Even ancient philosophers suggested that matter is built from atoms. However, the fact that the “bricks” of the universe themselves consist of the smallest particles, scientists began to guess only at the turn of the 19th and 20th centuries. Experiments proving this made a real revolution in science in its time. It is the proportion constituent parts distinguishes one chemical element from another. Each of them has its own place in according to the serial number. But there are varieties of atoms that occupy the same cells in the table, despite the difference in mass and properties. Why this is so and what isotopes are in chemistry will be discussed later.

Atom and its particles

Exploring the structure of matter through bombardment with alpha particles, E. Rutherford proved in 1910 that the main space of the atom is filled with emptiness. And only in the center is the core. Negative electrons move in orbits around it, making up the shell of this system. That's how it was created planetary model"bricks" of matter.

What are isotopes? Remember from the chemistry course that the nucleus also has complex structure. It consists of positive protons and uncharged neutrons. The number of the former determines the qualitative characteristics of the chemical element. It is the number of protons that distinguishes substances from each other, endowing their nuclei with a certain charge. And on this basis, they are assigned a serial number in the periodic table. But the number of neutrons in the same chemical element differentiates them into isotopes. Definition in chemistry this concept so the following can be given. These are varieties of atoms that differ in the composition of the nucleus, have the same charge and serial numbers, but have different mass numbers due to differences in the number of neutrons.

Notation

Studying chemistry in the 9th grade and isotopes, students will learn about accepted legend. The letter Z marks the charge of the nucleus. This figure coincides with the number of protons and therefore is their indicator. The sum of these elements with neutrons, marked with the sign N, is A - the mass number. The family of isotopes of one substance, as a rule, is indicated by the icon of that chemical element, which in the periodic table is endowed with a serial number coinciding with the number of protons in it. The left superscript added to the specified icon corresponds to the mass number. For example, 238 U. The charge of an element (in this case, uranium, marked with serial number 92) is indicated by a similar index below.

Knowing these data, one can easily calculate the number of neutrons in a given isotope. It is equal to the mass number minus the serial number: 238 - 92 \u003d 146. The number of neutrons could be less, from this this chemical element would not cease to be uranium. It should be noted that most often in other, simpler substances, the number of protons and neutrons is approximately the same. Such information helps to understand what an isotope is in chemistry.

Nucleons

It is the number of protons that gives individuality to a certain element, and the number of neutrons does not affect it in any way. But the atomic mass is made up of these two indicated elements, having common name"nucleons", representing their sum. However, this indicator does not depend on those forming the negatively charged shell of the atom. Why? It's worth just comparing.

The mass fraction of a proton in an atom is large and is approximately 1 AU. u m or 1.672 621 898 (21) 10 -27 kg. The neutron is close to the parameters of this particle (1.674 927 471(21) 10 -27 kg). But the mass of an electron is thousands of times smaller, it is considered negligible and is not taken into account. That is why, knowing the superscript of an element in chemistry, it is not difficult to find out the composition of the nucleus of isotopes.

Isotopes of hydrogen

The isotopes of certain elements are so well known and common in nature that they have received their own names. The clearest and simplest example of this is hydrogen. IN vivo it is found in its most abundant variety, protium. This element has a mass number of 1, and its nucleus consists of one proton.

So what are hydrogen isotopes in chemistry? As you know, the atoms of this substance have the first number in the periodic table and, accordingly, are endowed in nature with a charge number of 1. But the number of neutrons in the nucleus of an atom is different for them. Deuterium, being heavy hydrogen, in addition to the proton, has one more particle in the nucleus, that is, the neutron. As a result, this substance exhibits its own physical properties, unlike protium, having its own weight, melting point and boiling point.

Tritium

Tritium is the most complex of all. This is superheavy hydrogen. According to the definition of isotopes in chemistry, it has charge number 1, but the mass number is 3. It is often called a triton, because in addition to one proton, it has two neutrons in the nucleus, that is, it consists of three elements. The name of this element, discovered in 1934 by Rutherford, Oliphant and Harteck, was proposed even before its discovery.

It is an unstable substance exhibiting radioactive properties. Its nucleus has the ability to split with the release of a beta particle and an electron antineutrino. The decay energy of this substance is not very high and amounts to 18.59 keV. Therefore, such radiation is not too dangerous for humans. Ordinary clothing and surgical gloves can protect against it. And this radioactive element obtained with food is quickly excreted from the body.

