The existence of an almost point-like, but very heavy, positively charged nucleus in an atom was proven by the English physicist Ernest Rutherforth House.

In 1906-1912. he studied the passage of α-particles with energies of several MeV through thin plates (foils) of gold and other metals. Most of the particles flew through the foil, practically without changing the direction of their movement. But some of them sharply deviated from their path. With a foil thickness of 1 micron, on average, only 1 in 10,000 particles were deflected by an angle greater than 90°. This seemed quite strange, since, flying through the foil, an alpha particle must pass by several thousand atoms.

Such rare interactions forced Rutherford assume that the mass in the substance is not distributed evenly, but in the form of separate, very small clumps. The majority of particles fly between these clumps, and only those that fall into them are scattered. Since the atoms in a solid are located quite close to each other, the distances between them are approximately the same as the dimensions of the atom itself, they cannot be these clumps. That's why Rutherford I came to the conclusion that the substance is concentrated in the center of the atom, in its “nucleus”.

By the time of his experiments, the scientist had already established the charge and mass of α-particles. He knew that alpha particles carry a positive charge, twice the charge of an electron, and that they are quite heavy, about 7000 times heavier than electrons. If alpha particles are deflected by nuclei, then the nuclei also carry a positive charge.

Rutherford calculated the fractions of particles that should be scattered into certain angular intervals by point nuclei. The results of calculations and experiments are in excellent agreement if we set the nuclear charge equal to Z|e|, Where Z is the atomic number of the element from which the foil is made.

It is interesting to note that the data Rutherford's experiments compared with calculations performed within the framework of classical physics. However, as it turned out after the creation of quantum mechanics, the “classical” formula he obtained to describe the scattering of α-particles ( Rutherford's formula) is also true in quantum physics. He was very proud of this fact. After all, in order to do the calculations himself, Rutherford specially took a course in probability theory together with his students, although by that time he was already a Nobel laureate, a laboratory director, and a recognized master of experimental physics!

Based on the results Rutherford's experiments it is possible to estimate the upper limit of the size of the nucleus. To do this, we find the minimum distance R, to which an α-particle with energy E kin can approach the core. At maximum approach to the nucleus, the kinetic energy of the α particle transforms into the potential energy of the Coulomb interaction:

E kin = 2 keZe/R.

At E kin on the order of several MeV, and these were the energies of α-particles in Rutherford's experiments, we get: R~ 10 -14 m. In his calculations, Rutherford assumed that the nucleus was point-like, so it can be argued that the dimensions of the nuclei do not exceed the obtained figure and up to distances of ~10 -14 m the interaction of α-particles with nuclei is Coulomb in nature. True, for particles that experienced a head-on collision and deviated almost 180°, slight differences were observed with the distribution following from Coulomb's law. This indicated that at distances less than ~10 -14 m, some other, non-electrostatic forces begin to act. Now we know that at such distances a strong ( nuclear) interaction. Material from the site

Thus, Rutherford established in 1911 the presence in atoms of nuclei, the sizes of which are at least 104 times smaller than the sizes of atoms and in which almost the entire mass of the atom is concentrated. After Rutherford's experiments It became clear that matter mainly consists of “emptiness.” And for his research, Rutherford earned the title of “father of atomic theory” in the scientific world.

Rutherford studied the structure of atoms by bombarding them with alpha particles. He often said: “Smashtheatom" —"Break the atom." Until now, bombardment with high-energy particles remains the main method for studying the structure of micro-objects; only the tools have changed. More accurate recording instruments, methods for computer processing of results, and most importantly, modern powerful accelerators have been created that make it possible to obtain bombarding particles of very high energies.

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Rutherford proposed the use of atomic probing using alpha particles. The mass of the α particle is approximately 7300 greater than the mass of e, and the charge is equal in absolute value to 2e. Rutherford bombarded heavy metal atoms with these particles. Electrons entering an atom, due to their low mass, cannot greatly change the trajectory of the particle. Scattering can only be caused by the heavy, positively charged part of the atom. In the path of an alpha particle escaping from a radioactive source at a speed of cm/s, a thin gold foil was placed - a mesh 1 micron thick, which is equal to 10 atomic layers. At some distance from the target there is a fluorescent screen on which flashes from α-particles are recorded. Experience has shown that the overwhelming number of α-particles are deflected at small angles (2-3 deg), however, approximately one particle per 10 falling ones were deflected at a large angle, and even 180 degrees. Based on this, Rutherford suggested that an atom is a system of charges, in the center of which there is a heavy positive nucleus with a charge of Ze, having dimensions not exceeding 10

1. Derivation of Rutherford's formula for α-particle scattering.

Momentum due to scattering, where m is the mass of the particle and v is the initial velocity. According to Newton's 2nd law , wheref is the projection of force on Δp.F= , then, substitute it into the previous one and get
,;
;
;
;
; ; ;

;
The last expression is called Rutherford's formula for α-particle scattering.

