By studying the composition of matter, scientists came to the conclusion that all matter consists of molecules and atoms. For a long time, the atom (translated from Greek as “indivisible”) was considered the smallest structural unit of matter. However, further research showed that the atom has a complex structure and, in turn, includes smaller particles.
What does an atom consist of?
In 1911, the scientist Rutherford suggested that the atom has a central part with a positive charge. This is how the concept of the atomic nucleus first appeared.
According to Rutherford's scheme, called the planetary model, the atom consists of a nucleus and elementary particles with a negative charge - electrons, moving around the nucleus, just as the planets orbit the Sun.
In 1932, another scientist, Chadwick, discovered the neutron, a particle that has no electrical charge.
According to modern ideas, the nucleus corresponds to the planetary model proposed by Rutherford. The nucleus carries most of the atomic mass. It also has a positive charge. The atomic nucleus contains protons - positively charged particles and neutrons - particles that do not carry a charge. Protons and neutrons are called nucleons. Negatively charged particles - electrons - move in orbit around the nucleus.
The number of protons in the nucleus is equal to those moving in orbit. Therefore, the atom itself is a particle that does not carry a charge. If an atom gains electrons from others or loses its own, it becomes positive or negative and is called an ion.
Electrons, protons and neutrons are collectively called subatomic particles.
Charge of the atomic nucleus
The nucleus has a charge number Z. It is determined by the number of protons that make up the atomic nucleus. Finding out this quantity is easy: just turn to Mendeleev’s periodic table. The atomic number of the element to which the atom belongs is equal to the number of protons in the nucleus. Thus, if the chemical element oxygen has an atomic number of 8, then the number of protons will also be eight. Since the number of protons and electrons in an atom is the same, there will also be eight electrons.
The number of neutrons is called the isotopic number and is designated by the letter N. Their number can vary in an atom of the same chemical element.
The sum of protons and electrons in the nucleus is called the mass number of the atom and is denoted by the letter A. Thus, the formula for calculating the mass number looks like this: A = Z + N.
Isotopes
When elements have equal numbers of protons and electrons, but different numbers of neutrons, they are called isotopes of a chemical element. There can be one or more isotopes. They are placed in the same cell of the periodic table.
Isotopes are of great importance in chemistry and physics. For example, an isotope of hydrogen - deuterium - in combination with oxygen gives a completely new substance called heavy water. It has a different boiling and freezing point than normal. And the combination of deuterium with another isotope of hydrogen, tritium, leads to a thermonuclear fusion reaction and can be used to generate huge amounts of energy.
Mass of the nucleus and subatomic particles
The size and mass of atoms are negligible in human perception. The size of the nuclei is approximately 10 -12 cm. The mass of an atomic nucleus is measured in physics in the so-called atomic mass units - amu.
For one amu take one twelfth of the mass of a carbon atom. Using the usual units of measurement (kilograms and grams), mass can be expressed by the following equation: 1 amu. = 1.660540·10 -24 g. Expressed in this way, it is called the absolute atomic mass.
Despite the fact that the atomic nucleus is the most massive component of an atom, its size relative to the electron cloud surrounding it is extremely small.
Nuclear forces
Atomic nuclei are extremely stable. This means that protons and neutrons are held in the nucleus by some force. These cannot be electromagnetic forces, since protons are similarly charged particles, and it is known that particles with the same charge repel each other. Gravitational forces are too weak to hold nucleons together. Consequently, particles are held in the nucleus by another interaction - nuclear forces.
Nuclear force is considered the strongest of all existing in nature. Therefore, this type of interaction between the elements of the atomic nucleus is called strong. It is present in many elementary particles, just like electromagnetic forces.
Features of nuclear forces
- Short action. Nuclear forces, unlike electromagnetic ones, appear only at very small distances, comparable to the size of the nucleus.
- Charge independence. This feature is manifested in the fact that nuclear forces act equally on protons and neutrons.
- Saturation. The nucleons of the nucleus interact only with a certain number of other nucleons.
