The mass of electrons is several thousand times less than the mass of atoms. Since the atom as a whole is neutral, therefore, the bulk of the atom falls on its positively charged part.

For an experimental study of the distribution of a positive charge, and hence the mass inside the atom, Rutherford proposed in 1906 to apply the probing of the atom using α -particles. These particles arise from the decay of radium and some other elements. Their mass is about 8000 times the mass of the electron, and the positive charge is equal in modulus to twice the charge of the electron. These are nothing but fully ionized helium atoms. Speed α -particles is very large: it is 1/15 of the speed of light.

With these particles, Rutherford bombarded the atoms of heavy elements. Electrons, due to their small mass, cannot noticeably change the trajectory α -particles, like a pebble of several tens of grams in a collision with a car, are not able to noticeably change its speed. Scattering (changing direction of movement) α -particles can cause only the positively charged part of the atom. Thus, by scattering α -particles can determine the nature of the distribution of positive charge and mass inside the atom.

A radioactive preparation, such as radium, was placed inside lead cylinder 1, along which a narrow channel was drilled. bundle α -particles from the channel fell on thin foil 2 of the material under study (gold, copper, etc.). After scattering α -particles fell on a translucent screen 3 coated with zinc sulfide. The collision of each particle with the screen was accompanied by a flash of light (scintillation), which could be observed in a microscope 4. The entire device was placed in a vessel from which the air was evacuated.

With a good vacuum inside the device, in the absence of foil, a bright circle appeared on the screen, consisting of scintillations caused by a thin beam α -particles. But when foil was placed in the path of the beam, α -particles due to scattering were distributed on the screen in a circle of a larger area. Modifying the experimental setup, Rutherford tried to detect the deviation α -particles at large angles. Quite unexpectedly, it turned out that a small number α -particles (about one in two thousand) deviated at angles greater than 90°. Later, Rutherford admitted that, having offered his students an experiment to observe the scattering α -particles at large angles, he himself did not believe in a positive result. "It's almost as incredible," Rutherford said, "as if you fired a 15-inch projectile at a piece of thin paper, and the projectile came back to you and hit you." Indeed, it was impossible to predict this result on the basis of the Thomson model. When distributed throughout the atom, a positive charge cannot create a sufficiently intense electric field capable of throwing the a-particle back. The maximum repulsive force is determined by Coulomb's law:

where q α - charge α -particles; q is the positive charge of the atom; r is its radius; k - coefficient of proportionality. The electric field strength of a uniformly charged ball is maximum on the surface of the ball and decreases to zero as it approaches the center. Therefore, the smaller the radius r, the greater the repulsive force α -particles.

Determining the size of the atomic nucleus. Rutherford realized that α -particle could be thrown back only if the positive charge of the atom and its mass are concentrated in a very small region of space. So Rutherford came up with the idea of ​​the atomic nucleus - a body of small size, in which almost all the mass and all the positive charge of the atom are concentrated.

Planetary model of the atom, or Rutherford model, - the historical model of the structure of the atom, which was proposed by Ernest Rutherford as a result of an experiment with the scattering of alpha particles. 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 the 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 motion of electrons is of a quantum nature and is not described by the laws of classical mechanics. Historically, Rutherford's planetary model replaced Joseph John Thomson's "plum pudding model", which postulates that negatively charged electrons are placed inside a positively charged atom.

The first information about the complex the structure of the atom were obtained in the study of the processes of passage of 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, was 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 are not immutable, indestructible particles.

Rutherford model of the atom

Investigating the passage of a narrow beam of alpha particles through thin layers of matter, Rutherford found 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 obstacles in their path. no obstacles. However, some particles were deflected at large angles, having experienced the action of large forces.

Based on the results of experiments to observe 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 each atom is positively charged nucleus with a radius of ≈ 10 -10 m, like planets, they circulate 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, since most of the space inside atoms is empty, and collisions with light electrons have almost no effect on the motion of a heavy alpha particle. Scattering of alpha particles occurs in collisions with atomic nuclei.

Rutherford's model of the atom failed to explain all the properties of atoms.

According to the laws of classical physics, an atom consisting of a positively charged nucleus and electrons in circular orbits must radiate electromagnetic waves. The radiation of electromagnetic waves should lead to a decrease in the potential energy in the nucleus-electron system, to a gradual decrease in the radius of the electron orbit and the fall of the electron 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 ideas and laws when explaining the properties of atoms.

The basic properties of atoms receive a consistent qualitative explanation based on the adoption quantum postulates of N. Bohr.

1. The electron revolves 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 radiate energy in stationary states.

Stationary state of the atom with the minimum amount of energy is called main state, all other states are called excited (quantum) states. In the ground state, an atom can be infinitely long, 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 passes from one stationary state to another. The energy of an electromagnetic radiation quantum during the transition from a stationary state with energy E m into a state of energy E n is equal to the difference between the energies of an atom in two quantum states:

∆E = E m – E n = hv,

where v is the radiation frequency, h\u003d 2ph \u003d 6.62 ∙ 10 -34 J ∙ s.