1. Consequences from Rutherford's experiments.

Based on his experiments, Rutherford drew conclusions: an atom is a system of charges, in the center of which there is a heavy positive nucleus with a charge Ze, having dimensions not exceeding 10
cm, and around the nucleus there are Z electrons distributed throughout the entire volume occupied by the atom. Almost all the mass of an atom is concentrated in the nucleus.

1. Experimental determination of the nuclear charge using the Chadwick method.

The scattering foil had the shape of a ring A A", the radioactive preparation R (a source of alpha particles) and the fluorescent screen S made of ZnS were installed on the axis of the ring at equal distances from it. To count scintillations from alpha particles scattered by the foil, the hole of the ring A A" was covered with a screen that was opaque to alpha particles. On the contrary, to measure I, scintillations were counted when the hole was free and ring A A" was closed. Since in this case the number of scintillations was very large, to reduce it, a rotating disk with a narrow cutout was installed in front of the screen S. Knowing the width of the cutout and counting the number of scintillations can be calculated I. Substitute the data into
(reduced Rutherford formula). Chadwick found Z = 77.4 for platinum, Z = 46.3 for silver, and Z = 29.3 for copper.

1. Rutherford's planetary model of the atom.

An atom consists of a small, positively charged nucleus that contains almost all the mass of the atom, 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, because 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 would have to emit electromagnetic waves, lose energy and fall onto the core.

Education

Rutherford's Alpha Particle Scattering Experiment (briefly)

April 2, 2017

Ernest Rutherford is one of the founders of the fundamental doctrine of the internal structure of the atom. The scientist was born in England, in a family of immigrants from Scotland. Rutherford was the fourth child in his family, and turned out to be the most talented. He managed to make a special contribution to the theory of atomic structure.

Initial ideas about the structure of the atom

It should be noted that before Rutherford's famous experiment on the scattering of alpha particles was carried out, the dominant idea at that time about the structure of the atom was the Thompson model. This scientist was sure that the positive charge uniformly filled the entire volume of the atom. Negatively charged electrons, Thompson believed, were as if interspersed with it.

Prerequisites for a scientific revolution

After graduating from school, Rutherford, as the most talented student, received a grant of 50 pounds for further education. Thanks to this, he was able to go to college in New Zealand. Next, the young scientist passes exams at the University of Canterbury and begins to seriously study physics and chemistry. In 1891, Rutherford gave his first talk on "The Evolution of the Elements." For the first time in history, it outlined the idea that atoms are complex structures.

At that time, Dalton's idea that atoms were indivisible dominated scientific circles. To everyone around Rutherford, his idea seemed completely insane. The young scientist had to constantly apologize to his colleagues for his “nonsense.” But after 12 years, Rutherford still managed to prove he was right. Rutherford had the chance to continue his research at the Cavendish Laboratory in England, where he began to study the processes of air ionization. Rutherford's first discovery was alpha and beta rays.

Rutherford's experience

The discovery can be briefly described as follows: in 1912, Rutherford, together with his assistants, conducted his famous experiment - alpha particles were emitted from a lead source. All particles, except those that were absorbed by lead, moved along the installed channel. Their narrow stream fell on a thin layer of foil. This line was perpendicular to the sheet. Rutherford's experiment on alpha particle scattering proved that those particles that passed right through a sheet of foil caused so-called scintillations on the screen.

This screen was coated with a special substance that began to glow when alpha particles hit it. The space between the gold foil layer and the screen was filled with a vacuum to prevent alpha particles from scattering into the air. Such a device allowed researchers to observe particles scattering at an angle of about 150°.

If the foil was not used as an obstacle in front of the beam of alpha particles, then a light circle of scintillations formed on the screen. But as soon as a barrier of gold foil was placed in front of their beam, the picture changed greatly. Flashes appeared not only outside this circle, but also on the opposite side of the foil. Rutherford's experiment on alpha particle scattering showed that most particles passed through the foil without noticeable changes in their trajectory.

In this case, some particles were deflected at a rather large angle and were even thrown back. For every 10,000 particles freely passing through a layer of gold foil, only one was deflected by an angle exceeding 10° - as an exception, one of the particles was deflected by such an angle.

