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 electrons moving at stunning speeds through orbitals in the electron cloud, neutrons and protons that make up the nucleus.
Definition
Isotopes is 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), the atom is always neutral (this elementary particle does not carry a charge, it is zero). When an electron is lost or captured, an atom loses 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 vary. This does not in any way affect the neutrality of the atom, but it does affect its mass and properties. For example, any isotope of a hydrogen atom contains one electron and one proton. But 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
They have different numbers of neutrons, different masses and different properties. Isotopes have identical structures of electron shells. This means that they are quite similar in chemical properties. Therefore, they are given one place in the periodic table.
Stable and radioactive (unstable) isotopes have been found in nature. The nuclei of atoms of radioactive isotopes are capable of spontaneously transforming into other nuclei. During 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 varies slightly.
The existence of isotopes made it possible to understand why, in some cases, elements with lower atomic mass have a higher atomic number than elements with higher atomic mass. For example, in the argon-potassium pair, argon includes heavy isotopes, and potassium contains light isotopes. Therefore, the mass of argon is greater than that of potassium.
Conclusions website
- They have different numbers of neutrons.
- Isotopes have different atomic masses.
- The value of the mass of ion atoms affects their total energy and properties.
Repeat the main points of the topic “Basic concepts of chemistry” and solve the proposed problems. Use Nos. 6-17.
Basic provisions
1. Substance(simple and complex) is any collection of atoms and molecules located in a certain state of aggregation.
Transformations of substances accompanied by changes in their composition and (or) structure are called chemical reactions .
2. Structural units substances:
· Atom- the smallest electrically neutral particle of a chemical element or simple substance, possessing all its chemical properties and then physically and chemically indivisible.
· Molecule- the smallest electrically neutral particle of a substance, possessing all its chemical properties, physically indivisible, but chemically divisible.
3. Chemical element - This is a type of atom with a certain nuclear charge.
4. Compound atom :
|
Particle |
How to determine? |
Charge |
Weight |
||
|
Cl |
conventional units |
a.e.m. |
|||
|
Electron |
By ordinal Number (N) |
1.6 ∙ 10 -19 |
9.10 ∙ 10 -28 |
0.00055 |
|
|
Proton |
By ordinal number (N) |
1.6 ∙ 10 -19 |
1.67 ∙ 10 -24 |
1.00728 |
|
|
Neutron |
Ar–N |
1.67 ∙ 10 -24 |
1.00866 |
||
5. Compound atomic nucleus :
The nucleus contains elementary particles ( nucleons) –
protons(1 1 p ) and neutrons(1 0 n ).
· Because Almost all the mass of an atom is concentrated in the nucleus and m p ≈ m n≈ 1 amu, That rounded valueA rof a chemical element is equal to the total number of nucleons in the nucleus.
7. Isotopes- a variety of atoms of the same chemical element, differing from each other only in their mass.
· Isotopic notation: to the left of the element symbol indicate the mass number (top) and atomic number of the element (bottom)
· Why do isotopes have different masses?
Assignment: Determine the atomic composition of chlorine isotopes: 35 17Cland 37 17Cl?
· Isotopes have different masses due to different numbers of neutrons in their nuclei.
8. In nature, chemical elements exist in the form of mixtures of isotopes.
The isotopic composition of the same chemical element is expressed in atomic fractions(ω at.), which indicate what part the number of atoms of a given isotope makes up of the total number of atoms of all isotopes of a given element, taken as one or 100%.
For example:
ω at (35 17 Cl) = 0.754
ω at (37 17 Cl) = 0.246
9. The periodic table shows the average values of the relative atomic masses of chemical elements, taking into account their isotopic composition. Therefore, Ar indicated in the table are fractional.
A rWed= ω at.(1) ∙ Ar (1) + … + ω at.(n ) ∙ Ar ( n )
For example:
A rWed(Cl) = 0.754 ∙ 35 + 0.246 ∙ 37 = 35.453
10. Problem to solve:
No. 1. Determine the relative atomic mass of boron if it is known that the molar fraction of the 10 B isotope is 19.6%, and the 11 B isotope is 80.4%.
11. The masses of atoms and molecules are very small. Currently, a unified measurement system has been adopted in physics and chemistry.
1 amu =m(a.u.m.) = 1/12 m(12 C) = 1.66057 ∙ 10 -27 kg = 1.66057 ∙ 10 -24 g.
Absolute masses of some atoms:
m( C) =1.99268 ∙ 10 -23 g
m( H) =1.67375 ∙ 10 -24 g
m( O) =2.656812 ∙ 10 -23 g
A r– shows how many times a given atom is heavier than 1/12 of a 12 C atom. M r∙ 1.66 ∙ 10 -27 kg
13. The number of atoms and molecules in ordinary samples of substances is very large, therefore, when characterizing the amount of a substance, the unit of measurement is used -mole .
· Mole (ν)– a unit of quantity of a substance that contains the same number of particles (molecules, atoms, ions, electrons) as there are atoms in 12 g of isotope 12 C
· Mass of 1 atom 12 C is equal to 12 amu, so the number of atoms in 12 g of isotope 12 C equals:
N A= 12 g / 12 ∙ 1.66057 ∙ 10 -24 g = 6.0221 ∙ 10 23
· Physical quantity N A called Avogadro's constant (Avogadro's number) and has the dimension [N A] = mol -1.