Isotopes of uranium

Much more dangerous different types uranium, of which 26 are known to science today. Therefore, when talking about what isotopes are in chemistry, it is impossible not to mention this element. Despite the variety of types of uranium, only three of its isotopes occur in nature. These include 234 U, 235 U, 238 U. The first of them, having suitable properties, is actively used as fuel in nuclear reactors. And the latter - for the production of plutonium-239, which itself, in turn, is indispensable as the most valuable fuel.

Each of the radioactive elements is characterized by its own. This is the length of time during which the substance splits in the ratio of ½. That is, as a result of this process, the amount of the preserved part of the substance is halved. This period of time for uranium is huge. For example, for the isotope-234, it is estimated at 270 millennia, and for the other two indicated varieties, it is much more significant. The record half-life is that of uranium-238, lasting billions of years.

Nuclides

Not each of the types of atom, characterized by its own and strictly a certain number protons and electrons, is so stable that there is at least some long period sufficient for its study. Those that are relatively stable are called nuclides. Stable formations of this kind do not undergo radioactive decay. Unstable are called radionuclides and, in turn, are also divided into short-lived and long-lived. As is known from grade 11 chemistry lessons about the structure of isotope atoms, osmium and platinum have the largest number of radionuclides. Cobalt and gold have one stable each, and the largest number stable nuclides in tin.

Calculation of the serial number of the isotope

Now let's try to summarize the information described earlier. Having understood what isotopes are in chemistry, it's time to figure out how you can use the knowledge gained. Consider it on specific example. Suppose it is known that a certain chemical element has a mass number of 181. At the same time, the shell of an atom of a given substance contains 73 electrons. How can, using the periodic table, find out the name given element, as well as the number of protons and neutrons in its nucleus?

Let's start solving the problem. You can determine the name of a substance by knowing its serial number, which corresponds to the number of protons. Since the number of positive and negative charges in an atom is equal, it is 73. So, this is tantalum. Moreover, the total number of nucleons in total is 181, which means that the protons of this element are 181 - 73 = 108. Quite simply.

Isotopes of gallium

The element gallium in has an atomic number of 71. In nature, this substance has two isotopes - 69 Ga and 71 Ga. How to determine the percentage of varieties of gallium?

Solving problems on isotopes in chemistry is almost always associated with information that can be obtained from the periodic table. This time, you should do the same. Let us determine the average atomic mass from the specified source. It is equal to 69.72. Denoting for x and y the quantitative ratio of the first and second isotopes, we take their sum equal to 1. So, in the form of an equation, this will be written: x + y = 1. It follows that 69x + 71y = 69.72. Expressing y in terms of x and substituting the first equation into the second, we get that x = 0.64 and y = 0.36. This means that 69 Ga is contained in nature 64%, and the percentage of 71 Ga is 34%.

Isotope transformations

The radioactive fission of isotopes with their transformation into other elements is divided into three main types. The first of these is alpha decay. It occurs with the emission of a particle, which is the nucleus of a helium atom. That is, this formation, consisting of a set of pairs of neutrons and protons. Since the number of the latter determines the charge number and the number of an atom of a substance in the periodic system, as a result of this process, a qualitative transformation of one element into another occurs, and in the table it shifts to the left by two cells. In this case, the mass number of the element is reduced by 4 units. We know this from the structure of atoms of isotopes.

When the nucleus of an atom loses a beta particle, which is essentially an electron, its composition changes. One of the neutrons is transformed into a proton. This means that the qualitative characteristics of the substance change again, and the element moves one cell to the right in the table, practically without losing mass. Typically, such a transformation is associated with electromagnetic gamma radiation.

Radium isotope conversion

The above information and knowledge from grade 11 chemistry about isotopes again helps to solve practical problems. For example, the following: 226 Ra during decay turns into a chemical element of group IV, which has a mass number of 206. How many alpha and beta particles should it lose in this case?

Given the changes in the mass and group of the daughter element, using the periodic table, it is easy to determine that the isotope formed during the fission will be lead with a charge of 82 and a mass number of 206. And given the charge number of this element and the original radium, it should be assumed that its nucleus lost five alpha -particles and four beta particles.

