The first attempt to create a model of the atom based on accumulated experimental data (1903) belongs to J. Thomson. He believed that the atom is an electrically neutral spherical system with a radius of approximately 10–10 m. The positive charge of the atom is evenly distributed throughout the entire volume of the ball, and negatively charged electrons are located inside it (Fig. 6.1.1). To explain the line emission spectra of atoms, Thomson tried to determine the location of electrons in an atom and calculate the frequencies of their vibrations around equilibrium positions. However, these attempts were unsuccessful. A few years later, in the experiments of the great English physicist E. Rutherford, it was proven that Thomson's model was incorrect.
Figure 6.1.1.
J. Thomson's model of the atom
The first direct experiments to study the internal structure of atoms were carried out by E. Rutherford and his collaborators E. Marsden and H. Geiger in 1909–1911. Rutherford proposed using atomic probing using α-particles, which arise during the radioactive decay of radium and some other elements. The mass of alpha particles is approximately 7300 times the mass of an electron, and the positive charge is equal to twice the elementary charge. In his experiments, Rutherford used α-particles with a kinetic energy of about 5 MeV (the speed of such particles is very high - about 107 m/s, but still significantly less than the speed of light). 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. The diagram of Rutherford's experiment is shown in Fig. 6.1.2.
Figure 6.1.2.
Scheme of Rutherford's experiment on α-particle scattering. K – lead container with a radioactive substance, E – screen coated with zinc sulfide, F – gold foil, M – microscope
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°.
This result was completely unexpected even for Rutherford. His ideas were in sharp contradiction with Thomson's model of the atom, according to which the positive charge is distributed throughout the entire volume of the atom. With such a distribution, the positive charge cannot create a strong electric field that can throw α particles back. The electric field of a uniform charged ball is maximum on its surface and decreases to zero as it approaches the center of the ball. If the radius of the ball in which all the positive charge of the atom is concentrated decreased by a factor of n, then the maximum repulsive force acting on an α-particle, according to Coulomb’s law, would increase by a factor of n 2. Consequently, for a sufficiently large value of n, alpha particles could experience scattering at large angles up 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. Rice. 6.1.3 illustrates the scattering of an α particle in a Thomson atom and in a Rutherford atom.
Classic experiments on the study of the structure of the atom were carried out by Sir Ernest Rutherford in 1911. Rutherford conducted experiments to study the scattering of alpha particles by thin sheets of metal foil. The impact on atoms was carried out by bombarding them with a beam of massive particles. The experimental diagram is shown in Fig. 1.
Thin gold foil F (the thickness of the foil was about 10 -7 m, about 400 atoms were placed on it) was placed inside a spherical screen E. Through a hole in the screen, a beam of fast alpha particles emitted by a radioactive drug contained in a lead container fell perpendicularly onto the plate R. Alpha particles are a fully ionized helium atom with a mass equal to 4.0015 amu. and charge equal to + 2e
(e is the value of the elementary electric charge). The speed of the alpha particle was of the order of 10 7 m/s, the energy was 4.05 MeV. When the foil thickness is small, the collision of alpha particles is almost single, i.e. each particle collides with only one atom, changing the direction of its flight.
The inner walls of the screen were coated with phosphor, a substance in which flashes occurred where alpha particles hit. This made it possible to register alpha particles with the M device, scattered by atoms at various angles θ from the original direction. Experiments on the scattering of alpha particles made it possible to establish the following patterns.
1. The vast majority of alpha particles pass through the foil almost freely: they are not deflected and do not lose energy.
2. Only a small fraction of particles (≈ 0.01%, that is, one ten-thousandth) turned back, that is, changed the direction of movement by an angle greater than 90 degrees.
The results of Rutherford's experiments can be explained based on the assumption that all the positive charge and almost the entire mass of the atom are concentrated in a small region of the atom - the nucleus, the dimensions of which are about 10 -14 m. Negatively charged electrons move around the nucleus in a huge (compared to the nucleus ) area, the size of which is about 10 -10 m.
This assumption underlies nuclear model of the atom, which is also called planetary. The number of electrons in an atom is equal to the atomic number of the element in the Mendeleev periodic table. In addition, it was shown that the forces connecting electrons with the nucleus are subject to Coulomb's law.
