In 1903 (i.e., even before the discovery of the existence of atomic nuclei), Rutherford and his collaborator, the English chemist Frederick Soddy, discovered that the radioactive element radium in the process of α-decay (i.e., spontaneous emission of α-particles) turns into another chemical element is radon.

Radium and radon differ in their physical and chemical properties. Radium is a metal; under normal conditions it is in a solid state, and radon is an inert gas. The atoms of these chemical elements differ in mass, nuclear charge, and the number of electrons in the electron shell. They enter into chemical reactions in different ways.

Further experiments with various radioactive drugs showed that not only during α-decay, but also during β-decay, the transformation of one chemical element into another occurs.

After Rutherford proposed the nuclear model of the atom in 1911, it became obvious that it is the nucleus that undergoes changes during radioactive transformations. Indeed, if changes affected only the electron shell of an atom (for example, the loss of one or more electrons), then the atom would turn into an ion of the same chemical element, and not at all into an atom of another element, with different physical and chemical properties.

The reaction of α-decay of the nucleus of a radium atom with its transformation into the nucleus of a radon atom is written as follows:

where the sign denotes the nucleus of a radium atom, the sign denotes the nucleus of a radon atom, and the sign denotes an α-particle, or, which is the same, the nucleus of a helium atom (i.e., the nuclei of atoms are denoted in the same way as the atoms themselves in the table of D.I. Mendeleev).

The number preceding the letter designation of the nucleus at the top is called the mass number, and at the bottom the charge number (or atomic number).

The mass number of the nucleus of an atom of a given chemical element, accurate to whole numbers, is equal to the number of atomic mass units contained in the mass of this nucleus. Recall that one atomic mass unit (abbreviated 1 amu) is equal to 1/12 of the mass of a carbon atom.

The charge number of the nucleus of an atom of a given chemical element is equal to the number of elementary electrical charges contained in the charge of this nucleus. (Recall that the elementary electric charge is the smallest electric charge, positive or negative, equal in magnitude to the charge of the electron.)

We can say this: the charge number is equal to the charge of the nucleus, expressed in elementary electric charges.

Both of these numbers - mass and charge - are always integer and positive. They have no dimension (i.e., units of measurement), since they indicate how many times the mass and charge of the nucleus are greater than unit ones.

According to the reaction equation, it can be seen that the nucleus of a radium atom, as a result of its emission of an α-particle, loses approximately four atomic mass units and two elementary charges, thereby turning into the nucleus of a radon atom.

This entry is a consequence of the fact that during the process of radioactive decay the laws of conservation of mass number and charge are satisfied: the mass number (226) and charge (88) of the decaying nucleus of a radium atom are equal, respectively, to the sum of mass numbers (222 + 4 = 226) and the sum of charges (86 + 2 = 88) nuclei of radon and helium atoms formed as a result of this decay.

Thus, from the discovery made by Rutherford and Soddy, it followed that the nuclei of atoms have a complex composition, that is, they consist of some kind of particles. In addition, it became clear that radioactivity is the ability of some atomic nuclei to spontaneously transform into other nuclei by emitting particles.

Questions

1. What happens to radioactive chemical elements as a result of α- and β-decay? Give examples.
2. Which part of the atom - the nucleus or the electron shell - undergoes changes during radioactive decay? Why do you think so?
3. What is the mass number? charge number?
4. Using the example of the α-decay reaction of radium, explain what the laws of conservation of charge (charge number) and mass number are.
5. What conclusion followed from the discovery made by Rutherford and Soddy?
6. What is radioactivity?

Exercise 46

In the previous lesson we discussed the issue related to Rutherford's experiment, as a result of which we now know that the atom is a planetary model.

This is what is called the planetary model of the atom. At the center of the nucleus is a massive, positively charged nucleus. And electrons revolve around the nucleus in their orbits.

Frederick Soddy took part in the experiments together with Rutherford. Soddy is a chemist, so he carried out his work precisely in terms of identifying the obtained elements by their chemical properties. It was Soddy who managed to find out what the a-particles were, the flow of which fell on the gold plate in Rutherford’s experiments. When measurements were made, it turned out that the mass of an a-particle is 4 atomic mass units, and the charge of an a-particle is 2 elementary charges. By comparing these things, having accumulated a certain number of a-particles, scientists found that these particles turned into a chemical element - helium gas.

The chemical properties of helium were known, thanks to which Soddy argued that the nuclei, which are a-particles, captured electrons from the outside and turned into neutral helium atoms.

Subsequently, the main efforts of scientists were aimed at studying the nucleus of the atom. It became clear that all the processes that occur during radioactive radiation occur not with the electron shell, not with the electrons that surround the nuclei, but with the nuclei themselves. It is in the nuclei that some transformations occur, as a result of which new chemical elements are formed.

The first such chain was obtained to transform the element radium, which was used in experiments on radioactivity, into the inert gas radon with the emission of an a-particle; the reaction in this case is written as follows:

Firstly, an a-particle is 4 atomic mass units and a double, doubled elementary charge, and the charge is positive. Radium has a serial number of 88, its mass number is 226, and radon has a serial number of 86, a mass number of 222, and an a-particle appears. This is the nucleus of a helium atom. In this case, we simply write helium. Ordinal number 2, mass number 4.

Reactions as a result of which new chemical elements are formed and at the same time new radiations and other chemical elements are also formed are called nuclear reactions.