Quantum model of the structure of the atom

In the future, 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 an 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 that form the atom.

basis modern theory of the structure of the atom is a planetary model, supplemented and improved. According to this theory, the nucleus of an atom consists of protons (positively charged particles) and neurons (uncharged particles). And around the nucleus, electrons (negatively charged particles) move along indefinite trajectories.

Do you have any questions? Do you want to know more about atomic structure models?
To get the help of a tutor - register.
The first lesson is free!

site, with full or partial copying of the material, a link to the source is required.

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 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 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 provided an explanation of the phenomenon of radioactivity that won him a 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 at high speeds, resulting in centrifugal forces that balance the electrostatic forces of attraction between protons and electrons.

Rutherford's experiments

The planetary model of the 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 the discovery of the atomic nucleus is a good example of 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 can produce sheets as thin as 0.00004 cm. Behind a 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, it is used to 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 with more energy will be found at higher levels, farther 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 different ideas: Rutherford's atomic model with electrons revolving around the nucleus (essentially the planetary Bohr-Rutherford model of the atom), and Max Planck's idea of ​​quantizing the energy of matter, published in 1901. A quantum (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.

Lecture: Planetary model of the atom

The structure of the atom

The most accurate way to determine the structure of any substance is spectral analysis. The radiation of each atom of an element is exclusively individual. However, before understanding how spectral analysis occurs, let's figure out what structure an atom of any element has.

The first assumption about the structure of the atom was presented by J. Thomson. This scientist has been studying atoms for a long time. Moreover, it is he who owns the discovery of the electron - for which he received the Nobel Prize. The model that Thomson proposed had nothing to do with reality, but served as a strong enough incentive for Rutherford to study the structure of the atom. The model proposed by Thomson was called "raisin pudding".

Thomson believed that the atom is a solid ball with a negative electrical charge. To compensate for it, electrons are interspersed in the ball, like raisins. In sum, the charge of the electrons coincides with the charge of the entire nucleus, which makes the atom neutral.

During the study of the structure of the atom, it was found out that all atoms in solids make oscillatory motions. And, as you know, any moving particle radiates waves. That is why each atom has its own spectrum. However, these statements did not fit into the Thomson model in any way.

Rutherford's experience

To confirm or disprove Thomson's model, Rutherford proposed an experiment that resulted in the bombardment of an atom of some element by alpha particles. As a result of this experiment, it was important to see how the particle would behave.

Alpha particles were discovered as a result of the radioactive decay of radium. Their streams were alpha rays, each particle of which had a positive charge. As a result of numerous studies, it was determined that the alpha particle is like a helium atom, in which there are no electrons. Using current knowledge, we know that the alpha particle is the nucleus of helium, while Rutherford believed that these were helium ions.

Each alpha particle had tremendous energy, as a result of which it could fly at the atoms in question at high speed. Therefore, the main result of the experiment was to determine the particle deflection angle.

For the experiment, Rutherford used thin gold foil. He directed high-speed alpha particles at it. He assumed that as a result of this experiment, all particles would fly through the foil, and with small deviations. However, in order to find out for sure, he instructed his students to check if there were any large deviations in these particles.

The result of the experiment surprised absolutely everyone, because many particles not only deviated by a sufficiently large angle - some deflection angles reached more than 90 degrees.

These results surprised absolutely everyone, Rutherford said that it felt like a piece of paper was placed in the path of the projectiles, which did not allow the alpha particle to penetrate inside, as a result of which it turned back.

If the atom were really solid, then it should have some electric field, which slowed down the particle. However, the strength of the field was not enough to stop her completely, let alone push her back. This means that Thomson's model was refuted. So Rutherford started working on a new model.

To get this result of the experiment, it is necessary to concentrate the positive charge in a smaller amount, resulting in a larger electric field. Using the field potential formula, you can determine the required size of a positive particle that could repel an alpha particle in the opposite direction. Its radius should be of the order of maximum 10 -15 m. That is why Rutherford proposed the planetary model of the atom.

This model is named so for a reason. The fact is that inside the atom there is a positively charged nucleus, similar to the Sun in the solar system. Electrons revolve around the nucleus like planets. The solar system is arranged in such a way that the planets are attracted to the Sun with the help of gravitational forces, however, they do not fall on the surface of the Sun as a result of the available speed that keeps them in their orbit. The same thing happens with electrons - Coulomb forces attract electrons to the nucleus, but due to rotation, they do not fall on the surface of the nucleus.

One assumption of Thomson turned out to be absolutely correct - the total charge of electrons corresponds to the charge of the nucleus. However, as a result of a strong interaction, electrons can be knocked out of their orbit, as a result of which the charge is not compensated and the atom turns into a positively charged ion.

Very important information regarding the structure of the atom is that almost all the mass of the atom is concentrated in the nucleus. For example, a hydrogen atom has only one electron, whose mass is more than one and a half thousand times less than the mass of the nucleus.

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. To this day, no better model has been proposed, 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 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.