Nuclear binding energy
Another thing closely related to the concept of strong interaction is the binding energy of nuclei. Nuclear bond energy refers to the amount of energy required to split an atomic nucleus into its constituent nucleons. It equals the energy required to form a nucleus from individual particles.
To calculate the binding energy of a nucleus, it is necessary to know the mass of subatomic particles. Calculations show that the mass of a nucleus is always less than the sum of its constituent nucleons. A mass defect is the difference between the mass of a nucleus and the sum of its protons and electrons. Using the relationship between mass and energy (E=mc 2), one can calculate the energy generated during the formation of a nucleus.
The strength of the binding energy of a nucleus can be judged by the following example: the formation of several grams of helium produces the same amount of energy as the combustion of several tons of coal.
Nuclear reactions
The nuclei of atoms can interact with the nuclei of other atoms. Such interactions are called nuclear reactions. There are two types of reactions.
- Fission reactions. They occur when heavier nuclei, as a result of interaction, decay into lighter ones.
- Synthesis reactions. The reverse process of fission: nuclei collide, thereby forming heavier elements.
All nuclear reactions are accompanied by the release of energy, which is subsequently used in industry, the military, the energy sector, and so on.
Having familiarized ourselves with the composition of the atomic nucleus, we can draw the following conclusions.
- An atom consists of a nucleus containing protons and neutrons, and electrons around it.
- The mass number of an atom is equal to the sum of the nucleons in its nucleus.
- Nucleons are held together by strong interactions.
- The enormous forces that give stability to the atomic nucleus are called nuclear binding energies.
The nucleus of an atom consists of nucleons, which are divided into protons and neutrons.
Symbolic designation of the nucleus of an atom:
A is the number of nucleons, i.e. protons + neutrons (or atomic mass)
Z- number of protons (equal to the number of electrons)
N is the number of neutrons (or atomic number)
NUCLEAR FORCES
Act between all nucleons in the nucleus;
- forces of attraction;
- short-acting
Nucleons are attracted to each other by nuclear forces, which are completely unlike either gravitational or electrostatic forces. . Nuclear forces decay very quickly with distance. Their radius of action is about 0.000 000 000 000 001 meters.
For this ultra-small length, characterizing the size of atomic nuclei, a special designation was introduced - 1 fm (in honor of the Italian physicist E. Fermi, 1901-1954). All nuclei are several Fermi in size. The radius of nuclear forces is equal to the size of a nucleon, so nuclei are clumps of very dense matter. Perhaps the densest in terrestrial conditions.
Nuclear forces are strong interactions. They are many times greater than the Coulomb force (at the same distance). Short-range action limits the effect of nuclear forces. As the number of nucleons increases, nuclei become unstable, and therefore most heavy nuclei are radioactive, and very heavy ones cannot exist at all.
The finite number of elements in nature is a consequence of the short-range action of nuclear forces.
Structure of the atom - Cool physics
Did you know?
In the middle of the 20th century, nuclear theory predicted the existence of stable elements with atomic numbers Z = 110 -114.
In Dubna, the 114th element with atomic mass A = 289 was obtained, which “lived” for only 30 seconds, which is incredibly long for an atom with a nucleus of this size.
Today, theorists are already discussing the properties of superheavy nuclei weighing 300 and even 500.
Atoms with the same atomic numbers are called isotopes: in the periodic table
they are located in the same cell (in Greek isos - equal, topos - place).
The chemical properties of isotopes are almost identical.
If there are about 100 elements in nature, then there are more than 2000 isotopes. Many of them are unstable, that is, radioactive, and decay, emitting various types of radiation.
Isotopes of the same element differ in composition only in the number of neutrons in the nucleus.
Isotopes of hydrogen.
If you remove space from all the atoms of the human body, then what remains can fit through the eye of a needle.
For the curious
Planing cars
If, while driving a car on a wet road at high speed, you brake sharply, the car will behave like a glider; its tires will begin to slide on a thin film of water, practically without touching the road. Why is this happening? Why doesn't a car always slide on a wet road, even if the brake is not applied? Is there a tread pattern that reduces this effect?
