As already noted, an atom consists of three types of elementary particles: protons, neutrons and electrons. The atomic nucleus is the central part of the atom, consisting of protons and neutrons. Protons and neutrons have the common name nucleon, in the nucleus they can turn into each other. The nucleus of the simplest atom, the hydrogen atom, consists of one elementary particle, the proton.

The diameter of the nucleus of an atom is approximately 10 -13 - 10 -12 cm and is 0.0001 of the diameter of an atom. However, almost the entire mass of an atom (99.95 - 99.98%) is concentrated in the nucleus. If it were possible to obtain 1 cm 3 of pure nuclear matter, its mass would be 100 - 200 million tons. The mass of the nucleus of an atom is several thousand times greater than the mass of all the electrons that make up the atom.

Proton- an elementary particle, the nucleus of a hydrogen atom. The mass of a proton is 1.6721x10 -27 kg, it is 1836 times the mass of an electron. The electric charge is positive and equal to 1.66x10 -19 C. A pendant is a unit of electric charge equal to the amount of electricity passing through the cross section of a conductor in a time of 1s at a constant current strength of 1A (amperes).

Each atom of any element contains a certain number of protons in the nucleus. This number is constant for a given element and determines its physical and Chemical properties. That is, the number of protons depends on what chemical element we are dealing with. For example, if one proton in the nucleus is hydrogen, if 26 protons are iron. The number of protons in the atomic nucleus determines the charge of the nucleus (charge number Z) and the serial number of the element in the periodic system of elements D.I. Mendeleev (atomic number of the element).

Hneutron- an electrically neutral particle with a mass of 1.6749 x10 -27 kg, 1839 times the mass of an electron. A neuron in a free state is an unstable particle; it independently turns into a proton with the emission of an electron and an antineutrino. The half-life of neutrons (the time during which half of the original number of neutrons decays) is approximately 12 minutes. However, in a bound state inside stable atomic nuclei, it is stable. Total number nucleons (protons and neutrons) in the nucleus is called the mass number (atomic mass - A). The number of neutrons that make up the nucleus is equal to the difference between the mass and charge numbers: N = A - Z.

Electron- an elementary particle, the carrier of the smallest mass - 0.91095x10 -27 g and the smallest electric charge - 1.6021x10 -19 C. This is a negatively charged particle. The number of electrons in an atom is equal to the number of protons in the nucleus, i.e. the atom is electrically neutral.

Positron– an elementary particle with a positive electric charge, an antiparticle with respect to an electron. The mass of an electron and a positron are equal, and the electric charges are equal in absolute value, but opposite in sign.

Different types of nuclei are called nuclides. A nuclide is a type of atom with a given number of protons and neutrons. In nature, there are atoms of the same element with different atomic masses (mass numbers): 17 35 Cl, 17 37 Cl, etc. The nuclei of these atoms contain the same number of protons, but a different number of neutrons. Varieties of atoms of the same element that have the same nuclear charge but different mass numbers are called isotopes . Having the same number of protons, but differing in the number of neutrons, isotopes have the same structure of electron shells, i.e. very similar chemical properties and occupy the same place in the periodic table of chemical elements.

Isotopes are denoted by the symbol of the corresponding chemical element with the index A located at the top left - the mass number, sometimes the number of protons (Z) is also given at the bottom left. For example, the radioactive isotopes of phosphorus are 32 P, 33 P, or 15 32 P and 15 33 P, respectively. When designating an isotope without indicating the symbol of the element, the mass number is given after the designation of the element, for example, phosphorus - 32, phosphorus - 33.

Most chemical elements have several isotopes. In addition to the hydrogen isotope 1 H-protium, heavy hydrogen 2 H-deuterium and superheavy hydrogen 3 H-tritium are known. Uranium has 11 isotopes, in natural compounds there are three of them (uranium 238, uranium 235, uranium 233). They have 92 protons and 146.143 and 141 neutrons, respectively.

Currently, more than 1900 isotopes of 108 chemical elements are known. Of these, natural isotopes include all stable (there are approximately 280 of them) and natural isotopes that are part of radioactive families (there are 46 of them). The rest are artificial, they are obtained artificially as a result of various nuclear reactions.

The term "isotopes" should only be used when we are talking about atoms of the same element, for example, carbon isotopes 12 C and 14 C. If atoms of different chemical elements are meant, it is recommended to use the term "nuclides", for example, radionuclides 90 Sr, 131 J, 137 Cs.