Use of radioactive isotopes

Everyone is well aware of the harm that radioactive radiation can cause to living organisms. However, the properties of radioactive isotopes are useful for humans. They are successfully used in many industries. With their help, it is possible to detect leaks in engineering and building structures, underground pipelines and oil pipelines, storage tanks, heat exchangers in power plants.

These properties are also actively used in scientific experiments. For example, the tsetse fly is a carrier of many serious diseases for humans, livestock and domestic animals. In order to prevent this, the males of these insects are sterilized by means of weak radioactive radiation. Isotopes are also indispensable in the study of the mechanisms of some chemical reactions, because the atoms of these elements can label water and other substances.

In biological research, labeled isotopes are often also used. For example, it was in this way that it was established how phosphorus affects the soil, growth and development cultivated plants. With success, the properties of isotopes are also used in medicine, which made it possible to treat cancerous tumors and others severe illness, determine the age of biological organisms.

When studying the properties of radioactive elements, it was found that atoms with different nuclear masses can be found in the same chemical element. At the same time, they have the same nuclear charge, that is, these are not impurities of third-party substances, but the same substance.

What are isotopes and why do they exist

In Mendeleev's periodic system, both a given element and atoms of a substance with a different mass of the nucleus occupy one cell. Based on the above, such varieties of the same substance were given the name "isotopes" (from the Greek isos - the same and topos - place). So, isotopes- these are varieties of a given chemical element, differing in the mass of atomic nuclei.

According to the accepted neutron roton model of the nucleus explain the existence of isotopes as follows: the nuclei of some atoms of matter contain a different number of neutrons, but the same number of protons. In fact, the nuclear charge of the isotopes of one element is the same, therefore, the number of protons in the nucleus is the same. Nuclei differ in mass, respectively, they contain a different number of neutrons.

Stable and unstable isotopes

Isotopes are either stable or unstable. To date, about 270 stable isotopes and more than 2000 unstable ones are known. stable isotopes are varieties chemical elements that can exist on their own for a long time.

Most of unstable isotopes was obtained artificially. Unstable isotopes are radioactive, their nuclei are subject to the process of radioactive decay, that is, spontaneous transformation into other nuclei, accompanied by the emission of particles and / or radiation. Almost all radioactive artificial isotopes have very short half-lives, measured in seconds and even fractions of seconds.

How many isotopes can a nucleus contain

The nucleus cannot contain an arbitrary number of neutrons. Accordingly, the number of isotopes is limited. Even in the number of protons elements, the number of stable isotopes can reach ten. For example, tin has 10 isotopes, xenon has 9, mercury has 7, and so on.

Those elements the number of protons is odd, can only have two stable isotopes. Some elements have only one stable isotope. These are substances such as gold, aluminum, phosphorus, sodium, manganese and others. Such variations in the number of stable isotopes for different elements are associated with a complex dependence of the number of protons and neutrons on the binding energy of the nucleus.

Almost all substances in nature exist as a mixture of isotopes. The number of isotopes in the composition of a substance depends on the type of substance, atomic mass and the number of stable isotopes of a given chemical element.

It has been established that every chemical element found in nature is a mixture of isotopes (hence they have fractional atomic masses). To understand how isotopes differ from one another, it is necessary to consider in detail the structure of the atom. An atom forms a nucleus and an electron cloud. The mass of an atom is influenced by the electrons moving at a staggering speed in orbits in the electron cloud, the neutrons and protons that make up the nucleus.

What are isotopes

isotopes A type of atom of a chemical element. There are always equal numbers of electrons and protons in any atom. Since they have opposite charges (electrons are negative and protons are positive), an atom is always neutral (this elementary particle does not carry a charge, it is equal to zero). When an electron is lost or captured, the atom loses its neutrality, becoming either a negative or a positive ion.
Neutrons have no charge, but their number in the atomic nucleus of the same element can be different. This does not affect the neutrality of the atom, but it does affect its mass and properties. For example, each isotope of a hydrogen atom has one electron and one proton each. And the number of neutrons is different. Protium has only 1 neutron, deuterium has 2 neutrons, and tritium has 3 neutrons. These three isotopes differ markedly from each other in properties.