Turns out...
Several tread patterns were offered to reduce the likelihood of hydroplaning. For example, the groove may direct water to the rear contact point of the tread with the road, where the water will be thrown out. Other, smaller grooves can drain water to the sides. Finally, small depressions on the tread can, as it were, “wet” the water layer on the road, touching it just before the area of the main contact of the tread with the road surface. In all cases, the goal is to remove water from the contact zone as quickly as possible and prevent hydroplaning.
Topic: Composition of the atomic nucleus. Nuclear forces.
Purpose of the lesson: to introduce students to the structural features of the atomic nucleus.
Lesson objectives:
Educational:
) Repeat, generalize and deepen knowledge about the composition of atomic nuclei;
) Form the concept of “isotopes of substances”;
) Form the concept of “nuclear power”;
) Study the properties of nuclear forces;
Educational:
) Develop the ability to perform mental operations: analysis, synthesis, systematization, comparison, specification;
) Develop interest in physics;
) Show the connection between theoretical knowledge and practice;
) Teach to use Mendeleev’s Periodic System to determine the composition of the atomic nucleus;
) Continue to develop the ability to apply theoretical knowledge when solving problems;
) Contribute to the development of flexible thinking in students;
) To promote the development of attention in students;
Educators:
) Education of a holistic picture of the world;
) To develop the ability to use the knowledge acquired by students when studying other subjects.
Equipment: Mendeleev's periodic table, presentation for the lesson, handouts.
Epigraph for the lesson:
“Intelligence lies not only in knowledge, but also in the ability to apply knowledge in practice”
Aristotle.
During the classes.
I. Organizational moment.
The ancient Greek philosopher Aristotle said, “Intelligence lies not only in knowledge, but also in the ability to apply knowledge in practice.” Let these words, spoken back in the 4th century BC, become the motto of our lesson today. (Slide 1)
II. Homework check stage
Frontal survey:
1. Who was the first to put forward the hypothesis that the atomic nuclei of all chemical elements include the nucleus of a hydrogen atom? (English physicist Ernest Rutherford)
2. In what year were the facts confirming the validity of this hypothesis obtained? (In 1919, when observing the interaction of α - particles with the nuclei of nitrogen atoms)
3. What is another name for the nucleus of a hydrogen atom? (proton from the Greek word protos - first)
4. Thanks to the invention of which device was the existence of the proton finally proven? (Cloud chamber)
5. Write down the symbolic designation of the proton on the board (11H, 11p)
6. Ernest Rutherford hypothesized about the existence of what particles included in the atomic nucleus in 1920? (neutron)
7. By whom and when was this assumption proven? (in 1932 - English physicist James Chadwig (Rutherford's student))
8. Write the symbol for neutron (10n) on the board.
Take the assessment sheets (Appendix 1) and give yourself a grade for this stage of the lesson
III. The stage of learning new material.
1. Everyone should have at least a general idea of how the world in which they live works. Therefore, it is important to know that the world is knowable, that as knowledge deepens, the picture of the world becomes more complex.
Guys, what do you think we will talk about in class today?
And I think that we will study the structure of the atom.)
Yes, we will continue our work on studying the structure of the atomic nucleus. The topic of our lesson: “Structure of the atomic nucleus. Nuclear forces." Write down the topic of the lesson in your notebook (Slide 2).
Let's try to determine the goals and objectives of the lesson.
(Study the structure of atomic nuclei. What forces hold the particles that make up the nuclei) (Slide 3)
There is a year in the history of modern physics that is called the “year of miracles.” This is 1932. One of his “miracles” was the discovery of the neutron and the creation of a neutron-proton model of the atomic nucleus (by Soviet physicists - and Gapon; German physicist - Werner Heisenberg; Italian physicist - Majorana).
The nucleus has the shape of a ball R ≈ 10-15 m, approximately 99.96% of the total mass of the atom is concentrated in it, ρ = 2.7∙1017 kg/m³.