However, the nuclear model contradicts the laws of classical electrodynamics. In fact, if an electron is at rest in an atom, it must fall onto the nucleus under the influence of the Coulomb force of attraction. If an electron orbits a nucleus, it should emit an electromagnetic field. At the same time, it loses its energy through radiation, the speed of movement decreases, and the electron must ultimately fall onto the nucleus. The emission spectra of atoms in this case should be continuous, and the lifetime of an atom should not exceed 10 -7 s. In fact, atoms are stable, and the emission spectra of atoms are discrete.
Physics lesson in 11th grade
Subject:
"Rutherford's Alpha Particle Scattering Experiment"
Goals and objectives of the lesson:
Educational:
Explain the mechanism of Rutherford's experiments
Educational:
develop students’ cognitive independence;
contribute to their moral and aesthetic education.
Educational:
develop the ability to highlight the main, essential, compare the facts being studied, and logically express thoughts.
During the classes:
I.Org. moment.
Stand straight next to your desk and clean yourself up. Say hello to the teacher. Then sit quietly in your seat and maintain order in the classroom.
Setting the topic and purpose of the lesson.
II. Repetition
Line spectra
1.What does the word atom mean?2.Which scientist discovered the law of periodic repetition of the properties of chemical elements?
3.Is an atom indivisible?
4.What happens to rarefied gases when heated to a high temperature?
5.What are the names of multi-colored lines separated by dark spaces?
6.What is inherent in each gas?
7.Which gas has the simplest spectrum?
8.Which gas spectrum consists of 4 lines?
9.Which scientist selected the formula for spectral lines for the visible region?
10.Whose theory made it possible to combine the formulas of the visible, ultraviolet and infrared regions into one general formula?
Fizminutka according to the video.
III. New material
§ 7.2. Rutherford's experiment on alpha particle scattering.
By studying the scattering of alpha particles as they pass through gold foil,ErnestRutherford came to the conclusion that all the positive charge of atoms is concentrated at their center in a very massive and compact nucleus. And negatively charged particles (electrons) revolve around this nucleus.
This model was fundamentally different from the Thomson model of the atom, which was widespread at that time.
Joseph JohnThomson proposed a model of the atom in the form of a pudding (pie), in which a positive charge uniformly filled the entire volume of the atom, and electrons were interspersed with it.
Somewhat later, Rutherford’s model was called the planetary model of the atom (it is really similar to the Solar System: the heavy core is the Sun, and the electrons revolving around it are the planets).
In 1912, E. Rutherford and his collaborators conducted an experiment on the scattering of alpha particles in matter.
Scheme of Rutherford's experiments.
In the absence of foil, a bright circle appeared on the screen, consisting of scintillations caused by a thin beam of alpha particles. But when a thin gold foil with a thickness of approximately 0.1 μm (micron) was placed in the path of the alpha particles, the picture observed on the screen changed greatly: individual flashes appeared not only outside the previous circle, but they could even be observed from the opposite side of the gold foil.
By counting the number of scintillations per unit time in different places on the screen, it is possible to establish the distribution of scattered alpha particles in space. The number of alpha particles decreases rapidly with increasing scattering angle.
The picture observed on the screen led to the conclusion that the majority of alpha particles pass through the gold foil without a noticeable change in the direction of their movement. However, some particles deviated at large angles from the original direction of the alpha particles (about 135 o...150 o ) and were even thrown back. Studies have shown that when alpha particles pass through foil, for every 10,000 incident particles, only one is deflected by an angle of more than 10 O from the original direction of movement. Only as a rare exception does one of the huge number of alpha particles deviate from its original direction.
The fact that many alpha particles passed through the foil without deviating from their direction of motion suggests that the atom is not a solid entity. Since the mass of an alpha particle is almost 8000 times greater than the mass of an electron, the electrons included in the atoms of the foil cannot noticeably change the alpha particles. The scattering of alpha particles can be caused by a positively charged particle of an atom - the atomic nucleus.
IV.Reinforcement
Consideration of examples.
V. Reflection
Did you like our lesson today?.. What do you remember?..
VI. D/Z repeat §7.1, learn §7.2
In 1906, Rutherford discovered the scattering of α particles. Rutherford's method was as follows. A wire coated with radium C was placed in a recess in a piece of lead. A narrow slit was placed above the wire; α-particles passing through this slit fell on a photographic plate. All this was placed in a brass cylinder, from which air was pumped out. The cylinder was placed between the poles of an electromagnet, the lines of force of which ran parallel to the wire. The resulting stripes on the photographic plate were sharply limited in the void. If the cylinder was filled with air, then the stripes were wider and their edges blurred. If the gap is covered with a thin layer of some substance, the stripes become wider and their intensity gradually decreases from the center to the edges.