Comparison of isotopes

How are isotopes different? They have a different number of neutrons, different masses and different properties. Isotopes have the same structure electron shells. This means that they are quite similar in chemical properties. Therefore, they are assigned one place in the periodic system.
Stable and radioactive (unstable) isotopes have been found in nature. The nuclei of atoms of radioactive isotopes are able to spontaneously transform into other nuclei. In the process of radioactive decay, they emit various particles.
Most elements have over two dozen radioactive isotopes. In addition, radioactive isotopes are artificially synthesized for absolutely all elements. In a natural mixture of isotopes, their content fluctuates slightly.
The existence of isotopes made it possible to understand why, in some cases, elements with a lower atomic mass have a higher serial number than elements with a larger atomic mass. For example, in an argon-potassium pair, argon includes heavy isotopes, and potassium includes light isotopes. Therefore, the mass of argon is greater than that of potassium.

ImGist determined that the difference between isotopes from each other is as follows:

They possess different number neutrons.
Isotopes have different mass atoms.
The value of the mass of atoms of ions affects their total energy and properties.

The content of the article

ISOTOPS Varieties of the same chemical element that are similar in their physical and chemical properties but with different atomic masses. The name "isotopes" was proposed in 1912 by the English radiochemist Frederick Soddy, who formed it from two Greek words: isos - the same and topos - place. Isotopes occupy the same place in the cell periodic system elements of Mendeleev.

An atom of any chemical element consists of a positively charged nucleus and a cloud of negatively charged electrons surrounding it. The position of a chemical element in the periodic system of Mendeleev (its serial number) is determined by the charge of the nucleus of its atoms. isotopes are called therefore varieties of the same chemical element whose atoms have the same nuclear charge (and, therefore, almost the same electron shells), but differ in the values ​​of the mass of the nucleus. According to the figurative expression of F. Soddy, the atoms of isotopes are the same "outside", but different "inside".

The neutron was discovered in 1932 – a particle that has no charge, with a mass close to the mass of the nucleus of a hydrogen atom - a proton , and created proton-neutron model of the nucleus. As a result in science, the final modern definition concepts of isotopes: isotopes are substances whose atomic nuclei consist of the same number of protons and differ only in the number of neutrons in the nucleus . Each isotope is usually denoted by a set of symbols , where X is the symbol of a chemical element, Z is the charge of the atomic nucleus (the number of protons), A is the mass number of the isotope ( total number nucleons - protons and neutrons in the nucleus, A = Z + N). Since the charge of the nucleus is unambiguously associated with the symbol of the chemical element, often the notation A X is simply used for abbreviation.

Of all the isotopes known to us, only the isotopes of hydrogen have their own names. Thus, the 2 H and 3 H isotopes are called deuterium and tritium and are designated D and T, respectively (the 1 H isotope is sometimes called protium).

They occur naturally as stable isotopes. , and unstable - radioactive, the nuclei of atoms of which are subject to spontaneous transformation into other nuclei with the emission of various particles (or the processes of the so-called radioactive decay). Now about 270 stable isotopes are known, and stable isotopes are found only in elements with atomic number Z Ј 83. The number of unstable isotopes exceeds 2000, the vast majority of them were obtained artificially as a result of various nuclear reactions. The number of radioactive isotopes in many elements is very large and can exceed two dozen. The number of stable isotopes is much less. Some chemical elements consist of only one stable isotope (beryllium, fluorine, sodium, aluminum, phosphorus, manganese, gold and a number of other elements). The largest number of stable isotopes - 10 - was found in tin, in iron, for example, there are 4 of them, and in mercury - 7.

Discovery of isotopes, historical background.

In 1808, the English naturalist John Dalton first introduced the definition of a chemical element as a substance consisting of atoms of one kind. In 1869, the chemist DIMendeleev discovered the periodic law of chemical elements. One of the difficulties in substantiating the concept of an element as a substance that occupies a certain place in the cell of the periodic system was the experimentally observed non-integer atomic weights of elements. In 1866, the English physicist and chemist, Sir William Crookes, put forward the hypothesis that each natural chemical element is a mixture of substances that are identical in their properties, but have different atomic masses, but at that time such an assumption had not yet been experimentally confirmed and therefore little seen.