When it became clear that radioactive processes take place inside the nucleus, they turned to other elements, not just radium. Studying various chemical elements, scientists realized that there are not only reactions with the emission, radiation of an a-particle from the nucleus of a helium atom, but also other nuclear reactions. For example, reactions with the emission of a b-particle. We now know that these are electrons. In this case, a new chemical element is also formed, respectively, a new particle, this is a b-particle, it is also an electron. Of particular interest in this case are all chemical elements whose atomic number is greater than 83.

So, we can formulate the so-called Soddy's rules, or displacement rules for radioactive transformations:

Rice. 2. Alpha decay

During beta decay, the atomic number increases by 1, but the atomic weight does not change.

Rice. 3. Beta decay

List of additional literature

1. Bronstein M.P. Atoms and electrons. “Library “Kvant””. Vol. 1. M.: Nauka, 1980
2. Kikoin I.K., Kikoin A.K. Physics: Textbook for 9th grade of high school. M.: “Enlightenment”
3. Kitaygorodsky A.I. Physics for everyone. Photons and nuclei. Book 4. M.: Science
4. Myakishev G.Ya., Sinyakova A.Z. Physics. Optics Quantum physics. 11th grade: textbook for in-depth study of physics. M.: Bustard
5. Rutherford E. Selected scientific works. Radioactivity. M.: Science
6. Rutherford E. Selected scientific works. The structure of the atom and the artificial transformation of elements. M.: Science

Lesson type
Lesson Objectives:

Continue studying the phenomenon of radioactivity;

Study radioactive transformations (displacement rules and the law of conservation of charge and mass numbers).

Study fundamental experimental data in order to explain in an elementary form the basic principles of the use of nuclear energy.
Tasks:
educational
developing
educational

Download:

Preview:

Lesson on the topic “Radioactive transformations of atomic nuclei.”

Physics teacher I category Medvedeva Galina Lvovna

Lesson type : lesson in learning new material
Lesson objectives:

Continue studying the phenomenon of radioactivity;

Study radioactive transformations (displacement rules and the law of conservation of charge and mass numbers).

Study fundamental experimental data in order to explain in an elementary form the basic principles of the use of nuclear energy.
Tasks :
educational- familiarize students with the displacement rule; expanding students' understanding of the physical picture of the world;
developing – to develop skills in the physical nature of radioactivity, radioactive transformations, and the rules of displacement in the periodic table of chemical elements; continue to develop skills in working with tables and diagrams; continue to develop work skills: highlighting the main thing, presenting the material, developing attentiveness, skills to compare, analyze and summarize facts, promote the development of critical thinking.
educational – promote the development of curiosity, develop the ability to express one’s point of view and defend one’s rightness.

Lesson summary:

Text for the lesson.

Good afternoon everyone present at our lesson today.

Teacher: So, we are at the second stage of research work on the topic “Radioactivity”. What is it? That is, today we will study radioactive transformations and displacement rules. ----This is the subject of our research and, accordingly, the topic of the lesson

Research equipment: periodic table, work card, collection of problems, crossword puzzle (one for two).

Teacher, Epigraph:“At one time, when the phenomenon of radioactivity was discovered, Einstein compared it to the production of fire in ancient times, since he believed that fire and radioactivity are equally important milestones in the history of civilization.”

Why did he think so?

Students in our class conducted theoretical research and here is the result:

Student message:

1. Pierre Curie placed an ampoule of radium chloride in a calorimeter. α-, β-, γ-rays were absorbed in it, and due to their energy the calorimeter was heated. Curie determined that 1 g of radium releases about 582 J of energy in 1 hour. And such energy is released over a number of years.
2. The formation of 4g grams of helium is accompanied by the release of the same energy as during the combustion of 1.5-2 tons of coal.
3. The energy contained in 1g of uranium is equal to the energy released during the combustion of 2.5 tons of oil.

Over the course of days, months and years, the radiation intensity did not change noticeably. It was unaffected by ordinary influences such as heat or increased pressure. The chemical reactions into which radioactive substances entered also did not affect the intensity of the radiation.

Each of us is not only “under the supervision” of a vigilant radiation “nanny”, each of us is a little radioactive on our own. Sources of radiation are not only outside of us. When we drink, with each sip we introduce a certain number of atoms of radioactive substances into the body, the same thing happens when we eat. Moreover, when we breathe, our body again receives from the air something capable of radioactive decay - maybe the radioactive isotope of carbon C-14, maybe potassium K-40 or some other isotope.

Teacher: Where does such an amount of radioactivity, constantly present around and inside us, come from?

Student message:

According to nuclear geophysics, there are many sources of natural radioactivity in nature. In rocks of the earth's crust, on average, per ton of rocks there are 2.5 - 3 grams of uranium, 10 - 13 g of thorium, 15 - 25 g of potassium. True, radioactive K-40 is only up to 3 milligrams per ton. All this abundance of radioactive, unstable nuclei continuously, spontaneously decays. Every minute, an average of 60,000 K-40 nuclei, 15,000 Rb-87 isotope nuclei, 2,400 Th-232 nuclei, and 2,200 U-238 nuclei disintegrate in 1 kg of earthly rock matter. The total amount of natural radioactivity is about 200 thousand decays per minute. Did you know that natural radioactivity is different in men and women? The explanation for this fact is obvious - their soft and dense tissues have different structures, absorb and accumulate radioactive substances differently.

PROBLEM: What equations, rules, laws describe these reactions of decomposition of substances?

Teacher: What problem will we solve with you? What solutions to the problem do you propose?

Students work and make their guesses.

Student answers:

Solutions:

Student 1: Recall the basic definitions and properties of radioactive radiation.