In 1909-1910 G. Geiger carefully studied the scattering of α-particles using the scintillation method. The Geiger device is shown in a figure taken from Geiger's article, 1910. Radon is introduced into a conical tube L, closed with a thin layer of mica, and remains in it for several hours. The radon is then sucked into vessel B, and soon after all the a-particles are ejected by the radium: C deposited on the walls of the tube. Slit D selects from the stream of α-particles emitted by radium C a narrow beam, which gives a bright picture of scintillations on the zinc sulfide screen S. If a thin plate of the substance under study is then placed in E, the scintillations on the screen are reduced due to the scattering of α-particles. The results of the experiment are presented by curves, where the scattering angles are plotted along the abscissa axis, and the number of particles scattered at a given angle is plotted along the ordinate axis. From Geiger's experiments it follows that:
- The most probable scattering angle (i.e., the angle for which the number of scattered particles is greatest) increases for small thicknesses approximately in proportion to the square root of the thickness of the substance penetrated by α-particles. For larger thicknesses, scattering increases much faster.
- The most probable angle at which a particle is deflected when passing through an atom is proportional to the atomic weight. The actual value of this angle in the case of a gold atom is about 1/200 of a degree.
- The most probable scattering angle increases rapidly with decreasing α-particle velocity, being, to a first approximation, inversely proportional to the cube of the velocity.
The most striking phenomenon observed during the scattering of α particles was a fact discovered in 1909. Geiger And Marsden, that some small part of the particles are scattered at very large angles, such that the particles fly back towards the source. For alpha particles emitted by radium C, approximately one in 8000 particles is scattered at an angle greater than the right angle.
How to explain this fact? Assuming that the atom has the structure proposed D. D. Thomson, then the single deflections of an α particle upon collision with such an atom are very small, and large scattering angles can be interpreted as a cumulative effect resulting from many deflections. Calculations carried out by Thomson and Rutherford themselves showed that even with a larger number of collisions, the resulting deflection of the α particle should be very small. “I have shown,” wrote Rutherford in 1914, “that the model of the atom proposed by Lord Kelvin and worked out in great detail by Sir D. D. Thomson cannot give such large deviations unless the diameter of the positive sphere is assumed to be extremely small.” .
The need to interpret the results of the experiments of Geiger and Marsden led Rutherford to the nuclear model of the atom. He first reported his discovery in a paper “The Scattering of α- and β-Rays and the Structure of the Atom,” read at the Manchester Philosophical Society on March 7, 1911. We present this message in full in view of its enormous historical significance.
“It is well known that α- and β-particles will deviate from their straight paths during collisions with atoms of matter. The scattering of β-particles due to their small moment (i.e. momentum - P.K.) and energy in general is much greater, than the deflection of α particles. It seems certain that these rapidly moving particles do pass through the atomic system and a detailed study of the deflections that occur should throw light on the electrical structure of the atom. It is generally assumed that the observed scattering is the result of many small scatterings. Sir D. D. Thomson (Proc. Camb. Phil. Soc. 15, p. 5, 1910) recently advanced the theory of small scatterings, and the main conclusions of the theory were experimentally verified Grouter (Proc. Roy. Soc. 84, p. 226, 1910). According to this theory, the atom is assumed to consist of a positively electrified sphere containing an equal amount of negative electricity in the form of corpuscles. When comparing theory with experiment, Grouter concluded that the number of corpuscles in an atom is approximately three times greater than its atomic weight, expressed in the weight of hydrogen. However, there are a number of scattering experiments that show that α and β particles sometimes experience deflections of more than 90° in a single collision. For example, Geiger and Marsden (Proc. Roy. Soc. 82, p. 493, 1909) found that a small part of α-particles falling on a thin gold leaf experiences a deflection greater than a right angle. Such a large deviation cannot be explained by probability theory, taking into account the experimentally observed small scattering. It certainly appears that these large deflections occur in a single atomic collision.
To explain these and other results it is necessary to assume that the electrified particles pass through an intense electric field in the atom. The scattering of charged particles can be explained by supposing an atom which consists of a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity of equal magnitude. With this arrangement of the atom, α- and β-particles, when they pass at a close distance from the center of the atom, experience large deflections, although the probability of a large deflection is small. According to this theory, the fraction of the total number of charged particles experiencing a deviation between the angles Ф and Ф+dФ is given by the expression
where n is the number of atoms per unit volume of the scattering material, t is the assumed small thickness of the material and 
where Ne is the charge at the center of the atom, E is the charge of the electrified particle, m is its mass, and u is its speed.