>> Building atomic nucleus. nuclear forces

§ 104 STRUCTURE OF THE NUCLEAR. NUCLEAR FORCES

Immediately after the neutron was discovered in Chadwick's experiments, the Soviet physicist D. D. Ivanenko and the German scientist W. Heisenberg in 1932 proposed a proton-neutron model of the nucleus. It was confirmed by subsequent studies of nuclear transformations and is now generally accepted.

Proton-neutron model of the nucleus. According to the proton-neutron model, nuclei consist of elementary particles of two types - protons and neutrons.

Since the atom as a whole is electrically neutral, and the charge of the proton is equal to the modulus of the charge of the e-lectron, the number of protons in the nucleus is equal to the number of electrons in the atomic shell. Consequently, the number of protons in the nucleus is equal to the atomic number of the element Z in the periodic system of elements of D. I. Mendeleev.

The sum of the number of protons Z and the number of neutrons N in the nucleus is called the mass number and is denoted by the letter A:

A = Z + N. (13.2)

The masses of the proton and neutron are close to each other, and each of them is approximately equal to an atomic mass unit. The mass of electrons in an atom is much less than the mass of its nucleus. Therefore, the mass number of the nucleus is equal to the relative rounded to an integer atomic mass element. Mass numbers can be determined by approximate measurement of the mass of nuclei with instruments that do not have high accuracy.

Isotopes are nuclei with the same value but with different mass numbers A, i.e. with different numbers of neutrons N.

Nuclear forces. Since the nuclei are very stable, the protons and neutrons must be kept inside the nucleus by some forces, and very large ones. What are these forces? We can immediately say that this is not gravitational forces which are too weak. The stability of the nucleus cannot be explained by electromagnetic forces either, since there is an electrical repulsion between like-charged protons. And neutrons have no electric charge.

This means that between nuclear particles - protons and neutrons (they are called nucleons) - there are special forces called nuclear forces.

What are the main properties of nuclear forces? Nuclear forces are about 100 times greater than electrical (Coulomb) forces. These are the most powerful forces of all existing, bending in nature. Therefore, the interactions of nuclear particles are often called strong interactions.

Strong interactions are manifested not only in the interactions of nucleons in the nucleus. This is a special type of interaction inherent in most elementary particles along with electromagnetic interactions.

Another important feature of nuclear forces is their short-range action. Electromagnetic forces weaken relatively slowly with increasing distance. Nuclear forces manifest themselves appreciably only at distances equal to the size of the nucleus (10 -12 -10 -13 cm), which was already shown by Rutherford's experiments on the scattering of -particles by atomic nuclei. Nuclear forces are, so to speak, "a hero with very short arms." A complete quantitative theory of nuclear forces has not yet been developed. Significant progress in its development has been achieved quite recently - in the last 10-15 years.

The nuclei of atoms are made up of protons and neutrons. These particles are held in the nucleus by nuclear forces.

What are the main features of nuclear forces!

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atomic nucleus
Atomic nucleus

atomic nucleus - the central and very compact part of the atom, in which almost all of its mass and all positive electric charge are concentrated. The nucleus, holding close to itself by the Coulomb forces the electrons in an amount that compensates for its positive charge, forms a neutral atom. Most of the nuclei have a shape close to spherical and a diameter of ≈ 10 -12 cm, which is four orders of magnitude smaller than the diameter of an atom (10 -8 cm). The density of matter in the core is about 230 million tons/cm 3 .
The atomic nucleus was discovered in 1911 as a result of a series of experiments on the scattering of alpha particles by thin gold and platinum foils, carried out in Cambridge (England) under the direction of E. Rutherford. In 1932, after the discovery of the neutron by J. Chadwick, it became clear that the nucleus consists of protons and neutrons
(V. Heisenberg, D.D. Ivanenko, E. Majorana).
To designate the atomic nucleus, the symbol of the chemical element of the atom, which includes the nucleus, is used, and the upper left index of this symbol shows the number of nucleons (mass number) in this nucleus, and the lower left index shows the number of protons in it. For example, a nickel nucleus containing 58 nucleons, of which 28 are protons, is denoted. The same nucleus can also be designated 58 Ni, or nickel-58.