Student 2: Using the proposed reaction equations (from the map), obtain general equations for radioactive transformation reactions using the periodic table, formulate general displacement rules for alpha and beta decays.

Student 3 : Consolidate the acquired knowledge in order to apply it for further research (problem solving).

Teacher.

Fine. Let's get to the solution.

Stage 1. Working with cards. You have been given questions that you must answer in writing. answers.

Five questions - five correct answers. We evaluate using a five-point system.

(Give time to work, then verbally voice the answers, check them with the slides, and give yourself a grade according to the criteria).

1. Radioactivity is...
2. α-rays are...
3. β-rays are...
4. γ-radiation -….
5. Formulate the law of conservation of charge and mass numbers.

ANSWERS AND POINTS:

STAGE 2. Teacher.

We work independently and at the board (3 students).

A) We write down the equations of reactions that are accompanied by the release of alpha particles.

2. Write the reaction of α-decay of uranium 235 92 U.

3. .Write the alpha decay of the polonium nucleus

Teacher :

CONCLUSION #1:

As a result of alpha decay, the mass number of the resulting substance decreases by 4 amu, and the charge number by 2 elementary charges.

B) We write down the equations of reactions that are accompanied by the release of beta particles (3 study at the board).

1. . Write the β-decay reaction of plutonium 239 94 Pu.

2. Write the beta decay of the thorium isotope

3.Write the reaction of β-decay of curium 247 96 cm

Teacher : What general expression can we write down and draw the appropriate conclusion?

CONCLUSION #2:

As a result of beta decay, the mass number of the resulting substance does not change, but the charge number increases by 1 elementary charge.

STAGE 3.

Teacher: At one time after these expressions were obtained, Rutherford's student Frederick Soddy,proposed displacement rules for radioactive decays, with the help of which the resulting substances can be found in the periodic table. Let's look at the equations we obtained.

QUESTION:

1). WHAT REGULARITY IS OBSERVED DURING ALPHA DECAY?

ANSWER: During alpha decay, the resulting substance shifts two cells to the beginning of the periodic table.

2). WHAT REGULARITY IS OBSERVED IN BETA DECAY?

ANSWER: During beta decay, the resulting substance shifts one cell to the end of the periodic table.

STAGE 4.

Teacher. : And the last stage of our activity for today:

Independent work (based on Lukashik’s collection of problems):

Option 1.

Option2.

EXAMINATION: on the board, independently.

CRITERIA FOR EVALUATION:

“5” - tasks completed

“4” - 2 tasks completed

“3” - 1 task completed.

SELF-ASSESSMENT FOR THE LESSON:

IF YOU HAVE TIME LEFT:

Question for the class:

What topic did you study in class today? After solving the crossword puzzle, you will find out the name of the process of release of radioactive radiation.

1. Which scientist discovered the phenomenon of radioactivity?

2. Particle of matter.

3. The name of the scientist who determined the composition of radioactive radiation.

4. Nuclei with the same number of protons, but with a different number of neutrons are...

5. Radioactive element discovered by the Curies.

6. The isotope of polonium is alpha radioactive. What element is formed in this case?

7. The name of a woman scientist who became a Nobel laureate twice.

8. What is at the center of an atom?

1. RADIOACTIVE TRANSFORMATIONS

Ernest Rutherford was born in New Zealand to an English family. In New Zealand he received higher education, and then in 1895 he came to Cambridge and began scientific work as Thomson's assistant. In 1898, Rutherford was invited to the Department of Physics at Montreal's McGill University (Canada), where he continued the research on radioactivity that had begun in Cambridge.

In 1899, in Montreal, Rutherford's colleague Ownes informed him that the radioactivity of thorium was sensitive to air currents. This observation seemed curious, Rutherford became interested and discovered that the radioactivity of thorium compounds, if the thorium is in a closed ampoule, remains constant in intensity, but if the experiment is carried out in the open air, it quickly decreases, and even weak air currents affect the results. In addition, bodies located in the vicinity of thorium compounds, after some time, themselves begin to emit radiation, as if they were also radioactive. Rutherford called this property “excited activity.”

Rutherford soon realized that all these phenomena could be easily explained if we assume that thorium compounds emit, in addition to alpha particles, other particles, which in turn are radioactive. He called the substance consisting of these particles “emanation” and considered it similar to radioactive gas, which, located in a thin invisible layer on bodies located next to the thorium that emits this emanation, imparts apparent radioactivity to these bodies. Guided by this assumption, Rutherford was able to separate this radioactive gas by simply extracting air that had come into contact with the thorium preparation, and then, introducing it into an ionization chamber, thus determined its activity and basic physical properties. In particular, Rutherford showed that the degree of radioactivity of the emanation (later christened thoron, just as the radioactive gases emitted by radium and actinium were called radon and actinon) very quickly decreases exponentially depending on time: every minute the activity is halved, after ten minutes she already becomes completely unnoticeable.

Meanwhile, the Curies showed that radium also has the ability to excite the activity of nearby bodies. To explain the radioactivity of the sediments of radioactive solutions, they accepted the theory put forward by Becquerel and called this new phenomenon “induced radioactivity.” The Curies believed that induced radioactivity was caused by some special excitation of bodies by rays emitted by radium: something similar to phosphorescence, to which they directly likened this phenomenon. However, Rutherford, speaking of “excited activity,” at first must also have had in mind the phenomenon of induction, which 19th-century physics was quite ready to accept. But Rutherford already knew something more than the Curies: he knew that excitation, or induction, was not a direct consequence of the influence of thorium, but the result of the action of emanation. At that time, the Curies had not yet discovered the emanation of radium; it was obtained by Lather and Dorn in 1900, after they repeated the same studies of radium that Rutherford had previously carried out with thorium.