The reason why alpha particles were deflected

What Rutherford's experiment examined and proved in detail is the structure of the atom. This situation indicated that the atom is not a continuous formation. Most particles passed freely through the one-atom-thick foil. And since the mass of an alpha particle is almost 8,000 times greater than the mass of an electron, the latter could not significantly affect the trajectory of the alpha particle. This could be done only by the atomic nucleus - a body of small size, possessing almost all the mass and all the electrical charge of the atom. At that time, this became a significant breakthrough for the English physicist. Rutherford's experience is considered one of the most important steps in the development of the science of the internal structure of the atom.

Other discoveries made in the process of studying the atom

These studies provided direct evidence that the positive charge of an atom is located inside its nucleus. This area occupies a very small space compared to its overall dimensions. In such a small volume, scattering of alpha particles turned out to be very unlikely. And those particles that passed near the region of the atomic nucleus experienced sharp deviations from the trajectory, because the repulsive forces between the alpha particle and the atomic nucleus were very powerful. Rutherford's alpha particle scattering experiment proved the likelihood of an alpha particle hitting directly the nucleus. True, the probability was very small, but still not zero.

This was not the only fact that Rutherford's experience proved. The structure of the atom was briefly studied by his colleagues, who made a number of other important discoveries. Except for the teaching that alpha particles are fast moving helium nuclei.

The scientist was able to describe the structure of an atom in which the nucleus occupies a small part of the total volume. His experiments proved that almost the entire charge of an atom is concentrated inside its nucleus. In this case, both cases of deflection of alpha particles and cases of their collision with the nucleus occur.

Rutherford's experiments: nuclear model of the atom

In 1911, Rutherford, after numerous studies, proposed a model of the structure of the atom, which he called planetary. According to this model, inside the atom there is a nucleus that contains almost the entire mass of the particle. Electrons move around the nucleus in a similar way to how planets move around the Sun. From their combination a so-called electron cloud is formed. The atom has a neutral charge, as Rutherford's experiment showed.

The structure of the atom later became of interest to a scientist named Niels Bohr. It was he who finalized Rutherford’s teaching, because before Bohr the planetary model of the atom began to encounter difficulties of explanation. Since the electron moves around the nucleus in a certain orbit with acceleration, sooner or later it must fall onto the nucleus of the atom. However, Niels Bohr was able to prove that inside the atom the laws of classical mechanics no longer apply.

α particles are fully ionized helium atoms. They were discovered by Rutherford in 1899 while studying the phenomenon of radioactivity. Rutherford bombarded atoms of heavy elements (gold, silver, copper, etc.) with these particles. The electrons that make up the atoms, due to their low mass, cannot noticeably change the trajectory of the α particle. Scattering, that is, a change in the direction of motion of α-particles, can only be caused by the heavy, positively charged part of the atom.

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

These considerations led Rutherford to the conclusion that the atom is almost empty, and all its positive charge is concentrated in a small volume. Rutherford called this part of the atom the atomic nucleus. This is how the nuclear model of the atom arose.

Thus, the experiments of Rutherford and his colleagues led to the conclusion that at the center of the atom there is a dense, positively charged nucleus, the diameter of which does not exceed 10–14–10–15 m. This nucleus occupies only 10–12 of the total volume of the atom, but contains the entire positive charge and at least 99.95% of its mass. The substance constituting the nucleus of the atom should have been assigned a colossal density of the order of ρ ≈ 10 15 g/cm 3 . The charge of the nucleus must be equal to the total charge of all the electrons that make up the atom. Subsequently, it was possible to establish that if the charge of an electron is taken as one, then the charge of the nucleus is exactly equal to the number of a given element in the periodic table.

The radical conclusions about the structure of the atom that followed from Rutherford's experiments forced many scientists to doubt their validity. Rutherford himself was no exception, publishing the results of his research only in 1911, two years after the first experiments were performed. Based on classical ideas about the movement of microparticles, Rutherford proposed a planetary model of the atom. According to this model, at the center of the atom there is a positively charged nucleus, in which almost the entire mass of the atom is concentrated. The atom as a whole is neutral. Electrons rotate around the nucleus, like planets, under the influence of Coulomb forces from the nucleus (Fig. 6.1.4). Electrons cannot be at rest, since they would fall onto the nucleus.

An atom consists of a compact and massive positively charged nucleus and negatively charged light electrons around it.

Ernest Rutherford is a unique scientist in the sense that he had already made his main discoveries after receiving the Nobel Prize. In 1911, he succeeded in an experiment that not only allowed scientists to peer deep into the atom and gain insight into its structure, but also became a model of grace and depth of design.

