danish physicist Niels Bohr. He suggested that the electrons in an atom move only in fixed orbits, which are called "allowed orbits". Being on them, they do not radiate energy. And the emission or absorption of energy occurs only when an electron moves from one allowed orbit to another. If this is a transition from a distant orbit to one closer to the nucleus, then energy is radiated, and vice versa. The radiation occurs in portions, which are called quanta.

Although the model described by Rutherford could not explain the stability of the atom, it allowed significant progress in the study of its structure.

In 1903, the English scientist Thomson proposed a model of the atom, which was jokingly called the "bun with raisins." According to him, 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 recognized worldwide.

Rutherford's experience: the proposal of his model of the atom

In his experiments, Rutherford passed a beam of alpha particles through thin gold foil. Gold was chosen for its plasticity, which made it possible to create a very thin foil, almost one layer of molecules thick. Behind the foil was a special screen that was illuminated when bombarded by alpha particles falling on it. According to Thomson's theory, alpha particles should have passed through the foil unhindered, deviating quite a bit to the sides. However, it turned out that some of the particles behaved in this way, and a very small part bounced back, as if hitting something.

That is, it was found that inside the atom there is something solid and small, from which alpha particles bounced off. 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. Not offered to date best model, although some aspects of this theory are still not consistent with practice in some very narrow areas of science. But basically, the planetary model of the atom is the most useful of all. What is this model?

Planetary model of the structure of the atom

As the name implies, an atom is compared to a planet. In this case, the planet is the nucleus of an atom. And electrons revolve around the nucleus at a fairly large distance, just like satellites revolve around the planet. Only the speed of rotation of electrons is hundreds of thousands of times greater than the speed of rotation of the fastest satellite. Therefore, during its rotation, the electron creates, as it were, a 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 a certain distance from each other.

And when we talk about the collision of particles, we mean that they approach each other at a sufficiently large distance and are repelled by the fields of their charges. There is no direct contact. Particles in matter are generally very far apart. If by any means it were possible to implode together the particles of any body, it would be reduced by a billion 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 forces of interaction.