Proton: p (1919), lifetime 10³¹ years, m = 1836.2me, qp = +e
Neutron: n, q=0, lifetime outside the nucleus 15 min, m=1838.7me
Vadim Skorobogatko prepared a message for us about the composition of the atomic nucleus.
Both of these particles are often called nucleons. (Slide 4.)
Any chemical element is designated conventionally - X (Slide 5).
The number of particles that make up the atomic nucleus is called the mass number and is designated A. (Slide 6).
The number of protons in the nucleus is called the charge number and is denoted by Z. (Slide 7)
The number of neutrons included in the nucleus is designated N.
A= N + Z (Slide 8).
2. Further study of atomic nuclei led to the discovery that atoms of the same chemical element can have nuclei of different masses.
Moreover, all these atoms had the same chemical properties, and, therefore, have the same nuclear charge. If the charges of the nuclei are the same, it means that they have the same serial number in the table, that is, they occupy the same cell in the table.
(Slide 9). All varieties of one chemical element are called isotopes.
It has now been experimentally proven that almost all chemical elements have isotopes.
For example:
11H - protium 21H - deuterium 31Н – tritium. |
The presence of which particles included in the composition of the nucleus is different for isotopes? (neutrons)
It is the presence of different numbers of neutrons in the nuclei of isotopes that causes the different physical properties of chemical substances, which will be studied in more detail in grade 11.
3. The hypothesis about the proton-neutron composition of the atomic nucleus was confirmed, but the following question arises: why does the nucleus not decay into individual particles?
To answer this question, let’s recall the previously studied material:
There is mutual attraction between all bodies having mass. The force of gravity is calculated according to the law of universal gravitation: F=Gm1m2/r2.
The protons that make up the nucleus have a positive charge, which means that repulsion occurs between them, and the force of electric repulsion is 1039 times greater than the force of gravitational attraction. Only from this fact can we conclude that between the particles that make up the nucleus, an interaction occurs that is even stronger than electrical, otherwise the protons that make up the nucleus would fly away at enormous speed.
Scientists have come to the conclusion that there is another type of interaction in nature, which was called strong.
(Slide 10). The forces of attraction between the particles that make up the nucleus are called nuclear.
(Slide 11). Properties of nuclear forces:
Ø are only forces of attraction;
Ø many times greater than Coulomb forces;
Ø do not depend on the presence of charge;
Ø short-range: noticeable at a distance r ≈ 2.2∙10 -15 m;
Ø interact with a limited number of nucleons (saturation property).
https://pandia.ru/text/80/367/images/image003_45.gif" width="31" height="13">0 " style="border-collapse:collapse;border:none">
Substance name
Mass number, A
Charge number, Z
Number of neutrons, N
Germanium
Check how you completed the task and give yourself a grade on the evaluation sheet for this type of work.
2.Slide 14. Identify the missing chemical element.
Slide 15. Check how you completed the task and give yourself a grade on the evaluation sheet for this type of work.
Mg
Na
Li
C
O
3. Make up questions for the crossword puzzle (option 1 – for words located horizontally, option 2 – for words located vertically) (Appendix 1)
Give yourself a grade on the evaluation sheet for this type of work.
VI. Summing up the lesson
Finish the sentence:
1. An atom of any chemical element consists of...
2. The nucleus of any chemical element consists of...
3. The sum of protons and neutrons is called..., in the periodic table the mass number is....
4. In the periodic table, the number of protons in the nucleus is ..., and is called ....
5. The number of neutrons in the nucleus is equal to ... (the difference between the mass and charge numbers)
6. Protons and neutrons are held in the nucleus…. (nuclear forces)
7. Isotopes are... (varieties of the same chemical element that differ in the mass of atomic nuclei).
8. Binding energy is... (the energy required to split a nucleus into individual nucleons).
9. A nuclear reaction is called... (a change in atomic nuclei when they interact with elementary particles or with each other).
What goals did you set for yourself and did you manage to achieve them? Give yourself a grade on the score sheet for this type of work.
Calculate your average grade for the lesson.