An important step towards the discovery of isotopes was the discovery of the phenomenon of radioactivity and the hypothesis of radioactive decay formulated by Ernst Rutherford and Frederick Soddy: radioactivity is nothing more than the decay of an atom into a charged particle and an atom of another element, which differs in its chemical properties from the original one. As a result, the concept of radioactive series or radioactive families arose. , at the beginning of which there is the first parent element, which is radioactive, and at the end - the last stable element. An analysis of the chains of transformations showed that in their course one and the same radioactive elements, differing only in atomic masses, can appear in one cell of the periodic system. In fact, this meant the introduction of the concept of isotopes.

Independent confirmation of the existence of stable isotopes of chemical elements was then obtained in the experiments of J. J. Thomson and Aston in 1912-1920 with beams of positively charged particles (or so-called canal rays ) emerging from the discharge tube.

In 1919 Aston designed an instrument called the mass spectrograph. (or mass spectrometer) . The discharge tube was still used as the ion source, but Aston found a way in which the successive deflection of the particle beam in the electrical and magnetic fields led to the focusing of particles with the same value charge-to-mass ratio (regardless of their speed) at the same point on the screen. Along with Aston, a mass spectrometer of a slightly different design was created in the same years by the American Dempster. As a result of the subsequent use and improvement of mass spectrometers by the efforts of many researchers, by 1935 an almost complete table of the isotopic compositions of all chemical elements known by that time was compiled.

Isotope separation methods.

To study the properties of isotopes, and especially to use them for scientific and applied purposes, it is necessary to obtain them in more or less noticeable quantities. In conventional mass spectrometers, almost complete separation of isotopes is achieved, but their number is negligible. Therefore, the efforts of scientists and engineers were directed to the search for other possible methods isotope separation. First of all, physical and chemical separation methods were mastered, based on differences in such properties of isotopes of the same element as evaporation rates, equilibrium constants, rates of chemical reactions, etc. The most effective among them were the methods of rectification and isotope exchange, which are widely used in the industrial production of isotopes of light elements: hydrogen, lithium, boron, carbon, oxygen and nitrogen.

Another group of methods is formed by the so-called molecular-kinetic methods: gaseous diffusion, thermal diffusion, mass diffusion (diffusion in a vapor flow), and centrifugation. Gas diffusion methods based on different diffusion rates of isotopic components in highly dispersed porous media were used during the Second World War to organize industrial production separation of uranium isotopes in the United States in the framework of the so-called Manhattan project to create atomic bomb. For getting required quantities uranium, enriched up to 90% with the light isotope 235 U - the main "combustible" component of the atomic bomb, plants were built, occupying an area of ​​​​about four thousand hectares. More than 2 billion dollars were allocated for the creation of an atomic center with plants for the production of enriched uranium. After the war, plants for the production of enriched uranium for military purposes, also based on the diffusion method of separation, were developed and built in the USSR. IN last years this method has given way to a more efficient and less costly centrifugation method. In this method, the effect of separation of the isotope mixture is achieved due to the different action of centrifugal forces on the components of the isotope mixture that fills the centrifuge rotor, which is a thin-walled cylinder limited from above and below, rotating with a very high speed in vacuum chamber. Hundreds of thousands of centrifuges connected in cascades, the rotor of each of which makes more than a thousand revolutions per second, are currently used in modern separation plants both in Russia and in other developed countries of the world. Centrifuges are used for more than just getting the enriched uranium needed to run nuclear reactors nuclear power plants, but also for the production of isotopes of about thirty chemical elements of the middle part of the periodic table. For the separation of various isotopes, electromagnetic separation plants with powerful ion sources are also used; in recent years, laser methods separation.

The use of isotopes.

Various isotopes of chemical elements are widely used in scientific research, in various fields of industry and agriculture, in nuclear power, modern biology and medicine, in research environment and other areas. In scientific research (for example, in chemical analysis), as a rule, small amounts of rare isotopes of various elements are required, calculated in grams and even milligrams per year. At the same time, for a number of isotopes widely used in nuclear power engineering, medicine, and other industries, the need for their production can be many kilograms and even tons. Thus, in connection with the use of heavy water D 2 O in nuclear reactors, its global production by the beginning of the 1990s of the last century was about 5000 tons per year. The hydrogen isotope deuterium, which is part of heavy water, the concentration of which in the natural mixture of hydrogen is only 0.015%, along with tritium, in the future, according to scientists, will become the main fuel component of power thermonuclear reactors operating on the basis of nuclear fusion reactions. In this case, the need for the production of hydrogen isotopes will be enormous.