14. Basic formulas:
M = M r = ρ ∙ Vm(ρ – density; V m – volume at zero level)
Problems to solve independently
No. 1. Calculate the number of nitrogen atoms in 100 g of ammonium carbonate containing 10% non-nitrogen impurities.
No. 2. Under normal conditions, 12 liters of a gas mixture consisting of ammonia and carbon dioxide have a mass of 18 g. How many liters of each gas does the mixture contain?
No. 3. When exposed to excess hydrochloric acid, 8.24 g of a mixture of manganese oxide (IV) with the unknown oxide MO 2, which does not react with hydrochloric acid, 1.344 liters of gas were obtained at ambient conditions. In another experiment, it was established that the molar ratio of manganese oxide (IV) to the unknown oxide is 3:1. Determine the formula of the unknown oxide and calculate its mass fraction in the mixture.
Studying the phenomenon of radioactivity, scientists in the first decade of the 20th century. discovered a large number of radioactive substances - about 40. There were significantly more of them than there were free places in the periodic table of elements 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 became clear that some radioelements have exactly the same chemical properties, but differ in atomic masses. This circumstance contradicted the basic provisions of the periodic law. The English scientist F. Soddy resolved the contradiction. In 1913, he called chemically similar radioelements isotopes (from Greek words meaning “same” and “place”), that is, they occupy the same place in the periodic table. The radioelements turned out to be isotopes of natural radioactive elements. All of them are combined into three radioactive families, the ancestors of which are 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 credit for their discovery belongs to the English physicist F. Aston. He discovered stable isotopes of many elements.
From a modern point of view, 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 of protons, but different numbers of neutrons. For example, natural isotopes of oxygen 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. Consequently, the mass numbers of the indicated oxygen isotopes are 16, 17 and 18. Nowadays, the following designation for isotopes is accepted: the value Z is given below to the left of the element symbol, the value A is given to the upper left. For example: 16 8 O, 17 8 O, 18 8 O.
Since the discovery of the phenomenon of artificial radioactivity, approximately 1,800 artificial radioactive isotopes have been produced 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 a relatively long life expectancy (for example, 10 Be - 2.7 10 6 years, 26 Al - 8 10 5 years, etc.).
Stable elements are represented in nature by approximately 280 isotopes. However, some of them turned out to be weakly radioactive, with huge half-lives (for example, 40 K, 87 Rb, 138 La, l47 Sm, 176 Lu, 187 Re). The lifespan of these isotopes is so long that they can be considered stable.
There are still many challenges in the world of stable isotopes. Thus, it is unclear why their number varies so greatly among different elements. 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 type of atom. These are the so-called single elements. It is interesting that all of them (except Be) have odd Z values. In general, for odd elements the number of stable isotopes does not exceed two. In contrast, some even-Z elements consist of a large number of isotopes (for example, Xe has 9, Sn has 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 highest content is of isotopes with mass numbers that are multiples of four (12 C, 16 O, 20 Ca, etc.), although there are exceptions to this rule.
The discovery of stable isotopes made it possible to solve the long-standing mystery of atomic masses - their deviation from whole numbers, explained by the different percentages of stable isotopes of elements in the galaxy.
In nuclear physics the concept of “isobars” is known. Isobars are isotopes of different elements (that is, with different Z values) that have the same mass numbers. The study of isobars contributed to the establishment of many important patterns in the behavior and properties of atomic nuclei. One of these patterns is expressed by the rule formulated by the Soviet chemist S. A. Shchukarev and the German physicist I. Mattauch. It says: if two isobars differ in Z values by 1, then one of them will definitely 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 there are no stable isotopes in the elements technetium (Z = 43) and promethium (Z = 61). 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 prohibit the existence of stable isotopes of technetium and promethium. This is why these elements do not actually exist in nature and had to be synthesized artificially.
Scientists have long been trying to develop a periodic system of isotopes. Of course, it is based on different principles than the basis of the periodic table of elements. But these attempts have not yet led to satisfactory results. True, physicists have proven that the sequence of filling proton and neutron shells in atomic nuclei is, in principle, similar to the construction of electron shells and subshells in atoms (see Atom).
The electron shells of isotopes of a given element are constructed in exactly the same way. Therefore, their chemical and physical properties are almost identical. Only hydrogen isotopes (protium and deuterium) and their compounds exhibit 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, and does not support the life of animals and plant organisms. During the electrolysis of water into hydrogen and oxygen, predominantly H 2 0 molecules are decomposed, while heavy water molecules remain in the electrolyzer.
Separating isotopes of other elements is an extremely difficult task. However, in many cases, isotopes of individual elements with significantly altered abundances compared to natural abundance are required. 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 used, with the help of which the first kilograms of uranium-235 were obtained in the USA in 1944. However, this method proved to be too expensive and was replaced by the gas diffusion method, which used UF 6. There are now several methods for separating isotopes, but they are all quite complex and expensive. And yet the problem of “dividing the inseparable” is being successfully solved.