It follows that the number of scattered particles per unit area for a constant distance from the point of incidence of a beam of rays varies as cosec 4 Ф / 2. This distribution law was tested experimentally by Geiger for α-particles and was found to be valid within the limits of experimental errors.
From a discussion of the general results of scattering by various materials, the central charge of an atom was found to be very closely proportional to its atomic weight. The exact charge on the central nucleus has not been determined, but for a gold atom it is approximately 100 units of charge."
In such a classically clear and concise form, the world learned about one of the greatest discoveries in the history of science.
Two years later, Rutherford described in more detail his work and the work of his collaborators, which led to the discovery of the nuclear model of the atom, in the book “Radioactive Substances and Their Radiations.”
Here are calculations that make it possible to determine the number of α-particles scattered at an angle φ to the initial direction of the beam:
where n is the number of atoms per unit volume of the scattering substance, t is the thickness of the scattering plate, Q is the number of α-particles incident per unit area of the scattering plate, r is the distance from the source to the screen, b is the value determined by the equality
where Ne is the charge of the scattering nucleus, E is the charge of the α-particle, m is its mass, V is the velocity.
Rutherford's law allows us not only to check the validity of the hypothesis of the nuclear structure of the atom, but also to determine the charge of the nucleus (Ne). Geiger immediately began testing it in the same year, 1911. The experiment confirmed the validity of the cosec 4 φ / 2 law and indicated that the magnitude of the charge is approximately proportional to the atomic weight. In 1913, Geiger and Marsden undertook a new experimental test of Rutherford's formula using the scintillation method. “It was a very difficult and painstaking work,” wrote Rutherford, “since many thousands of particles had to be counted. The results of Geiger and Marsden agree very closely with the theory.”
Here is some data from Geiger and Marsden.
With a change in speed V and other constant parameters, Rutherford's formula gives that yV 4 = const. Geiger-Marsden data:
>
1 / V 4 (Relative value) 1.0 1.21 1.50 1.91 2.84 4.32 9.22
For angle dependence, Rutherford's formula gives
According to Geiger and Marsden,
And finally, for the dependence on charge (Ne), Rutherford’s formula gives the constancy of the ratios v" / A 2, where A is the atomic weight, v" = v / nt - "the reduced number of scintillations." According to Geiger and Marsden,
“Geiger and Marsden found,” Rutherford points out, “that the scattering by various atoms of a substance is approximately proportional to the square of the atomic weight, from which it follows that the charge of the atom is approximately proportional to the atomic weight. Having determined the number of α particles scattered by thin films of gold, they concluded that the nuclear charge approximately equal to half the atomic weight multiplied by the charge of the electron.Due to the difficulties of experiment, the real number could only be determined with an accuracy not exceeding 20%.
“Thus,” Rutherford concludes his presentation of the results of the experiments of Geiger and Marsden, “the experimental results of Geiger and Marsden turned out to be in complete agreement with the predictions of the theory and indicated that the hypothesis I made about the structure of the atom is correct in its simplest features.” It is interesting to note that back in 1913, Rutherford accepted the charge of the nucleus as equal to +Ne, i.e., he allowed the possibility of both positive and negative charge of the nucleus. Indeed, deflection mechanics allows for both positive and negative charge of the atom. But a number of facts, and in particular the studies of D. D. Thomson with positive rays, which we will discuss shortly, have shown that the carriers of positive electricity are always associated with a mass greater than or equal to the mass of the hydrogen atom. A massive nucleus can only carry positive charges. True, already in 1913 Bohr came to the conclusion that the nucleus must also contain electrons. This hypothesis was first expressed by Marie Skłodowska-Curie. In any case, by 1913 the connection between the charge of the nucleus and the ordinal number of the element in the periodic table was finally clarified (van den Broek, Moseley).
Rutherford's experience.
Ernst RUTHERFORD (1871-1937), English physicist, one of the founders of the doctrine of radioactivity and the structure of the atom, founder of a scientific school, foreign corresponding member of the Russian Academy of Sciences (1922) and honorary member of the USSR Academy of Sciences (1925). Director of the Cavendish Laboratory (since 1919). Discovered (1899) alpha and beta rays and established their nature. Created (1903, together with F. Soddy) the theory of radioactivity. Proposed (1911) a planetary model of the atom. Carried out (1919) the first artificial nuclear reaction. Predicted (1921) the existence of the neutron. Nobel Prize (1908).