In the spring of 1900, having published his discovery, Rutherford interrupted his research and returned to New Zealand, where his wedding was to take place. On his return to Montreal that same year, he met Frederick Soddy (1877-1956), who had graduated in chemistry at Oxford in 1898 and had also recently arrived in Montreal. The meeting of these two young people was a happy event for the history of physics. Rutherford told Soddy about his discovery, that he had managed to isolate thoron, emphasized the wide field of research that was opening up here, and invited him to team up for a joint chemical and physical study of the thorium compound. Soddy agreed.

This research took the young scientists two years. Soddy, in particular, studied the chemical nature of thorium emanation. As a result of his research, he showed that the new gas does not enter into any known chemical reactions at all. Therefore, it remained to assume that it belongs to the number of inert gases, namely (as Soddy definitely showed at the beginning of 1901) the new gas is similar in its chemical properties to argon (it is now known that this is one of its isotopes), which Rayleigh and Ramsay discovered in the air in 1894

The hard work of two young scientists culminated in a new significant discovery: along with thorium, another element was discovered in their preparations, which differed in chemical properties from thorium, and was at least several thousand times more active than thorium. This element was chemically separated from thorium by precipitation with ammonia. Following the example of William Crookes, who in 1900 named the radioactive element he obtained from uranium uranium X, the young scientists named the new radioactive element thorium X. The activity of this new element is reduced by half within four days; this time was enough to study it in detail. Research has made it possible to draw an undeniable conclusion: the emanation of thorium is not obtained from thorium at all, as it seemed, but from thorium X. If in a certain sample of thorium thorium X was separated from thorium, then the intensity of thorium radiation was at first much less than before the separation, but it gradually increased over time according to an exponential law due to the constant formation of new radioactive substance.

In the first work of 1902, scientists, explaining all these phenomena, came to the conclusion that

“...radioactivity is an atomic phenomenon accompanied by chemical changes, in which new types of matter are generated. These changes must occur inside the atom, and radioactive elements must be spontaneous transformations of atoms... Therefore, radioactivity must be considered as a manifestation of an intra-atomic chemical process.” (Philosophical Magazine, (6), 4, 395 (1902)).

And the next year they wrote more definitely:

“Radioactive elements have the highest atomic weight among all other elements. This, in fact, is their only common chemical property. As a result of atomic decay and the ejection of heavy charged particles with a mass of the same order as the mass of the hydrogen atom, a new system is left, lighter than the original, with physical and chemical properties completely different from those of the original element. The process of decay, having begun once, then moves from one stage to another at certain rates, which are quite measurable. At each stage, one or more α particles are emitted until the last stages are reached, when the α particles or electrons have already been emitted. It would seem advisable to give special names to these new fragments of atoms and new atoms which are obtained from the original atom after the emission of a particle and exist only for a limited period of time, constantly undergoing further changes. Their distinguishing property is instability. The quantities in which they can accumulate are very small, so that it is unlikely that they can be studied by ordinary means. Instability and the associated emission of rays give us a way to study them. Therefore, we propose to call these fragments of atoms “metabolons”." (Philosophical Magazine, (6), 5, 536 (1903)).

The proposed term did not survive, because this first cautious attempt to formulate a theory was soon corrected by the authors themselves and clarified in a number of unclear points, which the reader himself probably noted. In its corrected form, the theory no longer needed a new term, and ten years later one of these young scientists, who by that time had already become a world-renowned scientist and Nobel Prize laureate in physics, was expressed as follows:

“Atoms of a radioactive substance are subject to spontaneous modifications. At each moment, a small portion of the total number of atoms becomes unstable and disintegrates explosively. In the vast majority of cases, a fragment of an atom - an α-particle - is ejected at enormous speed; in some other cases, the explosion is accompanied by the ejection of a fast electron and the appearance of X-rays, which have great penetrating power and are known as γ-radiation. Radiation accompanies the transformations of atoms and serves as a measure that determines the degree of their decay. It was discovered that as a result of an atomic transformation, a completely new type of substance is formed, completely different in its physical and chemical properties from the original substance. This new substance, however, is itself also unstable and undergoes a transformation with the emission of characteristic radioactive radiation...

Thus, it is precisely established that the atoms of some elements are subject to spontaneous disintegration, accompanied by the emission of energy in quantities enormous in comparison with the energy released during ordinary molecular modifications" ( E. Rutherford, The structure of the atom, Scientia, 16, 339 (1914)).

In the 1903 paper already cited, Rutherford and Soddy compiled a table of "metabolons" which, according to their theory, are formed, according to their own experiments and the experiences of other scientists, as decay products:

These are the first “family trees” of radioactive substances. Gradually other substances took their place in these families of natural radioactive elements, and it was found that there are only three such families, of which two have uranium as their parent, and the third has thorium. The first family has 14 “descendants”, i.e. 14 elements resulting from one another as a result of sequential decay, the second - 10, the third - 11; in any modern physics textbook you can find a detailed description of these “family trees”.