The nucleus is a system of densely packed protons and neutrons moving at a speed of 10 9 -10 10 cm/sec and held by powerful and short-range nuclear forces of mutual attraction (their area of ​​action is limited by distances of ≈ 10 -13 cm). Protons and neutrons are about 10 -13 cm in size and are considered as two different states of a single particle called a nucleon. The radius of the nucleus can be approximately estimated by the formula R ≈ (1.0-1.1)·10 -13 A 1/3 cm, where A is the number of nucleons (the total number of protons and neutrons) in the nucleus. On fig. 1 shows how the density of matter changes (in units of 10 14 g/cm3) inside the nickel nucleus, consisting of 28 protons and 30 neutrons, depending on the distance r (in units of 10 -13 cm) to the center of the nucleus.
Nuclear interaction (interaction between nucleons in the nucleus) occurs due to the fact that nucleons exchange mesons. This interaction is a manifestation of the more fundamental strong interaction between quarks that make up nucleons and mesons (similarly, chemical bonding forces in molecules are a manifestation of more fundamental electromagnetic forces).
The world of nuclei is very diverse. About 3000 nuclei are known, differing from each other either in the number of protons, or in the number of neutrons, or both. Most of them are obtained artificially.
Only 264 cores are stable, ie. do not experience any spontaneous transformations, called decays, over time. The rest experience various forms of decay - alpha decay (emission of an alpha particle, i.e. the nucleus of a helium atom); beta decay (simultaneous emission of an electron and an antineutrino or a positron and a neutrino, as well as the absorption of an atomic electron with the emission of a neutrino); gamma decay (photon emission) and others.
Different types of nuclei are often referred to as nuclides. Nuclides with the same number of protons and different number neutrons are called isotopes. Nuclides with the same number of nucleons but different ratios of protons and neutrons are called isobars. Light nuclei contain approximately equal numbers of protons and neutrons. In heavy nuclei, the number of neutrons is about 1.5 times the number of protons. The lightest nucleus is the nucleus of the hydrogen atom, which consists of one proton. The heaviest known nuclei (they are obtained artificially) have a number of nucleons of ≈290. Of these, 116-118 are protons.
Different combinations of the number of protons Z and neutrons correspond to different atomic nuclei. Atomic nuclei exist (i.e. their lifetime t > 10 -23 s) in a rather narrow range of changes in the numbers Z and N. In this case, all atomic nuclei are divided into two large groups - stable and radioactive (unstable). Stable nuclei cluster near the line of stability, which is given by the equation

Rice. 2. NZ-diagram of atomic nuclei.

On fig. 2 shows an NZ diagram of atomic nuclei. Black dots show stable nuclei. The area where stable nuclei are located is usually called the stability valley. On the left side of the stable nuclei are nuclei overloaded with protons (proton-rich nuclei), on the right - nuclei overloaded with neutrons (neutron-rich nuclei). Atomic nuclei currently discovered are highlighted in color. There are about 3.5 thousand of them. It is believed that there should be 7 - 7.5 thousand of them in total. Proton-rich nuclei (crimson color) are radioactive and turn into stable ones mainly as a result of β + decays, the proton that is part of the nucleus turns into a neutron. Neutron-rich nuclei (blue) are also radioactive and become stable as a result of - -decays, with the transformation of a nucleus neutron into a proton.
The heaviest stable isotopes are those of lead (Z = 82) and bismuth (Z = 83). Heavy nuclei, along with the processes of β + and β - decay, are also subject to α decay ( yellow) and spontaneous fission, which become their main decay channels. The dotted line in fig. 2 outlines the region of possible existence of atomic nuclei. The line B p = 0 (B p is the proton separation energy) limits the region of existence of atomic nuclei on the left (proton drip-line). The line B n = 0 (B n is the neutron separation energy) is on the right (neutron drip-line). Outside these boundaries, atomic nuclei cannot exist, since they decay in a characteristic nuclear time(~10 -23 – 10 -22 s) with nucleon emission.
When connecting (synthesis) of two light nuclei and fission of a heavy nucleus into two lighter fragments, a lot of energy is released. These two methods of obtaining energy are the most efficient of all known. So 1 gram of nuclear fuel is equivalent to 10 tons chemical fuel. The fusion of nuclei (thermonuclear reactions) is the source of energy for stars. Uncontrolled (explosive) fusion is carried out when a thermonuclear (or so-called “hydrogen”) bomb is detonated. Controlled (slow) synthesis underlies a promising energy source being developed - a thermonuclear reactor.
Uncontrolled (explosive) fission occurs during the explosion of an atomic bomb. Controlled fission is carried out in nuclear reactors, which are sources of energy in nuclear power plants.
For the theoretical description of atomic nuclei, quantum mechanics and various models are used.
The nucleus can behave both as a gas (quantum gas) and as a liquid (quantum liquid). Cold nuclear liquid has the properties of superfluidity. In a strongly heated nucleus, nucleons decay into their constituent quarks. These quarks interact by exchanging gluons. As a result of such a decay, the set of nucleons inside the nucleus turns into a new state of matter - quark-gluon plasma

The atomic nucleus is the central part of the atom, in which its main mass is concentrated (more than 99.9%). The nucleus is positively charged, the charge of the nucleus determines the chemical element to which the atom is assigned. The dimensions of the nuclei of various atoms are several femtometers, which is more than 10 thousand times smaller than the size of the atom itself.