A new scientific discipline has emerged - isotope chemistry. She 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 is 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. She studies variations in the isotopic composition of different elements in the earth's crust.
The most widely used are so-called labeled atoms - artificial radioactive isotopes of stable elements or stable isotopes. With the help of isotopic indicators - labeled atoms - they study the paths 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 nuclear fuel (isotopes of thorium, uranium, plutonium); in thermonuclear fusion (deuterium, 6 Li, 3 He). Radioactive isotopes are also widely used as radiation sources.
ISOTOPES(Greek, isos equal, identical + topos place) - varieties of the same chemical element, occupying the same place in Mendeleev’s periodic table of elements, i.e. having the same nuclear charge, but differing in atomic masses. When mentioning I., be sure to indicate which isotope of the chemical. element he is. The term "isotope" is sometimes used in a broader sense - to describe atoms of various elements. However, to designate any of the atoms, regardless of its belonging to a particular element, it is customary to use the term “nuclide”.
I.'s belonging to a specific element and basic chemicals. properties are determined by its atomic number Z or the number of protons contained in the nucleus (respectively, the same number of electrons in the shell of the atom), and its nuclear physical. properties are determined by the totality and ratio of the number of protons and neutrons included in it. Each nucleus consists of Z protons and N neutrons, and the total number of these particles, or nucleons, is the mass number A = Z + N, which determines the mass of the nucleus. It is equal to the mass value of a given nuclide rounded to a whole number. Any nuclide, therefore, is determined by the values of Z and N, although some radioactive nuclides with the same Z and N may be in different nuclear energy states and differ in their nuclear physics. properties; such nuclides are called isomers. Nuclides with the same number of protons are called isotopes.
I. are designated by the symbol of the corresponding chemical. element with index A located at the top left - mass number; sometimes the number of protons (Z) is also given at the bottom left. For example, radioactive phosphorus with mass numbers 32 and 33 are designated: 32 P and 33 P or 32 P and 33 P, respectively. When designating I. without indicating the symbol of the element, the mass number is given after the designation of the element, for example. phosphorus-32, phosphorus-33.
I. different elements can have the same mass number. Atoms with different numbers of protons Z and neutrons N, but with the same mass number A are called isobars (for example, 14 32 Si, 15 32 P, 16 32 S, 17 32 Cl isobars).
The name "isotope" was suggested by the English. scientist Soddy (F. Soddy). The existence of iron was first discovered in 1906 during the study of the radioactive decay of heavy naturally radioactive elements; in 1913, they were also discovered in the non-radioactive element neon, and then the isotopic composition of all elements of the periodic system was determined using mass spectrometry. In 1934, I. Joliot-Curie and F. Joliot-Curie first obtained artificially radioactive ionizers of nitrogen, silicon, and phosphorus, and subsequently, using various nuclear reactions on neutrons, charged particles, and high-energy photons, radioactive ionizers of all types were obtained. known elements and synthesized radioactive 13 superheavy - transuranium elements (with Z ≥ 93). There are 280 known stable, characterized by stability, and more than 1,500 radioactive, i.e., unstable, I., which undergo radioactive transformations at one rate or another. The duration of existence of radioactive radiation is characterized by a half-life (see) - a period of time T 1/2, during which the number of radioactive nuclei is halved.
In a natural mixture I. chemical. Different elements are contained in different quantities. The percentage of i. in a given chemical. element is called their relative abundance. So, for example, natural oxygen contains three stable oxygen: 16O (99.759%), 17O (0.037%) and 18O (0.204%). Many chem. elements have only one stable I. (9 Be, 19 F, 23 Na, 31 P, 89 Y, 127 I, etc.), and some (Tc, Pm, Lu and all elements with Z greater than 82) have neither one stable I.
The isotopic composition of naturally occurring elements on our planet (and throughout the solar system) is largely constant, but there are slight variations in the abundance of light element atoms. This is explained by the fact that the differences in the masses of their elements are relatively large, and therefore the isotopic composition of these elements changes under the influence of various natural processes, as a result of isotope effects (i.e., differences in the properties of the chemical substances that contain these isotopes). Thus, the isotopic composition of a number of biologically important elements (H, C, N, O, S) is associated, in particular, with the presence of the biosphere and the vital activity of plant and animal organisms.
Differences in the composition and structure of atomic nuclei of the same chemical. element (different number of neutrons) determines the difference in their nuclear physics. properties, in particular the fact that some of its i. can be stable, while others can be radioactive.
Radioactive transformations. The following types of radioactive transformations are known.
Alpha decay is a spontaneous transformation of nuclei, accompanied by the emission of alpha particles, i.e. two protons and two neutrons forming the helium nucleus 2 4 He. As a result, the charge Z of the original nucleus decreases by 2, and the total number of nuclides or mass number decreases by 4 units, for example:
88 226 Ra -> 86 222 Ra + 2 4 He
In this case, the kinetic energy of the escaping alpha particle is determined by the masses of the initial and final nuclei (taking into account the mass of the alpha particle itself) and their energy state. If the final nucleus is formed in an excited state, then the kinetic energy of the alpha particle decreases somewhat, and if the excited nucleus decays, then the energy of the alpha particle increases accordingly (in this case, so-called long-range alpha particles are formed). The energy spectrum of alpha particles is discrete and lies in the range of 4-9 MeV for approximately 200 I. heavy elements and 2-4.5 MeV for almost 20 alpha radioactive I. rare earth elements.