In scientific research, stable and radioactive isotopes are widely used as isotope indicators (labels) in the study of various processes occurring in nature.

IN agriculture isotopes ("labeled" atoms) are used, for example, to study the processes of photosynthesis, the digestibility of fertilizers, and to determine the efficiency of the use of nitrogen, phosphorus, potassium, trace elements, and other substances by plants.

Isotope technologies are widely used in medicine. So in the US, according to statistics, more than 36 thousand medical procedures are performed per day and about 100 million laboratory tests using isotopes. The most common procedures associated with computed tomography. The carbon isotope C 13 enriched up to 99% (natural content about 1%) is actively used in the so-called "diagnostic control of breathing". The essence of the test is very simple. The enriched isotope is introduced into the patient's food and, after participating in the metabolic process in various organs of the body, is released as carbon dioxide CO 2 exhaled by the patient, which is collected and analyzed using a spectrometer. The difference in the rates of processes associated with the release of various amounts of carbon dioxide labeled with the isotope C 13 makes it possible to judge the state of various organs of the patient. In the US, the number of patients who will undergo this test is estimated at 5 million people a year. Laser separation methods are now used to produce the highly enriched C 13 isotope on an industrial scale.

Vladimir Zhdanov

Studying the phenomenon of radioactivity, scientists in the first decade of the XX century. discovered a large number of radioactive substances - about 40. There were significantly more of them than free places in the periodic table of elements in the interval between bismuth and uranium. The nature of these substances has been controversial. Some researchers considered them to be independent chemical elements, but in this case the question of their placement in the periodic table turned out to be insoluble. Others generally denied them the right to be called elements in the classical sense. In 1902, the English physicist D. Martin called such substances radioelements. As they were studied, it turned out that some radio elements have exactly the same Chemical properties, but differ in magnitude atomic masses. This circumstance was contrary to the fundamental principles periodic law. The English scientist F. Soddy resolved the contradiction. In 1913, he called chemically similar radioelements isotopes (from the Greek words meaning "same" and "place"), i.e., occupying the same place in the periodic system. Radioelements turned out to be isotopes of natural radioactive elements. All of them are combined into three radioactive families, the ancestors of which are the isotopes of thorium and uranium.

Isotopes of oxygen. Isobars of potassium and argon (isobars are atoms of different elements with the same mass number).

Number of stable isotopes for even and odd elements.

It soon became clear that other stable chemical elements also have isotopes. The main merit in their discovery belongs to the English physicist F. Aston. He discovered stable isotopes in many elements.

FROM modern point Isotopes are varieties of atoms of a chemical element: they have different atomic masses, but the same nuclear charge.

Their nuclei thus contain the same number protons, but different number neutrons. For example, natural oxygen isotopes with Z = 8 contain 8, 9, and 10 neutrons in their nuclei, respectively. The sum of the numbers of protons and neutrons in the nucleus of an isotope is called the mass number A. Therefore, the mass numbers of the indicated oxygen isotopes are 16, 17 and 18. The following designation of isotopes is now accepted: the Z value is given at the bottom left of the element symbol, the A value is given at the top left. For example: 16 8 O, 17 8 O, 18 8 O.

After the discovery of the phenomenon of artificial radioactivity, about 1800 artificial radioactive isotopes were obtained using nuclear reactions for elements with Z from 1 to 110. The vast majority of artificial radioisotopes have very short half-lives, measured in seconds and fractions of seconds; only a few have relatively longer duration life (for example, 10 Be - 2.7 10 6 years, 26 Al - 8 10 5 years, etc.).

Stable elements are present in nature with about 280 isotopes. However, some of them turned out to be slightly radioactive, with huge half-lives (for example, 40 K, 87 Rb, 138 La, l47 Sm, 176 Lu, 187 Re). The lifetime of these isotopes is so long that they can be considered stable.

There are still many problems in the world of stable isotopes. So, it is not clear why their number in different elements varies so much. About 25% of stable elements (Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pt, Tb, Ho, Tu, Ta, Au) are present in nature only one kind of atom. These are the so-called single elements. Interestingly, all of them (except Be) have odd Z values. In general, for odd elements, the number of stable isotopes does not exceed two. On the contrary, some elements with even Z consist of a large number isotopes (for example, Xe has 9, Sn - 10 stable isotopes).