VII. Slide 17. D/z: §61, 62 ex. 45 (textbook: ,)
VIII. Reflection.
Continue the sentence
Today in class
) I felt …
I understand …
I will …
Physics is the science of nature - shows us how great the world in which we live is, but this world is knowable, which means that physics gives a person extraordinary strength.
From the thought of the smallest particles, in the end, all the benefits that we have today emerged: new materials, televisions, lasers, computers. And the main idea about the smallest particles helped to understand the world from a single point of view.
Guys, our lesson has come to an end. I would like to end it with the words of the proverb “It’s not a shame not to know, it’s a shame not to learn!” And how much is still unknown around! What a field of activity for an inquisitive mind. So start your “perpetual motion machine” and go!
Annex 1.
Evaluation paper__________________________________________________
Type of work | ||
Checking homework | ||
Learning new material | ||
Consolidation | ||
Preparing for the State Examination a) filling out the table | ||
Lecture 18. Elements of physics of the atomic nucleus
Lecture outline
Atomic nucleus. Mass defect, nuclear binding energy.
Radioactive radiation and its types. Law of radioactive decay.
Conservation laws for radioactive decays and nuclear reactions.
1.Atomic nucleus. Mass defect, nuclear binding energy.
Composition of the atomic nucleus
Nuclear physics- the science of the structure, properties and transformations of atomic nuclei. In 1911, E. Rutherford established in experiments on the scattering of α-particles as they pass through matter that a neutral atom consists of a compact positively charged nucleus and a negative electron cloud. W. Heisenberg and D.D. Ivanenko (independently) hypothesized that the nucleus consists of protons and neutrons.
Atomic nucleus- the central massive part of an atom, consisting of protons and neutrons, which are collectively called nucleons. Almost the entire mass of the atom is concentrated in the nucleus (more than 99.95%). The dimensions of the nuclei are on the order of 10 -13 - 10 -12 cm and depend on the number of nucleons in the nucleus. The density of nuclear matter for both light and heavy nuclei is almost the same and is about 10 17 kg/m 3, i.e. 1 cm 3 of nuclear matter would weigh 100 million tons. Nuclei have a positive electric charge equal to the absolute value of the total charge of electrons in the atom.
Proton (symbol p) is an elementary particle, the nucleus of a hydrogen atom. A proton has a positive charge equal in magnitude to the charge of an electron. Proton mass m p = 1.6726 10 -27 kg = 1836 m e, where m e is the mass of the electron.
In nuclear physics, it is customary to express masses in atomic mass units:
1 amu = 1.65976 10 -27 kg.
Therefore, the proton mass, expressed in amu, is equal to
m p = 1.0075957 a.m.u.
The number of protons in the nucleus is called charge number Z. It is equal to the atomic number of a given element and, therefore, determines the element’s place in Mendeleev’s periodic table of elements.
Neutron (symbol n) is an elementary particle that does not have an electric charge, the mass of which is slightly greater than the mass of a proton.
Neutron mass m n = 1.675 10 -27 kg = 1.008982 amu The number of neutrons in the nucleus is denoted N.
The total number of protons and neutrons in the nucleus (number of nucleons) is called mass number and is designated by the letter A,
To designate nuclei, the symbol is used, where X is the chemical symbol of the element.
Isotopes- varieties of atoms of the same chemical element, the atomic nuclei of which have the same number of protons (Z) and a different number of neutrons (N). The nuclei of such atoms are also called isotopes. Isotopes occupy the same place in the periodic table of elements. As an example, here are the isotopes of hydrogen:
The concept of nuclear forces.
The nuclei of atoms are extremely strong formations, despite the fact that similarly charged protons, being at very small distances in the atomic nucleus, must repel each other with enormous force. Consequently, extremely strong attractive forces between nucleons act inside the nucleus, many times greater than the electrical repulsive forces between protons. Nuclear forces are a special type of force; they are the strongest of all known interactions in nature.