Beta decay is a spontaneous transformation of nuclei, in which the charge Z of the original nucleus changes by one, but the mass number A remains the same. beta decay is the interconversion of protons (p) and neutrons (n) included in the nucleus, accompanied by the emission or absorption of electrons (e -) or positrons (e +), as well as neutrinos (v) and antineutrinos (v -). There are three types of beta decay:
1) electronic beta decay n -> p + e - + v - , accompanied by an increase in charge Z by 1 unit, with the transformation of one of the neutrons of the nucleus into a proton, for example.
2) positron beta decay p -> n + e + + v, accompanied by a decrease in charge Z by 1 unit, with the transformation of one of the protons of the nucleus into a neutron, for example.
3) electron capture p + e - -> n + v with the simultaneous transformation of one of the protons of the nucleus into a neutron, as in the case of decay with positron emission, also accompanied by a decrease in charge by 1 unit, for example.
In this case, electron capture occurs from one of the electron shells of the atom, most often from the K-shell closest to the nucleus (K-capture).
Beta-minus decay is characteristic of neutron-rich nuclei, in which the number of neutrons is greater than in stable nuclei, and beta-plus decay and, accordingly, electron capture are characteristic of neutron-deficient nuclei, in which the number of neutrons is less than in stable nuclei, or so called beta-stable nuclei. The decay energy is distributed between the beta particle and the neutrino, and therefore the beta spectrum is not discrete, like that of alpha particles, but continuous and contains beta particles with energies from close to zero to a certain Emax, characteristic of each radioactive And Beta-radioactive ions are found in all elements of the periodic table.
Spontaneous fission is the spontaneous decay of heavy nuclei into two (sometimes 3-4) fragments, representing the nuclei of the middle elements of the periodic table (the phenomenon was discovered in 1940 by Soviet scientists G.N. Flerov and K.A. Petrzhak).
Gamma radiation is photon radiation with a discrete energy spectrum that occurs during nuclear transformations, a change in the energy state of atomic nuclei, or during the annihilation of particles. The emission of gamma rays accompanies radioactive transformation in cases where a new nucleus is formed in an excited energy state. The lifetime of such nuclei is determined by nuclear physics. properties of the mother and daughter nuclei, in particular, increases with decreasing energy of gamma transitions and can reach relatively large values for cases of a metastable excited state. The energy of gamma radiation emitted by different lasers ranges from tens of keV to several MeV.
Stability of nuclei. During beta decay, mutual transformations of protons and neutrons occur until the most energetically favorable ratio of p and n is achieved, which corresponds to the stable state of the nucleus. All nuclides are divided with respect to beta decay into beta radioactive and beta stable nuclei. Beta-stable refers to either stable or alpha radioactive nuclides for which beta decay is energetically impossible. All beta-resistant I. in chem. elements with atomic numbers Z up to 83 are stable (with a few exceptions), but heavy elements do not have stable i.s., and all of their beta-stable i.s. are alpha radioactive.
During a radioactive transformation, energy is released corresponding to the ratio of the masses of the initial and final nuclei, the mass and energy of the emitted radiation. The possibility of p-decay occurring without changing the mass number A depends on the ratio of the masses of the corresponding isobars. Isobars with higher mass are transformed into isobars with lower mass as a result of beta decay; Moreover, the smaller the mass of the isobar, the closer it is to the P-stable state. The reverse process, due to the law of conservation of energy, cannot occur. So, for example, for the isobars mentioned above, transformations proceed in the following directions with the formation of the stable isotope of sulfur-32:
The nuclei of beta-decay-resistant nuclides contain at least one neutron for each proton (the exceptions are 1 1 H and 2 3 He), and as the atomic number increases, the N/Z ratio increases and reaches a value of 1.6 for uranium.
As the number N increases, the nucleus of a given element becomes unstable with respect to electron beta-minus decay (with the transformation n->p), therefore neutron-enriched nuclei are beta-active. Accordingly, neutron-deficient nuclei are unstable to positron beta+ decay or electron capture (with the p->n transformation), and alpha decay and spontaneous fission are also observed in heavy nuclei.
Separation of stable and production of artificial radioactive isotopes. Separation of i. is the enrichment of a natural mixture of i. of a given chemical. element by the individual constituents of its composition and the isolation of pure compounds from this mixture. All separation methods are based on isotope effects, i.e., on differences in physical-chemical. properties of different i. and chemicals containing them. compounds (strength of chemical bonds, density, viscosity, heat capacity, melting point, evaporation, diffusion rate, etc.). The methods of separation are based on differences in the behavior of i. and the compounds containing them in physical chemistry. processes. Electrolysis, centrifugation, gas and thermal diffusion, diffusion in a steam flow, rectification, chemical are practically used. and isotope exchanges, electromagnetic separation, laser separation, etc. If a single process produces a low effect, i.e., a low I. separation coefficient, it is repeated many times until a sufficient degree of enrichment is obtained. The separation of light elements is most efficient due to the large relative differences in the masses of their isotopes. For example, “heavy water,” i.e., water enriched with heavy hydrogen-deuterium, the mass of which is twice as large, is produced on an industrial scale in electrolysis plants; The isolation of deuterium by low-temperature distillation is also highly effective. The separation of i. uranium (to obtain nuclear fuel - 235 U) is carried out at gaseous diffusion plants. A wide range of enriched stable iodine is obtained using electromagnetic separation plants. In some cases, separation and enrichment of a mixture of radioactive iron is used, for example, to obtain radioactive iron-55 with high specific activity and radionuclide purity.