Let us make one remark. Now it may seem quite natural, moreover, self-evident, the conclusion that Rutherford and Soddy came to as a result of their experiments. Essentially, what were we talking about? The fact that after some time, initially pure thorium contained an admixture of a new element, from which, in turn, a gas was formed, which was also radioactive. The formation of new elements can be seen clearly. Visually, but not very much. It must be borne in mind that the quantities in which new elements were formed were very far from the minimum doses that were necessary at that time for the most accurate chemical analysis. We were talking about barely noticeable traces that can only be detected by radioactive methods, photography and ionization. But all these effects could be explained in another way (induction, the presence of new elements in the original preparations from the very beginning, as was the case with the discovery of radium, etc.). That the decay was not at all so obvious is clear from the fact that neither Crookes nor Curie saw the slightest hint of it, although they observed similar phenomena. It is also impossible to remain silent about the fact that it took great courage to talk about the transformations of elements in 1903, at the very height of the triumph of atomism. This hypothesis was by no means protected from all kinds of criticism and, perhaps, would not have stood up if Rutherford and Soddy had not defended it with amazing tenacity for entire decades, resorting to new evidence, which we will talk about later.

It seems appropriate to us to add here that the theory of radioactive induction has also rendered a great service to science by preventing the scattering of efforts in the search for new radioactive elements with each manifestation of radioactivity in non-radioactive elements.

2. NATURE OF α-PARTICLES

A very important point in the theory of radioactive decay, which we have so far passed over, however, in silence for the sake of simplicity of presentation, is the nature of the α-particles emitted by radioactive substances, for the hypothesis attributing to them corpuscular properties is of decisive importance for the theory of Rutherford and Soddy.

At first, α-particles - a slow component of radiation that is easily absorbed by matter - after their discovery by Rutherford did not attract much attention from physicists who were interested mainly in fast β-rays, which have a hundred times greater penetrating power than α-particles.

The fact that Rutherford foresaw the importance of α particles in explaining radioactive processes and devoted many years to studying them is one of the clearest manifestations of Rutherford's genius and one of the main factors determining the success of his work.

In 1900, Robert Rayleigh (Robert Strett, son of John William Rayleigh) and independently of him Crookes put forward a hypothesis, not supported by any experimental evidence, that α particles carry a positive charge. Today we can very well understand the difficulties that stood in the way of the experimental study of α-particles. These difficulties are twofold: first, α particles are much heavier than β particles, so they are slightly deflected by electric and magnetic fields, and, of course, a simple magnet was not enough to produce a noticeable deflection; secondly, α-particles are quickly absorbed by the air, making them even more difficult to observe.

For two years, Rutherford tried to deflect alpha particles in a magnetic field, but all the time he received uncertain results. Finally, at the end of 1902, when, thanks to the kind mediation of Pierre Curie, he was able to obtain a sufficient amount of radium, he was able to reliably establish the deflection of α particles in magnetic and electric fields using the device shown on page 364.

The deviation he observed allowed him to determine that the α particle carried a positive charge; by the nature of the deviation, Rutherford also determined that the speed of the α particle is approximately equal to half the speed of light (later refinements reduced the speed to approximately one tenth the speed of light); the e/m ratio turned out to be approximately 6000 electromagnetic units. It followed from this that if an alpha particle carries an elementary charge, then its mass should be twice the mass of a hydrogen atom. Rutherford was aware that all these data were extremely approximate, but they still allowed one qualitative conclusion to be drawn: α-particles have a mass of the same order as atomic masses, and therefore are similar to the channel rays that Goldstein observed, but have significantly greater speed. The results obtained, says Rutherford, “shed light on radioactive processes,” and we have already seen the reflection of this light in the passages quoted from the papers of Rutherford and Soddy.

In 1903, Marie Curie confirmed Rutherford's discovery with the help of an installation now described in all physics textbooks, in which, thanks to the scintillation caused by all the rays that radium emits, it was possible to simultaneously observe the opposite deflections of α-particles and β-rays and the immunity of γ-radiation to electric and magnetic fields.

The theory of radioactive decay led Rutherford and Soddy to the idea that all stable substances resulting from radioactive transformations of elements must be present in radioactive ores, in which these transformations have been occurring for many thousands of years. Shouldn't the helium found by Ramsay and Travers in uranium ores then be considered a product of radioactive decay?

From the beginning of 1903, the study of radioactivity received an unexpected new impetus thanks to the fact that Giesel (the company "Hininfabrik", Braunschweig) released such pure radium compounds as radium bromide hydrate, containing 50% of the pure element, at relatively reasonable prices. Previously, one had to work with compounds containing at most 0.1% of the pure element!

By that time, Soddy had returned to London to continue studying the properties of emanation in the Ramsey Chemical Laboratory - the only laboratory in the world at that time where research of this kind could be carried out. He bought 30 mg of the drug that went on sale, and this amount was enough for him to prove, together with Ramsey in the same 1903, that helium is present in radium that is several months old, and that helium is formed during the decay of the emanation.

But what place did helium occupy in the table of radioactive transformations? Was it the final product of the transformations of radium or the product of some stage of its evolution? Rutherford very soon realized that helium was formed by α particles emitted by radium, that each α particle was an atom of helium with two positive charges. But it took years of work to prove this. The proof was obtained only when Rutherford and Geiger invented the α-particle counter, which we discussed in Chapter. 13. Measuring the charge of an individual α particle and determining the ratio e/m immediately gave its mass m a value equal to the mass of a helium atom.