Rutherford's experiment (1906) on the scattering of fast charged particles passing through thin layers of matter made it possible to study the internal structure of atoms. In these experiments, alpha particles were used to probe atoms - fully ionized helium atoms - resulting from the radioactive decay of radium and some other elements. Rutherford bombarded heavy metal atoms with these particles.
Rutherford knew that atoms consist of light negatively charged particles - electrons and a heavy positively charged particle. The main goal of the experiments is to find out how the positive charge is distributed inside the atom. The scattering of α - particles (that is, a change in the direction of movement) can only be caused by the positively charged part of the atom.
Experiments have shown that some of the α particles are scattered at large angles, close to 180˚, that is, they are thrown back. This is only possible if the positive charge of the atom is concentrated in a very small central part of the atom - the atomic nucleus. Almost the entire mass of the atom is also concentrated in the nucleus.
It turned out that the nuclei of various atoms have diameters of the order of 10 -14 – 10 -15 cm, while the size of the atom itself is ≈10 -8 cm, that is, 10 4 – 10 5 times the size of the nucleus.
Thus, the atom turned out to be “empty”.
Based on experiments on the scattering of α - particles on atomic nuclei, Rutherford came to to the planetary model of the atom. According to this model, an atom consists of a small positively charged nucleus and electrons orbiting around it.
From the point of view of classical physics, such an atom must be unstable, since electrons moving in orbits with acceleration must continuously emit electromagnetic energy.
Further development of ideas about the structure of atoms was made by N. Bohr (1913) on the basis of quantum concepts.
Laboratory work.
This experiment can be carried out using a special device, the drawing of which is shown in Figure 1. This device is a lead box with a complete vacuum inside it and a microscope.
Scattering (change in direction of movement) of α-particles can only be caused by the positively charged part of the atom. Thus, from the scattering of α particles, it is possible to determine the nature of the distribution of positive charge and mass inside the atom. The diagram of Rutherford's experiments is shown in Figure 1. A beam of α-particles emitted by a radioactive drug was released by a diaphragm and then fell on a thin foil of the material under study (in this case, gold). After scattering, the α-particles fell on a screen coated with zinc sulfide. The collision of each particle with the screen was accompanied by a flash of light (scintillation), which could be observed through a microscope.
With a good vacuum inside the device and in the absence of foil, a strip of light appeared on the screen, consisting of scintillations caused by a thin beam of α particles. But when foil was placed in the path of the beam, α-particles, due to scattering, were distributed over a larger area of the screen.
In our experiment, we need to examine the α-particle, which is directed at the gold core when making an angle of 180° (Fig. 2) and monitor the reaction of the α-particle, i.e. at what minimum distance will the α-particle approach the gold core (Fig. 3).
Rice. 2
Fig.3
Given:
V 0 =1.6*10 7 m/s – initial speed
d = 10 -13
= 180°
r min =?
Questions:
What is the minimum distance r min between the α particle and the nucleus that can be achieved in this experiment? (Fig. 4)
Fig.4
Solution:
In our experiment, the α-particle is represented as an atom
m neutr kg
Z=2 – protons
N=Au –Z = 4 – 2 = 2 neutrons
m p =kg
Z=79 – number of protons
N=Au –Z = 196 – 79 =117 (neutrons)
Cl 2 /H ∙m 2 – electrical constant
m 2 =6.6∙10 -27 kg
- the charge of an α-particle is equal to 2 elementary.
Answer: r min =4.3·10 -14 m
Conclusion: During this experiment, it was possible to find out that the a-particle was able to approach the atomic nucleus to a minimum distance, which was r min =4.3·10 -14 m and return back along the same trajectory along which it began to move.
When Rutherford performed the same experiment for the first time, with such an a-particle positioned relative to an angle of 180°, he said in surprise: “This is almost as incredible as if you fired a 15-inch projectile at a piece of tissue paper, and the projectile returned would come to you and strike you.”
And in truth, this is not probable, the fact is that when carrying out this experiment at smaller angles, the a-particle will certainly jump to the side, just as a pebble of several tens of grams when colliding with a car is not able to noticeably change its speed (Fig. 5). Since their mass is approximately 8000 times greater than the mass of the electron, and the positive charge is equal in magnitude to twice the charge of the electron. These are nothing more than fully ionized helium atoms. The speed of α particles is very high: it is 1/15 the speed of light. Consequently, electrons, due to their low mass, cannot noticeably change the trajectory of the α particle.