Research has shown that nuclear forces have the following properties:
nuclear attractive forces act between any nucleons, regardless of their charge state;
nuclear attractive forces are short-range: they act between any two nucleons at a distance between the centers of particles of about 2·10 -15 m and decrease sharply with increasing distance (at distances greater than 3·10 -15 m they are practically equal to zero);
Nuclear forces are characterized by saturation, i.e. each nucleon can interact only with the nucleons of the nucleus closest to it;
nuclear forces are not central, i.e. they do not act along the line connecting the centers of interacting nucleons.
At present, the nature of nuclear forces is not fully understood. It has been established that they are the so-called exchange forces. Exchange forces are quantum in nature and have no analogue in classical physics. Nucleons are connected to each other by a third particle, which they constantly exchange. In 1935, Japanese physicist H. Yukawa showed that nucleons exchange particles whose mass is approximately 250 times greater than the mass of an electron. The predicted particles were discovered in 1947 by the English scientist S. Powell while studying cosmic rays and were subsequently called -mesons or pions.
The mutual transformations of the neutron and proton are confirmed by various experiments.
Defect in the masses of atomic nuclei. Binding energy of the atomic nucleus.
The nucleons in the atomic nucleus are interconnected by nuclear forces, therefore, in order to divide the nucleus into its individual protons and neutrons, it is necessary to expend a lot of energy.
The minimum energy required to separate a nucleus into its constituent nucleons is called nuclear binding energy. The same amount of energy is released if free neutrons and protons combine and form a nucleus.
Accurate mass spectroscopic measurements of nuclear masses have shown that the rest mass of an atomic nucleus is less than the sum of the rest masses of free neutrons and protons from which the nucleus was formed. The difference between the sum of the rest masses of free nucleons from which the nucleus is formed and the mass of the nucleus is called mass defect:
This mass difference m corresponds to the binding energy of the nucleus E St., determined by the Einstein relation:
or, substituting the expression for m, we get:
Binding energy is usually expressed in megaelectronvolts (MeV). Let us determine the binding energy corresponding to one atomic mass unit (, the speed of light in vacuum 
):
Let's convert the resulting value into electronvolts:
In this regard, in practice it is more convenient to use the following expression for the binding energy:
where the factor m is expressed in atomic mass units.
An important characteristic of the nucleus is the specific binding energy of the nucleus, i.e. binding energy per nucleon:
.
The more 
, the more strongly the nucleons are connected to each other.
The dependence of the value on the mass number of the nucleus is shown in Figure 1. As can be seen from the graph, nucleons in nuclei with mass numbers of the order of 50-60 (Cr-Zn) are most strongly bound. The binding energy for these nuclei reaches
The atomic nucleus consists of protons and neutrons. The number of protons determines the charge of the nucleus (ordinal number in the periodic table).
The mass of the nucleus of an arbitrary element is determined by a value close to the sum of the masses of protons and neutrons included in its composition. Therefore, the mass number of the nucleus, denoted by the letter A and expressed in atomic mass units, is rounded equal to A = N + Z. Z– nuclear charge, determines the number of protons in the nucleus and the number of electrons in the electron shell of a neutral atom. N– number of neutrons in the nucleus. Proton and neutron have a common name - nucleon. The symbol is used to denote the core, where X is a symbol of a chemical element. For example, what does it mean Z = 82, N = 126, A = 208.
Different combinations of numbers of protons and neutrons correspond to different nuclei. In this case, the following groups of atoms can be distinguished.
Isotopes– atoms whose nuclei have the same number of protons Z and a different number of neutrons N. Such elements occupy the same place in the periodic table. For example, a group of hydrogen isotopes common in nature: – light hydrogen, – deuterium and – tritium. The nuclei of hydrogen isotopes also have their own names: proton, deuteron, triton.
Isobars– atoms whose nuclei have the same number A ().
Along with the term atomic nucleus term used nuclide
Approximate sizes of atoms and their components:
nucleus size ~ 10–14 m, neutron and proton size ~ 10–15 m, atom ~ 10–10 m, electron< 10 –18 м.