And yet all these studies and calculations have not yet decisively proven that α-particles are identical with helium ions. In fact, if, say, simultaneously with the ejection of an α-particle, a helium atom was released, then all experiments and calculations would remain valid, but the α-particle could also be an atom of hydrogen or some other unknown substance. Rutherford was well aware of the possibility of such criticism and, in order to reject it, in 1908, together with Royds, gave decisive proof of his hypothesis using the installation schematically depicted in the above figure: α-particles emitted by radon are collected and accumulated in a tube for spectroscopic analysis; in this case, a characteristic spectrum of helium is observed.

Thus, starting from 1908, there was no longer any doubt that α particles were helium ions and that helium was a constituent of naturally occurring radioactive substances.

Before moving on to another question, let us add that several years after the discovery of helium in uranium ores, the American chemist Boltwood, examining ores containing uranium and thorium, came to the conclusion that the last non-radioactive product of a successive series of transformations of uranium is lead and that, in addition Moreover, radium and actinium are themselves decay products of uranium. Rutherford and Soddy's table of "metabolons" must therefore have undergone a significant change.

The theory of atomic decay led to another new interesting consequence. Since radioactive transformations occur at a constant rate, which could not be changed by any physical factor known at that time (1930), then by the ratio of the amounts of uranium, lead and helium present in uranium ore, the age of the ore itself can be determined, i.e. age of the Earth. The first calculation gave a figure of one billion eight hundred million years, but John Joly (1857-1933) and Robert Rayleigh (1875-1947), who carried out important research in this area, considered this estimate to be very inaccurate. Now the age of uranium ores is considered to be approximately one and a half billion years, which is not very different from the original estimate.

3. BASIC LAW OF RADIOACTIVITY

We have already said that Rutherford experimentally established the exponential law of decrease in the activity of thorium emanation with time: the activity decreases by half in about one minute. All radioactive substances studied by Rutherford and others obeyed qualitatively the same law, but each of them had its own half-life. This experimental fact is expressed by the simple formula ( This formula looks like

where λ is the half-life constant, and its inverse is the average lifetime of the element. The time required for the number of atoms to be reduced by half is called the half-life. As we have already said, A varies greatly from element to element and, therefore, all other quantities dependent on it also change. For example, the average lifetime of uranium I is 6 billion 600 million years, and actinium A is three thousandths of a second), establishing the relationship between the number N 0 of radioactive atoms at the initial moment and the number of atoms that have not yet decayed at moment t. This law can be expressed differently: the fraction of atoms that decay over a certain period of time is a constant characterizing the element and is called the radioactive decay constant, and its inverse is called the average lifetime.

Before 1930, no factor was known that would influence in the slightest degree the natural rate of this phenomenon. Beginning in 1902, Rutherford and Soddy, and then many other physicists, placed radioactive bodies in a wide variety of physical conditions, but never obtained the slightest change in the radioactive decay constant.

“Radioactivity,” wrote Rutherford and Soddy, “according to our present knowledge of it, must be considered as the result of a process that remains completely outside the sphere of action of forces known and controlled by us; it can neither be created nor changed nor stopped.” (Philosophical Magazine, (6), 5, 582 (1903).).

The average lifetime of an element is a precisely defined constant, unchanged for each element, but the individual lifetime of an individual atom of a given element is completely uncertain. The average lifetime does not decrease with time: it is the same both for a group of newly formed atoms and for a group of atoms formed in early geological epochs. In short, using an anthropomorphic comparison, we can say that the atoms of radioactive elements die, but do not age. In general, from the very beginning, the basic law of radioactivity seemed completely incomprehensible, as it remains to this day.

From all that has been said, it is clear, and it was immediately clear, that the law of radioactivity is a probabilistic law. He argues that the possibility of an atom decaying at a given moment is the same for all existing radioactive atoms. We are thus talking about a statistical law, which becomes clearer the greater the number of atoms considered. If the phenomenon of radioactivity was influenced by external causes, then the explanation of this law would be quite simple: in this case, the atoms decaying at a given moment would be precisely those atoms that are in particularly favorable conditions in relation to the influencing external cause. These special conditions leading to the disintegration of an atom could, for example, be explained by the thermal excitation of atoms. In other words, the statistical law of radioactivity would then have the same meaning as the statistical laws of classical physics, considered as a synthesis of particular dynamic laws, which, due to their large number, are simply convenient to consider statistically.

But the experimental data made it absolutely impossible to reduce this statistical law to the sum of particular laws determined by external causes. Having excluded external causes, they began to look for the reasons for the transformation of an atom in the atom itself.

“Since,” wrote Marie Curie, “in the aggregate of a large number of atoms, some of them are immediately destroyed, while others continue to exist for a very long time, it is no longer possible to consider all the atoms of the same simple substance as completely identical, but it should be recognized that the difference in their fate is determined by individual differences. But then a new difficulty arises. The differences that we want to take into account should be of such a kind that they should not determine, so to speak, the “aging” of the substance. They must be such that the probability that the atom will live for some given time does not depend on the time during which it already exists. Any theory of the structure of atoms must satisfy this requirement if it is based on the considerations expressed above." (Rapports et discussions du Conseil Solvay tenu a Bruxelles du 27 au 30 avril 1913, Paris, 1921, p. 68-69).

Marie Curie's point of view was also shared by her student Debierne, who put forward the assumption that each radioactive atom continuously passes quickly through numerous different states, maintaining a certain average state unchanged and independent of external conditions. It follows that, on the average, all atoms of the same kind have the same properties and the same probability of decay due to the unstable state through which the atom passes from time to time. But the presence of a constant probability of decay of an atom implies its extreme complexity, since it must consist of a large number of elements subject to random movements. This intra-atomic excitation, limited to the central part of the atom, can lead to the need to introduce an internal temperature of the atom, which is significantly higher than the external one.