The atomic nucleus, considered as a class of particles with a certain number of protons and neutrons, is commonly called a nuclide. The number of protons in the nucleus is called its charge number - this number is equal to the ordinal number of the element to which the atom belongs, in the table ( Periodic system elements) Mendeleev. The number of protons in the nucleus determines the structure electron shell neutral atom and thus the chemical properties of the corresponding element. The number of neutrons in a nucleus is called its isotopic number. Nuclei with the same number of protons and different numbers of neutrons are called isotopes.

In 1911, Rutherford, in his report "Scattering of α- and β-rays and the structure of the atom" in the Philosophical Society of Manchester, stated:

The scattering of charged particles can be explained by assuming an atom that consists of a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity of equal magnitude. With such a structure of the atom, α- and β-particles, when they pass at a close distance from the center of the atom, experience large deviations, although the probability of such a deviation is small.

Thus, Rutherford discovered the atomic nucleus, from that moment nuclear physics began, studying the structure and properties of atomic nuclei.

After the discovery of stable isotopes of elements, the nucleus of the lightest atom was assigned the role of a structural particle of all nuclei. Since 1920, the nucleus of the hydrogen atom has been officially termed proton. After the intermediate proton-electron theory of the structure of the nucleus, which had many obvious shortcomings, it first of all contradicted the experimental results of measurements of spins and magnetic moments nuclei, in 1932 James Chadwick discovered a new electrically neutral particle called the neutron. In the same year, Ivanenko and, independently, Heisenberg put forward a hypothesis about the proton-neutron structure of the nucleus. Later, with the development of nuclear physics and its applications, this hypothesis was fully confirmed.

Radioactivity

Radioactive decay (from Latin radius "beam" and āctīvus "effective") is a spontaneous change in the composition (charge Z, mass number A) or the internal structure of unstable atomic nuclei by emitting elementary particles, gamma quanta and / or nuclear fragments. The process of radioactive decay is also called radioactivity, and the corresponding nuclei (nuclides, isotopes and chemical elements) are radioactive. Substances containing radioactive nuclei are also called radioactive.

The law of radioactive decay is a law discovered experimentally by Frederick Soddy and Ernest Rutherford and formulated in 1903. The modern wording of the law:

which means that the number of decays in a time interval t in an arbitrary substance is proportional to the number N of radioactive atoms of a given type present in the sample.

In that mathematical expressionλ is the decay constant, which characterizes the probability of radioactive decay per unit time and has the dimension c −1 . The minus sign indicates a decrease in the number of radioactive nuclei over time. The law expresses the independence of the decay of radioactive nuclei from each other and from time: the probability of decay of a given nucleus in each subsequent unit of time does not depend on the time that has elapsed since the beginning of the experiment, and on the number of nuclei remaining in the sample.

Solution to this differential equation looks like:

Or , where T is the half-life equal to the time during which the number of radioactive atoms or the activity of the sample decreases by 2 times.

12. Nuclear reactions.

A nuclear reaction is the process of interaction of an atomic nucleus with another nucleus or elementary particle, accompanied by a change in the composition and structure of the nucleus. The consequence of the interaction may be the fission of the nucleus, the emission of elementary particles or photons. The kinetic energy of the newly formed particles can be much higher than the initial one, and one speaks of the release of energy by a nuclear reaction.

Types of nuclear reactions

Nuclear fission reaction - the process of splitting an atomic nucleus into two (rarely three) nuclei with similar masses, called fission fragments. As a result of fission, other reaction products can also appear: light nuclei (mainly alpha particles), neutrons and gamma quanta. Fission can be spontaneous (spontaneous) and forced (as a result of interaction with other particles, primarily with neutrons). The fission of heavy nuclei is an exoenergetic process, as a result of which a large number of energy in the form of kinetic energy of reaction products, as well as radiation.

Nuclear fission serves as a source of energy in nuclear reactors and nuclear weapons.

Nuclear fusion reaction - the process of fusion of two atomic nuclei with the formation of a new, heavier nucleus.

In addition to the new nucleus, during the fusion reaction, as a rule, various elementary particles and (or) quanta of electromagnetic radiation.