The size of the nucleus is characterized by the radius of the nucleus, which has a conventional meaning, since the boundaries of the nucleus are blurred, like any quantum system. It has been experimentally established that each nucleus has an internal region where the density of matter is constant. This area is surrounded by a surface layer where the density of the substance drops to zero. Empirical formula for core radius
1 fm (femtometer) =10 –15 m (1)
This expression can be interpreted as the proportionality of the volume of the nucleus to the number of nucleons in it V ~ A. (1) means that the average density of the nucleus is independent of the mass number.
Nuclear mass is expressed in atomic mass units or MeV/ With 2 .
1a.u.m. =1/12 the mass of a carbon atom with atomic mass 12,000. 1a.u.m. = 1.66×10 –27 kg » 931.5 MeV/ With 2 .
When a nucleus is formed from nucleons, its mass decreases by the amount D m, which is called mass defect.
Dm is expressed in atomic mass units or MeV/ With 2 .
An important characteristic of the nucleus is the binding energy of the nucleus W(A,Z) is the energy that must be expended to split a nucleus into its individual constituent protons and neutrons without imparting kinetic energy to them.
W(A,Z) = Δ ts 2 = [Zm p +(A–Z)m n – M i(A,Z)]· With 2 , (3)
Specific binding energy is the average energy per 1 nucleon: . (4)
For most nuclei, the specific binding energy is almost the same and is ~8 MeV. Therefore, the total binding energy is approximately proportional to the mass number, i.e. number of nucleons in the nucleus. This speaks to a property of nuclear forces called saturation. It lies in the fact that each nucleon interacts only with a limited number of neighboring nucleons.
Nucleons in the nucleus are held together by specific nuclear forces, which are a manifestation of the strong interaction. Nuclear forces have the following properties:
– are short-range, their range of action is 10–14 m;
– the most intense, they are 2-3 orders of magnitude more powerful than electromagnetic forces. Nuclear forces ensure the existence of nuclei with a specific binding energy of about 8 MeV.
– They have the property of saturation. This is manifested in the fact that in the nucleus a proton can form a bound state with no more than two neutrons. For this reason, the hydrogen isotope tritium is no longer stable.
– They have charge independence, i.e. the forces acting between a proton and a neutron, a proton and a proton, a neutron and a neutron are the same. This property does not mean complete identity of systems p – p, p – p, p – p, since protons and neutrons are fermions and systems r - r, p - p consist of identical particles, and the system p – p – from different.
– They have an exchange nature. When interacting, nucleons can exchange their coordinates, charges, and spin projections.
– Depends on the spin of nucleons. This dependence is indicated by the fact that there is no state of the deuteron with spin 0. I.e. The spins of the proton and neutron in this state are only parallel.
– They are non-central, i.e. they depend on the orientation of the spins of the nucleons relative to the straight line connecting the nucleons.
In 1935, Japanese physicist H. Yukawa hypothesized that nuclear interaction is the result of the exchange of nucleons by a virtual particle. These particles must have a mass greater than the mass of an electron, but less than the mass of a proton, which is why they were called mesons. (From Greek . mesos– intermediate, average). Mesons began to be sought experimentally. In 1947 they were discovered in cosmic radiation. These particles were called pi-mesons (from the English. primary– primary). Now these particles are called more briefly - pions. Pion exists in the form p 0 , p – , p + .
Pi mesons play an important role in nucleon-nucleon interaction at distances of 1.5–2 fm. The essence of the meson theory of nuclear forces is as follows. Two nucleons, being at distances r£ h/2 m p c, exchange pions, which is the cause of nuclear interaction. There are 4 types of exchange possible:
p « p+ p 0 , (5)
n « n+ p 0 , (6)
p « n+ p + , n « p+ p – , (7)
in which nucleons find themselves surrounded by a cloud of virtual pions, forming a field of nuclear forces. The absorption of mesons by another nucleon leads to a strong interaction between the nucleons.
At distances less than 1.5 fm, nucleons exchange heavier mesons: h (549 MeV), r (770 MeV), w (782 MeV), which determine the repulsion of nucleons.