These considerations of Marie Curie and Debierne, which, however, were not confirmed by any experimental data and did not lead to any real consequences, did not find a response among physicists. We remember them because the unsuccessful attempt at a classical interpretation of the law of radioactive decay was the first, or at least the most convincing, example of a statistical law that cannot be derived from the laws of the individual behavior of individual objects. A new concept of a statistical law arises, given directly, without regard to the behavior of the individual objects that make up the totality. Such a concept would become clear only ten years after the unsuccessful efforts of Curie and Debierne.

4. RADIOACTIVE ISOTOPES

In the first half of the last century, some chemists, in particular Jean Baptiste Dumas (1800-1884), noticed a certain connection between the atomic weight of elements and their chemical and physical properties. These observations were completed by Dmitri Ivanovich Mendeleev (1834-1907), who in 1868 published his ingenious theory of the periodic table of the elements, one of the most profound generalizations in chemistry. Mendeleev arranged the elements known at that time in order of increasing atomic weight. Here are the first of them, indicating their atomic weight according to the data of that time:

7Li; 9.4Ве; 11B; 12C; 14N; 160; 19F;

23Na; 24Mg; 27.3Al; 28Si; 31P; 32S; 35.50Cl.

Mendeleev noted that the chemical and physical properties of elements are periodic functions of atomic weight. For example, in the first row of elements written out, the density regularly increases with increasing atomic weight, reaches a maximum in the middle of the row, and then decreases; the same periodicity, although not so clear, can be seen in relation to other chemical and physical properties (melting point, expansion coefficient, conductivity, oxidation, etc.) for elements of both the first and second row. These changes occur according to the same law in both rows, so that elements that are in the same column (Li and Na, Be and Mg, etc.) have similar chemical properties. These two series are called periods. Thus, all elements can be distributed over periods in accordance with their properties. From this follows Mendeleev's law: the properties of elements periodically depend on their atomic weights.

This is not the place to relate the lively discussion which the periodic classification gave rise to, and its gradual establishment through the invaluable services which it rendered to the development of science. It is enough only to point out that by the end of the last century it was accepted by almost all chemists, who accepted it as an experimental fact, having become convinced of the futility of all attempts to interpret it theoretically.

At the very beginning of the 20th century, during the processing of precious stones in Ceylon, a new mineral was discovered, thorianite, which, as is now known, is a thorium-uranium mineral. Some thorianite was sent to England for analysis. However, in the first analysis, due to an error, which Soddy attributes to famous German work on analytical chemistry, thorium was confused with zirconium, due to which the substance under investigation, believed to be uranium ore, was subjected to the Curie method to separate radium from the uranium ore. In 1905, using this method, Wilhelm Ramsey and Otto Hahn (the latter immortalized his name thirty years later by discovering the fission reaction of uranium) obtained a substance that chemical analysis determined to be thorium, but which differed from it by much more intense radioactivity. As with thorium, its decay resulted in the formation of thorium X; thoron and other radioactive elements. Intense radioactivity indicated the presence in the resulting substance of a new radioactive element, not yet chemically determined. It was called radiothorium. It soon became clear that it was an element from the decay series of thorium, that it had eluded the previous analysis of Rutherford and Soddy and had to be inserted between thorium and thorium X. The average lifetime of radiothorium was found to be about two years. This is a long enough period for radiothorium to replace expensive radium in laboratories. Apart from purely scientific interest, this economic reason has prompted many chemists to try to isolate it, but all attempts have been unsuccessful. It was not possible to separate it from thorium by any chemical process; moreover, in 1907 the problem seemed to become even more complicated because Khan discovered mesothorium, an element that generates radiothorium, which also turned out to be inseparable from thorium. The American chemists McCoy and Ross, having failed, had the courage to explain it and the failures of other experimenters by the fundamental impossibility of separation, but to their contemporaries such an explanation seemed only a convenient excuse. Meanwhile, in the period 1907-1910. There have been other cases where some radioactive elements could not be separated from others. The most typical examples were thorium and ionium, mesothorium I and radium, radium D and lead.

Some chemists likened the inseparability of the new radioelements to the case with rare earth elements that chemistry encountered in the 19th century. At first, the similar chemical properties of rare earths made it possible to consider the properties of these elements to be the same, and only later, as chemical methods improved, it was gradually possible to separate them. However, Soddy believed that this analogy was far-fetched: in the case of rare earths, the difficulty was not in separating the elements, but in establishing the fact of their separation. On the contrary, in the case of radioactive elements, the difference between the two elements is clear from the very beginning, but it is not possible to separate them.

In 1911, Soddy conducted a systematic study of a commercial preparation of mesothorium, which also contained radium, and found that the relative content of either of these two elements could not be increased, even by resorting to repeated fractional crystallization. Soddy concluded that two elements could have different radioactive properties and yet have other chemical and physical properties so similar that they could not be separated by ordinary chemical processes. If two such elements have the same chemical properties, they should be placed in the same place on the periodic table of elements; that's why he called them isotopes.

From this basic idea, Soddy attempted to provide a theoretical explanation by formulating the "rule of displacement in radioactive transformations": the emission of one α particle causes the element to shift two places to the left in the periodic table. But the transformed element can subsequently return to the same cell of the periodic table with the subsequent emission of two β particles, as a result of which the two elements will have the same chemical properties, despite different atomic weights. In 1911, the chemical properties of radioactive elements that emit β-rays and have, as a rule, a very short lifespan were still little known, so before accepting this explanation, it was necessary to better understand the properties of the elements that emit β-rays. Soddy entrusted this work to his assistant Fleck. The work took a lot of time, and both of Rutherford's assistants, Ressel and Hevesy, took part in it; later Faience also took up this task.