Artificially radioactive radiation is obtained as a result of nuclear reactions—the interaction of nuclides with each other and with nuclear particles or photons, as a result of which the formation of other nuclides and particles occurs. A nuclear reaction is conventionally designated as follows: first, the symbol of the initial isotope is indicated, and then the symbol formed as a result of this nuclear reaction. In parentheses between them, the influencing particle is indicated first, and then the emitted particle or radiation quantum (see table, column 2).
The probability of nuclear reactions occurring is quantitatively characterized by the so-called effective cross section (or cross section) of the reaction, denoted by the Greek letter o and expressed in barns (10 -24 cm 2). To produce artificially radioactive nuclides, nuclear reactors (see Nuclear reactors) and charged particle accelerators (see). Many radionuclides used in biology and medicine are produced in a nuclear reactor through nuclear radiation capture reactions, i.e., the capture of a neutron by a nucleus with the emission of a gamma quantum (n, gamma), resulting in the formation of an isotope of the same element with a mass number of unit larger than the original one, for example. 23 Na (n, γ) 24 Na, 31 P(n, γ) 32 P; by reaction (n, γ) with subsequent decay of the resulting radionuclide and the formation of a “daughter”, for example. 130 Te (n, γ) 131 Te -> 131 I; by reactions with the release of charged particles (n, p), (n, 2n), (n, α); eg 14 N (n, p) 14 C; by secondary reactions with tritons (t, p) and (t, n), for example. 7 Li (n, α) 3 H and then 16O (t, n) 18 F; by fission reaction U (n, f), for example. 90 Sr, 133 Xe, etc. (see Nuclear reactions).
Some radionuclides either cannot be produced in a nuclear reactor at all, or such production is irrational for medical purposes. In most cases, the (n, γ) reaction cannot produce isotopes without a carrier; Some reactions have too small a cross-sectional value, and the irradiated targets have a low relative content of the initial isotope in the natural mixture, which leads to low reaction yields and insufficient specific activity of the drugs. Therefore, many important radionuclides used clinically. radiodiagnostics, are obtained with sufficient specific activity using isotope-enriched targets. For example, to obtain calcium-47, a target enriched in calcium-46 from 0.003 to 10-20% is irradiated, to obtain iron-59, a target with iron-58 enriched from 0.31 to 80% is irradiated, to obtain mercury-197 - target with mercury-196, enriched from 0.15 to 40%, etc.
In the reactor ch. arr. radionuclides with an excess of neutrons are obtained, decaying with beta-radiation. Neutron-deficient radionuclides, which are formed in nuclear reactions on charged particles (p, d, alpha) and photons and decay with the emission of positrons or through the capture of electrons, are in most cases produced in cyclotrons, linear accelerators of protons and electrons (in the latter case, bremsstrahlung is used) at energies of accelerated particles of the order of tens and hundreds of MeV. This is how they get it for honey. targets radionuclides by reactions: 51 V (p, n) 51 Cr, 67 Zn (p, n) 67 Ga, 109 Ag (α, 2n) 111 In, 44 Ca (γ, p) 43 K, 68 Zn (γ, p) 67 Cu, etc. An important advantage of this method of obtaining radionuclides is that they, as a rule, have a different chemical. nature than the material of the irradiated target can be isolated from the latter without a carrier. This allows you to obtain the necessary radiopharmaceuticals. drugs with high specific activity and radionuclide purity.
To obtain many short-lived radionuclides directly in clinical institutions, the so-called. isotope generators containing a long-lived parent radionuclide, the decay of which produces the desired short-lived daughter radionuclide, for example. 99m Tc, 87m Sr, 113m In, 132 I. The latter can be repeatedly released from the generator during the lifetime of the parent nuclide (see Generators of radioactive isotopes).
Application of isotopes in biology and medicine. Radioactive and stable ionizers are widely used in scientific research. They are used as a label for the preparation of isotopic indicators (see Labeled compounds) - substances and compounds that have an isotopic composition different from natural ones. Using the method of isotopic indicators, the distribution, paths and nature of movement of labeled substances in various environments and systems are studied, their quantitative analysis is carried out, and the structure of chemicals is studied. compounds and biologically active substances, mechanisms of various dynamic processes, including their metabolism in the body of plants, animals and humans (see Radioisotope research). Using the method of isotope indicators, research is carried out in biochemistry (the study of metabolism, the structure and mechanism of biosynthesis of proteins, nucleic acids, fats and carbohydrates in a living organism, the rate of biochemical reactions, etc.); in physiology (migration of ions and various substances, absorption processes from the gastrointestinal tract of fats and carbohydrates, excretion, blood circulation, behavior and role of microelements, etc.); in pharmacology and toxicology (study of the behavior of drugs and toxic substances, their absorption, pathways and rates of accumulation, distribution, excretion, mechanism of action, etc.); in microbiology, immunology, virology (study of the biochemistry of microorganisms, enzymatic and immunochemical mechanisms, reactions, interactions of viruses and cells, mechanisms of action of antibiotics, etc.); in hygiene and ecology (study of pollution with harmful substances and decontamination of industries and the environment, the ecological chain of various substances, their migration, etc.). I. is also used in other medical biol. research (to study the pathogenesis of various diseases, study early changes in metabolism, etc.).