Using a natural source of radioactive radiation, Rutherford built a cannon that produced a directed and focused stream of particles. The gun was a lead box with a narrow slot, inside of which radioactive material was placed. Due to this, particles (in this case alpha particles, consisting of two protons and two neutrons) emitted by the radioactive substance in all directions except one were absorbed by the lead screen, and only a directed beam of alpha particles was released through the slot. Further along the path of the beam there were several more lead screens with narrow slits that cut off particles deviating from a strictly specified direction. As a result, a perfectly focused beam of alpha particles flew towards the target, and the target itself was a thin sheet of gold foil. It was the alpha ray that hit her. After colliding with the foil atoms, the alpha particles continued their path and hit a luminescent screen installed behind the target, on which flashes were recorded when alpha particles hit it. From them, the experimenter could judge in what quantity and how much alpha particles deviate from the direction of rectilinear motion as a result of collisions with foil atoms.

Experiments of this kind have been carried out before. Their main idea was to accumulate enough information from the angles of particle deflection so that something definite could be said about the structure of the atom. At the beginning of the twentieth century, scientists already knew that the atom contains negatively charged electrons. However, the prevailing idea was that the atom was something like a positively charged fine grid filled with negatively charged raisin electrons—a model called the “raisin grid model.” Based on the results of such experiments, scientists were able to learn some properties of atoms - in particular, estimate the order of their geometric sizes.

Rutherford, however, noted that none of his predecessors had even tried to test experimentally whether some alpha particles were deflected at very large angles. The raisin grid model simply did not allow for the existence of structural elements in the atom so dense and heavy that they could deflect fast alpha particles at significant angles, so no one bothered to test this possibility. Rutherford asked one of his students to re-equip the installation in such a way that it was possible to observe the scattering of alpha particles at large deflection angles - just to clear his conscience, to completely exclude this possibility. The detector was a screen coated with sodium sulfide, a material that produces a fluorescent flash when an alpha particle hits it. Imagine the surprise not only of the student who directly carried out the experiment, but also of Rutherford himself when it turned out that some particles were deflected at angles up to 180°!

Within the framework of the established model of the atom, the result could not be interpreted: there is simply nothing in the raisin grid that could reflect a powerful, fast and heavy alpha particle. Rutherford was forced to conclude that in an atom most of the mass is concentrated in an incredibly dense substance located at the center of the atom. And the rest of the atom turned out to be many orders of magnitude less dense than previously thought. It also followed from the behavior of scattered alpha particles that in these superdense centers of the atom, which Rutherford called cores, the entire positive electric charge of the atom is also concentrated, since only the forces of electric repulsion can cause the scattering of particles at angles greater than 90°.

Years later, Rutherford liked to use this analogy about his discovery. In one southern African country, customs officials were warned that a large shipment of weapons for rebels was about to be smuggled into the country, hidden in bales of cotton. And now, after unloading, the customs officer faces a whole warehouse filled with bales of cotton. How can he determine which bales contain rifles? The customs officer solved the problem simply: he began to shoot at the bales, and if the bullets ricocheted from any bale, he identified the bales with smuggled weapons based on this sign. So Rutherford, seeing how alpha particles ricocheted off gold foil, realized that a much denser structure was hidden inside the atom than expected.

The picture of the atom drawn by Rutherford based on the results of his experiment is well known to us today. An atom consists of a super-dense, compact nucleus that carries a positive charge, and negatively charged light electrons around it. Later, scientists provided a reliable theoretical basis for this picture ( cm. Bohr Atom), but it all started with a simple experiment with a small sample of radioactive material and a piece of gold foil.

See also:

Ernest Rutherford, First Baron Rutherford of Nelson, 1871-1937

New Zealand physicist. Born in Nelson, the son of an artisan farmer. Won a scholarship to study at the University of Cambridge in England. After graduating, he was appointed to the Canadian McGill University, where, together with Frederick Soddy (1877-1966), he established the basic laws of the phenomenon of radioactivity, for which he was awarded the Nobel Prize in Chemistry in 1908. Soon the scientist moved to the University of Manchester, where, under his leadership, Hans Geiger (1882-1945) invented his famous Geiger counter, began researching the structure of the atom, and in 1911 discovered the existence of the atomic nucleus. During the First World War, he was involved in the development of sonars (acoustic radars) to detect enemy submarines. In 1919 he was appointed professor of physics and director of the Cavendish Laboratory at the University of Cambridge and in the same year discovered nuclear decay as a result of bombardment by high-energy heavy particles. Rutherford remained in this position until the end of his life, at the same time being for many years president of the Royal Scientific Society. He was buried in Westminster Abbey next to Newton, Darwin and Faraday.