In the spring of 1913 the work was completed and Soddy's rule was confirmed without any exceptions. It could be formulated very simply: the emission of an alpha particle reduces the atomic weight of a given element by 4 units and shifts the element two places to the left in the periodic table; the emission of a β-particle does not significantly change the atomic weight of the element, but shifts it one place to the right in the periodic table. Therefore, if a transformation caused by the emission of an α particle is followed by two transformations with the emission of β particles, then after three transformations the element returns to its original place in the table and acquires the same chemical properties as the original element, however, having an atomic weight less by 4 units. It also clearly follows from this that isotopes of two different elements can have the same atomic weight, but different chemical properties. Stewart called them isobars. On page 371 a diagram is reproduced illustrating the rule of displacement during radioactive transformations in the form given by Soddy in 1913. Now we know, of course, much more radioactive isotopes than Soddy knew in 1913. But we probably do not need to trace all these subsequent technical achievements. It is more important to once again emphasize the main thing: α-particles carry two positive charges, and β-particles carry one negative charge; the emission of any of these particles changes the chemical properties of the element. The deep meaning of Soddy's rule is, therefore, that the chemical properties of elements, or at least radioactive elements until this rule is extended further, are related not to atomic weight, as classical chemistry asserted, but to intra-atomic electric charge.

What happens to matter during radioactive radiation? To answer this question at the beginning of the 20th century. it wasn't very easy. Already at the very beginning of radioactivity research, many strange and unusual things were discovered.

First, the amazing consistency with which the radioactive elements uranium, thorium and radium emit radiation. Over the course of days, months and years, the radiation intensity did not change noticeably. It was unaffected by ordinary influences such as heat or increased pressure.

The chemical reactions into which radioactive substances entered also did not affect the intensity of the radiation.

Secondly, very soon after the discovery of radioactivity it became clear that radioactivity is accompanied by the release of energy. Pierre Curie placed an ampoule of radium chloride in a calorimeter. α-, β- and γ-rays were absorbed in it, and due to their energy the calorimeter was heated. Curie determined that 1 g of radium releases 582 J of energy in 1 hour. And this energy is released continuously over a number of years.

Where does the energy come from, the release of which is not affected by all known influences? Apparently, during radioactivity, a substance experiences some profound changes, completely different from ordinary chemical transformations. It was assumed that the atoms themselves undergo transformations!

Now this thought may not cause much surprise, since a child can hear about it even before he learns to read. But at the beginning of the 20th century. it seemed fantastic and it took great courage to decide to express it. At that time, indisputable evidence for the existence of atoms had just been obtained. The centuries-old idea of ​​Democritus about the atomic structure of matter finally triumphed. And almost immediately after this, the immutability of atoms is called into question.

We will not talk in detail about those experiments that ultimately led to complete confidence that during radioactive decay a chain of successive transformations of atoms occurs. Let us dwell only on the very first experiments begun by Rutherford and continued by him together with the English chemist F. Soddy (1877-1956).

Rutherford discovered that thorium activity, defined as the number of decays per unit time, remains unchanged in a closed ampoule. If the preparation is blown with even very weak air currents, then the activity of thorium is greatly reduced. Rutherford suggested that, simultaneously with the alpha particles, thorium emits some kind of gas, which is also radioactive. He called this gas emanation. By sucking air from an ampoule containing thorium, Rutherford isolated the radioactive gas and examined its ionizing ability. It turned out that the activity of this gas decreases rapidly with time. Every minute the activity decreases by half, and after ten minutes it is practically equal to zero. Soddy studied the chemical properties of this gas and found that it does not enter into any reactions, i.e., it is an inert gas. Subsequently, the gas was named radon and placed in the periodic table under serial number 86. Other radioactive elements also experienced transformations: uranium, actinium, radium. The general conclusion that scientists came to was accurately formulated by Rutherford: “The atoms of a radioactive substance are subject to spontaneous modifications. At each moment, a small portion of the total number of atoms becomes unstable and disintegrates explosively. In the overwhelming majority of cases, a fragment of an atom - an α-particle - is ejected at enormous speed. In some other cases, the explosion is accompanied by the ejection of a fast electron and the appearance of rays, which, like X-rays, have high penetrating power and are called γ-radiation. It was discovered that as a result of an atomic transformation, a completely new type of substance is formed, completely different in its physical and chemical properties from the original substance. This new substance, however, is itself also unstable and undergoes a transformation with the emission of characteristic radioactive radiation.

Thus, it is precisely established that the atoms of certain elements are subject to spontaneous disintegration, accompanied by the emission of energy in quantities enormous in comparison with the energy released during ordinary molecular modifications.”

After the atomic nucleus was discovered, it immediately became clear that it was this nucleus that underwent changes during radioactive transformations. After all, there are no os-particles in the electron shell at all, and a decrease in the number of shell electrons by one turns the atom into an ion, and not into a new chemical element. The ejection of an electron from the nucleus changes the charge of the nucleus (increases it) by one. The charge of the nucleus determines the atomic number of the element in the periodic table and all its chemical properties.

Note

Literature

Myakishev G.Ya. Physics: Optics. The quantum physics. 11th grade: Educational. for in-depth study of physics. - M.: Bustard, 2002. - P. 351-353.