The set of stable isotopes of a given element is called a galaxy. Their content in the galaxy often fluctuates greatly. It is interesting to note that the abundance of isotopes with mass numbers that are multiples of four (12 C, 16 O, 20 Ca, etc.) is the highest, although there are exceptions to this rule.

The discovery of stable isotopes made it possible to solve the long-term mystery of atomic masses - their deviation from integers, due to the different percentages of stable isotopes of elements in the galaxy.

IN nuclear physics the concept of "isobar" is known. Isobars are called isotopes of various elements (i.e., with different values Z) having the same mass numbers. The study of isobars contributed to the establishment of many important regularities in the behavior and properties of atomic nuclei. One of these regularities is expressed by the rule formulated by the Soviet chemist S. A. Shchukarev and the Yemenite physicist I. Mattauch. It says: if the two isobars differ in Z values ​​by 1, then one of them will necessarily be radioactive. A classic example of a pair of isobars is 40 18 Ar - 40 19 K. In it, the potassium isotope is radioactive. The Shchukarev-Mattauch rule made it possible to explain why the elements technetium (Z = 43) and promethium (Z = 61) have no stable isotopes. Since they have odd Z values, more than two stable isotopes could not be expected for them. But it turned out that the isobars of technetium and promethium, respectively, the isotopes of molybdenum (Z = 42) and ruthenium (Z = 44), neodymium (Z = 60) and samarium (Z = 62), are represented in nature by stable varieties of atoms in a wide range of mass numbers . Thus, physical laws impose a ban on the existence of stable isotopes of technetium and promethium. That is why these elements do not actually exist in nature and they had to be synthesized artificially.

Scientists have long been trying to develop a periodic system of isotopes. Of course, it is based on other principles than the basis of the periodic system of elements. But these attempts have not yet led to satisfactory results. True, physicists have proved that the sequence of filling proton and neutron shells in atomic nuclei in principle it is similar to the construction of electron shells and subshells in atoms (see Atom).

The electron shells of the isotopes of a given element are built in exactly the same way. Therefore, their chemical and physical properties are almost identical. Only the isotopes of hydrogen (protium and deuterium) and their compounds show noticeable differences in properties. For example, heavy water (D 2 O) freezes at +3.8, boils at 101.4 ° C, has a density of 1.1059 g / cm 3, does not support the life of animal and plant organisms. During the electrolysis of water into hydrogen and oxygen, H 2 0 molecules are predominantly decomposed, while heavy water molecules remain in the electrolyzer.

The separation of isotopes of other elements is an extremely difficult task. However, in many cases, isotopes are needed individual elements with a significant change compared to the natural content. For example, when solving the problem of atomic energy, it became necessary to separate the isotopes 235 U and 238 U. For this purpose, the mass spectrometry method was first applied, with the help of which the first kilograms of uranium-235 were obtained in 1944 in the USA. However, this method turned out to be too expensive and was replaced by the gaseous diffusion method, which used UF 6 . Now there are several methods for separating isotopes, but all of them are quite complex and expensive. Nevertheless, the problem of “separation of the inseparable” is being successfully solved.

A new scientific discipline appeared - the chemistry of isotopes. It studies the behavior of various isotopes of chemical elements in chemical reactions and isotope exchange processes. As a result of these processes, the isotopes of a given element are redistributed between the reacting substances. Here the simplest example: H 2 0 + HD = HD0 + H 2 (a water molecule exchanges a protium atom for a deuterium atom). The geochemistry of isotopes is also developing. It investigates fluctuations in the isotopic composition of various elements in the earth's crust.

The most widely used are the so-called labeled atoms - artificial radioactive isotopes of stable elements or stable isotopes. With the help of isotope indicators - labeled atoms - they study the ways of movement of elements in inanimate and living nature, the nature of the distribution of substances and elements in various objects. Isotopes are used in nuclear technology: as materials for the construction of nuclear reactors; as a nuclear fuel (isotopes of thorium, uranium, plutonium); in thermonuclear fusion (deuterium, 6 Li, 3 He). Radioactive isotopes are also widely used as radiation sources.