Isotopes have different numbers of things. What are isotopes in chemistry? Definition, structure

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.

What are isotopes

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 of isotopes

How are isotopes different? 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.

ImGist determined that the differences between isotopes are as follows:

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.

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.

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 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.

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 :

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. Mr∙ 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 = Mr = 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.