Without the supply of external energy, the fusion of nuclei is impossible, since positively charged nuclei experience electrostatic repulsion forces - this is the so-called "Coulomb barrier". To synthesize nuclei, it is necessary to bring them closer to a distance of about 10 −15 m, at which the strong interaction will exceed the electrostatic repulsion forces. This is possible if the kinetic energy of the approaching nuclei exceeds the Coulomb barrier.

photonuclear reaction

When a gamma quantum is absorbed, the nucleus receives an excess of energy without changing its nucleon composition, and a nucleus with an excess of energy is a compound nucleus. Like other nuclear reactions, the absorption of a gamma-quantum by a nucleus is possible only if the necessary energy and spin ratios are met. If the energy transferred to the nucleus exceeds the binding energy of the nucleon in the nucleus, then the decay of the resulting compound nucleus occurs most often with the emission of nucleons, mainly neutrons.

Recording nuclear reactions

the method of writing formulas for nuclear reactions is similar to writing formulas for chemical reactions, that is, the sum of the initial particles is written on the left, the sum of the resulting particles (reaction products) is written on the right, and an arrow is placed between them.

Thus, the reaction of radiative capture of a neutron by a cadmium-113 nucleus is written as follows:

We see that the number of protons and neutrons on the right and left remains the same (the baryon number is preserved). The same applies to electric charges, lepton numbers and other quantities (energy, momentum, angular momentum, ...). In some reactions where the weak interaction is involved, protons can turn into neutrons and vice versa, but their total number does not change.

STRUCTURE OF THE NUCLEAR OF THE ATOM

In 1932 after the discovery of the proton and neutron by scientists D.D. Ivanenko (USSR) and W. Heisenberg (Germany) was nominated proton-neutron model of the atomic nucleus.

According to this model:
- the nuclei of all chemical elements consist of nucleons: protons and neutrons
- the charge of the nucleus is due only to protons
- the number of protons in the nucleus is equal to the atomic number of the element
- the number of neutrons is equal to the difference between the mass number and the number of protons (N=A-Z)

Kernel symbol atom of a chemical element:

X - symbol of a chemical element

A is the mass number, which shows:
- the mass of the nucleus in whole atomic mass units (a.m.u.)
(1a.m.u. = 1/12 of the mass of a carbon atom)
- number of nucleons in the nucleus
- (A = N + Z) , where N is the number of neutrons in the nucleus of an atom

Z - charge number, which shows:
- nuclear charge in elementary electric charges(e.e.z.)
(1e.e.z. \u003d electron charge \u003d 1.6 x 10 -19 C)
- number of protons
- number of electrons in an atom
- serial number in the periodic table

The mass of the nucleus is always less than the sum of the rest masses of the free protons and neutrons that make it up.
This is because protons and neutrons in the nucleus are very strongly attracted to each other. It takes a lot of work to separate them. Therefore, the total rest energy of the nucleus is not equal to the rest energy of its constituent particles. It is less by the amount of work to overcome the nuclear forces of attraction.
The difference between the mass of the nucleus and the sum of the masses of protons and neutrons is called the mass defect.

Remember the topic "Atomic Physics" for grade 9:

Radioactivity.
radioactive transformations.
The composition of the atomic nucleus. Nuclear forces.
Communication energy. mass defect.
Fission of uranium nuclei.
Nuclear chain reaction.
Nuclear reactor.
thermonuclear reaction.

Other pages on the topic "Atomic Physics" for grades 10-11:

HOW THE ATOM IS STUDYED

An atom is a nucleus of protons and neutrons around which electrons revolve. The sizes of atoms are thousandths of a micron. But there are beyond giant "atoms" about 10 kilometers in diameter. For the first time such an "atom" was discovered in 1967, and now there are more than a thousand of them. This is neutron stars - supernova remnants, which are actually huge atomic nuclei, consisting of 90% neutrons and 10% protons, and surrounded by an "atmosphere" of electrons.
___

In the 1920s, a young physicist was trained by E. Rutherford. Two months later, Rutherford invited him to his place and said that nothing would work out. "Why? After all, I work 20 hours a day!?" the young man retorted. "This is bad! You no time left to think! Rutherford replied.

In 1908 the famous physicist Ernest Rutherford said that he had dealt with many transformations in nature, but he would hardly have been able to foresee such a momentary transformation. - From physicists to chemists! In 1908, E. Rutherford received Nobel Prize in chemistry for his work in the study of the atom. In those years, research on the structure of the atom and radioactivity was classified as chemistry.