In honey In practice, radionuclides are used for the diagnosis and treatment of various diseases, as well as for radiation sterilization of honey. materials, products and medicines. Clinics use more than 130 radiodiagnostic and 20 radiotherapeutic techniques using open radiopharmaceuticals. drugs (RP) and sealed isotope radiation sources. For these purposes, St. 60 radionuclides, approx. 30 of them are the most widespread (table). Radiodiagnostic drugs allow you to obtain information about the functions and anatomical state of organs and systems of the human body. The basis of radioisotope diagnostics (see) is the ability to monitor biol, the behavior of chemicals labeled with radionuclides. substances and compounds in a living organism without violating its integrity and changing its functions. Introduction of the desired radioisotope of the corresponding element into the structure of a chemical. a compound, practically without changing its properties, allows one to monitor its behavior in a living organism by external detection of radiation, which is one of the very important advantages of the radioisotope diagnostic method.
Dynamic indicators of the behavior of a labeled compound make it possible to assess the function and condition of the organ or system being studied. Thus, according to the degree of dilution of radiopharmaceuticals with 24 Na, 42 K, 51 Cr, 52 Fe, 131 I, etc. in liquid media, the volume of circulating blood, erythrocytes, albumin, iron exchange, water exchange of electrolytes, etc. are determined. According to indicators of accumulation, movement and removal of radiopharmaceuticals in organs, body systems or in the lesion, you can assess the state of central and peripheral hemodynamics, determine the function of the liver, kidneys, lungs, study iodine metabolism, etc. Radiopharmaceuticals with radioisotopes of iodine and technetium allow you to study all functions of the thyroid gland. Using 99m Tc, 113m In, 123 I, 131 I, 133 Xe, you can conduct a comprehensive study of the lungs - study the distribution of blood flow, the state of ventilation of the lungs and bronchi. Radiopharmaceuticals with 43 K, 86 Rb, 99m Tc, 67 Ga, 131 I, 113m In, 197 Hg, etc. make it possible to determine blood flow and blood supply to the brain, heart, liver, kidneys and other organs. Radioactive colloidal solutions and some organoiodine preparations make it possible to assess the state of polygonal cells and hepatocytes (Kupffer cells) and the antitoxic function of the liver. Using radioisotope scanning, anatomical and topographical study and determination of the presence, size, shape and position of space-occupying lesions of the liver, kidneys, bone marrow, thyroid, parathyroid and salivary glands, lungs, lymph nodes are carried out; radionuclides 18 F, 67 Ga, 85 Sr, 87M Sr, 99M Tc make it possible to study skeletal diseases, etc.
In the USSR, radiation safety standards have been developed and put into effect for patients when using radioactive substances for diagnostic purposes, which strictly regulate these procedures in terms of permissible levels of exposure. Thanks to this, as well as the rational choice of methods and equipment for different types of examinations and the use in radiopharmaceuticals of short-lived radionuclides, which have favorable radiation characteristics in terms of the efficiency of their registration with minimal radiation exposure, radiation loads on the patient’s body during radioisotope diagnostic procedures are much lower doses , obtained during radiographs, examinations, and in most cases do not exceed hundredths and tenths of a rad.
In the 70s 20th century Radioisotope preparations have become increasingly used for in vitro studies, mainly for immunochemical studies. analysis. Radioimmunohim. methods are based on highly specific immunochemical. antigen-antibody reactions, as a result of which a stable complex of antibodies and antigens is formed. After separation of the resulting complex from unreacted antibodies or antigens, quantification is carried out by measuring their radioactivity. Use of antigens or antibodies labeled with radioisotopes, e.g. 125 I, increases the sensitivity of immunochemical. tests tens and hundreds of times. Using these tests, you can determine the content of hormones, antibodies, antigens, enzymes, enzymes, vitamins and other biologically active substances in the body in concentrations of up to 0.1 mg/ml. In this way, it is possible to determine not only various patol conditions, but also very small changes reflecting the initial stages of the disease. For example, these methods are successfully used for early in vitro diagnosis of diabetes mellitus, infectious hepatitis, carbohydrate metabolism disorders, some allergic and a number of other diseases. Such radioisotope tests are not only more sensitive and simpler, but also allow for mass research and are completely safe for patients (see Radioisotope diagnostics).
With lech. For the purpose of radiopharmaceuticals and radionuclide radiation sources, Ch. arr. in oncology, as well as in the treatment of inflammatory diseases, eczema, etc. (see Radiation therapy). For these purposes, both open radiopharmaceuticals are used, introduced into the body, into tissues, serous cavities, joint cavities, intravenously, intraarterially and into the lymph system, and closed radiation sources for external, intracavitary and interstitial therapy. With the help of appropriate radiopharmaceuticals, ch. arr. colloids and suspensions containing 32 P, 90 Y, 131 I, 198 Au and other radionuclides treat diseases of the hematopoietic system and various tumors, acting locally on the patol, the focus. For contact irradiation (dermatol and ophthalmic beta applicators) 32 P, 90 Sr, 90 Y, 147 Pm, 204 Tl are used, in remote gamma therapeutic devices - sources of 60 Co or 137 Cs of high activity (hundreds and thousands of curies) . For interstitial and intracavity irradiation, needles, granules, wires and other special types of sealed sources with 60 Co, 137 Cs, 182 Ta, 192 Ir, 198 Au are used (see Radioactive drugs).
Radioactive nuclides are also used to sterilize materials and medical products. prescriptions and medications. The practical use of radiation sterilization has become possible since the 50s, when powerful sources of ionizing radiation appeared. Compared with traditional sterilization methods (see), the radiation method has a number of advantages. Since with the usual sterilizing dose of radiation (2-3 Mrad) there is no significant increase in the temperature of the irradiated object, radiation sterilization of thermolabile objects, including biol, drugs and products made from certain types of plastics, becomes possible. The effect of radiation on the irradiated sample occurs simultaneously throughout its entire volume, and sterilization is carried out with a high degree of reliability. In this case, for control, color indicators of the received dose are used, placed on the surface of the packaging of the sterilized object. Honey. products and products are sterilized at the end of technol. cycle already in finished form and in hermetic packaging, including those made from polymer materials, which eliminates the need to create strictly aseptic production conditions and guarantees sterility after the production of products by the enterprise. Radiation sterilization is especially effective for honey. disposable products (syringes, needles, catheters, gloves, suture and dressing materials, blood collection and transfusion systems, biological products, surgical instruments, etc.), non-injectable medicines, tablets and ointments. During radiation sterilization of medicinal solutions, one should take into account the possibility of their radiation decomposition, leading to a change in composition and properties (see Sterilization, cold).
Toxicology of radioactive isotopes is a branch of toxicology that studies the effect of incorporated radioactive substances on living organisms. Its main objectives are: establishing acceptable levels of content and intake of radionuclides into the human body with air, water and food, as well as the degree of harmlessness of radioactive substances introduced into the body during wedges, radiodiagnostic studies; clarification of the specifics of damage by radionuclides depending on the nature of their distribution, energy and type of radiation, half-life, dose, routes and rhythm of entry and finding effective means to prevent damage.
The influence on the human body of radionuclides widely used in industry, research and medicine is studied most deeply. research, as well as those formed as a result of the fission of nuclear fuel.
The toxicology of radioactive isotopes is organically connected with radiobiology (see), radiation hygiene (see) and medical radiology (see).
Radioactive substances can penetrate into the human body through the respiratory tract, yellow-kish. tract, skin, wound surfaces, and during injections - through blood vessels, muscle tissue, articular surfaces. The nature of the distribution of radionuclides in the body depends on the basic chemicals. properties of the element, the form of the administered compound, the route of entry and physiol, the state of the body.
Quite significant differences have been discovered in the distribution and routes of elimination of individual radionuclides. Soluble compounds Ca, Sr, Ba, Ra, Y, Zr selectively accumulate in bone tissue; La, Ce, Pr, Pu, Am, Cm, Cf, Np - in the liver and bone tissue; K, Cs, Rb - in muscle tissue; Nb, Ru, Te, Po are distributed relatively evenly, although they tend to accumulate in the reticuloendothelial tissue of the spleen, bone marrow, adrenal glands and lymph nodes; I and At - in the thyroid gland.
The distribution in the body of elements belonging to a certain group of Mendeleev’s periodic system has much in common. The elements of the first main group (Li, Na, K, Rb, Cs) are completely absorbed from the intestine, distributed relatively evenly throughout the organs and excreted mainly in the urine. The elements of the second main group (Ca, Sr, Ba, Ra) are well absorbed from the intestines, are selectively deposited in the skeleton, and are excreted in slightly larger quantities with feces. Elements of the third main and fourth secondary groups, including light lanthanides, actinides and transuranium elements, are practically not absorbed from the intestine; as a rule, they are selectively deposited in the liver and, to a lesser extent, in the skeleton, and are excreted mainly in feces. The elements of the fifth and sixth main groups of the periodic table, with the exception of Po, are relatively well absorbed from the intestine and are excreted almost exclusively in the urine during the first day, due to which they are found in relatively small quantities in organs.
The deposition of radionuclides in lung tissue during inhalation depends on the size of the inhaled particles and their solubility. The larger the aerosols, the greater the proportion of them that is retained in the nasopharynx and the less that penetrates into the lungs. Poorly soluble compounds leave the lungs slowly. A high concentration of such radionuclides is often found in the lymph nodes of the roots of the lungs. Tritium oxide and soluble compounds of alkaline and alkaline earth elements are absorbed very quickly into the lungs. Pu, Am, Ce, Cm and other heavy metals are slowly absorbed into the lungs.
Radiation safety standards (RSS) regulate the intake and content of radionuclides in the body of persons whose work is associated with occupational hazards, and individuals from the population, as well as the population as a whole, and the permissible concentrations of radionuclides in atmospheric air and water, and food products. These standards are based on the values of maximum permissible doses (MAD) of radiation established for four groups of critical organs and tissues (see Critical organ, Maximum permissible doses).
For persons working in occupational hazard conditions, the accepted value for maximum irradiation of the whole body, gonads and red bone marrow is 5 rem/year, muscle and adipose tissue, liver, kidneys, spleen, gland. tract, lungs, eye lenses - 15 rem/year, bone tissue, thyroid gland and skin -30 rem/year, hands, forearms, ankles and feet -75 rem/year.
Standards for individuals from the population are recommended to be 10 times lower than for persons working in occupational hazard conditions. Irradiation of the entire population is regulated by a genetically significant dose, which should not exceed 5 rem in 30 years. This dose does not include possible radiation doses caused by honey. procedures and natural background radiation.
The value of the annual maximum permissible intake of soluble and insoluble compounds (μCi/year) through the respiratory system for personnel, the limit of the annual intake of radionuclides through the respiratory and digestive system for individuals from the population, average annual permissible concentrations (AAC) of radionuclides in atmospheric air and water (curies/ k) for individuals from the population, as well as the content of radionuclides in a critical organ corresponding to the maximum permissible intake level (μCi) for personnel are given in the standards.
When calculating the permissible levels of radionuclides entering the body, the often uneven distribution of radionuclides in individual organs and tissues is also taken into account. The uneven distribution of radionuclides, leading to the creation of high local doses, underlies the high toxicity of alpha emitters, which is largely facilitated by the absence of recovery processes and the almost complete summation of damage caused by this type of radiation.
Designations: β- - beta radiation; β+ - positron radiation; n - neutron; p - proton; d - deuteron; t - tritone; α - alpha particle; E.Z. - decay by electron capture; γ - gamma radiation (as a rule, only the main lines of the γ spectrum are given); I.P. - isomeric transition; U (n, f) - uranium fission reaction. The specified isotope is isolated from a mixture of fission products; 90 Sr-> 90 Y - production of a daughter isotope (90 Y) as a result of the decay of the parent isotope (90 Sr), including using an isotope generator.
Bibliography: Ivanov I.I. et al. Radioactive isotopes in medicine and biology, M., 1955; Kam e n M. Radioactive tracers in biology, trans. from English, M., 1948, bibliogr.; Levin V.I. Obtaining radioactive isotopes, M., 1972; Radiation Safety Standards (NRB-69), M., 1972; Preparation in a reactor and use of short-lived isotopes, trans. with in., ed. V.V. Bochkareva and B.V. Kurchatova, M., 1965; Production of isotopes, ed. V. V. Bochkareva, M., 1973; Selinov I.P. Atomic nuclei and nuclear transformations, vol. 1, M.-L., 1951, bibliogr.; Tumanyan M. A. and K and u-shansky D. A. Radiation sterilization, M., 1974, bibliogr.; Fateeva M. N. Essays on radioisotope diagnostics, M., 1960, bibliogr.; Hevesi G. Radioactive tracers, trans. from English, M., 1950, bibliogr.; Dynamic studies with radioisotopes in medicine 1974, Proc, symp., v. 1-2, Vienna, IAEA, 1975; L e d e g e g Ch. M., Hollander J. M. a. P e g 1 m a n I. Tables of isotopes, N. Y., 1967; Silver S. Radioactive isotopes in clinical medicine, New Engl. J. Med., v. 272, p. 569, 1965, bibliogr.
V. V. Bochkarev; Yu. I. Moskalev (current), compiler of the table. V.V. Bochkarev.
When studying the properties of radioactive elements, it was discovered that the same chemical element can contain atoms with different nuclear masses. At the same time, they have the same nuclear charge, that is, these are not impurities of foreign substances, but the same substance.
What are isotopes and why do they exist?
In Mendeleev's periodic table, both this element and atoms of a substance with different nuclear masses occupy one cell. Based on the above, such varieties of the same substance were given the name “isotopes” (from the Greek isos - identical 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-proton model of the nucleus, it was possible to explain the existence of isotopes as follows: the nuclei of some atoms of a substance contain different numbers of neutrons, but the same number of protons. In fact, the nuclear charge of isotopes of one element is the same, therefore, the number of protons in the nucleus is the same. Nuclei differ in mass; accordingly, they contain different numbers of neutrons.
Stable and unstable isotopes
Isotopes can be stable or unstable. To date, about 270 stable isotopes and more than 2000 unstable ones are known. Stable isotopes- These are varieties of chemical elements that can exist independently 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 or 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 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 have only 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 of different elements are associated with the complex dependence of the number of protons and neutrons on the binding energy of the nucleus.
Almost all substances in nature exist in the form of a mixture of isotopes. The number of isotopes in a substance depends on the type of substance, atomic mass and the number of stable isotopes of a given chemical element.