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Is the atomic nucleus divisible? And if so, what particles does it consist of? Many physicists have tried to answer this question.
In 1909, the British physicist Ernest Rutherford, together with the German physicist Hans Geiger and the New Zealand physicist Ernst Marsden, conducted his famous experiment on the scattering of α-particles, which resulted in the conclusion that the atom is not an indivisible particle at all. It consists of a positively charged nucleus and electrons revolving around it. Moreover, despite the fact that the size of the nucleus is approximately 10,000 times smaller than the size of the atom itself, 99.9% of the mass of the atom is concentrated in it.
But what is the nucleus of an atom? What particles are in it? Now we know that the core of any element consists of protons and neutrons, common name which nucleons. And at the beginning of the 20th century, after the appearance of the planetary, or nuclear, model of the atom, this was a mystery to many scientists. Different hypotheses have been put forward and different models have been proposed. But the correct answer to this question was again given by Rutherford.
Discovery of the proton
Rutherford's experience
The nucleus of a hydrogen atom is a hydrogen atom from which its single electron has been removed.
By 1913, the mass and charge of the nucleus of the hydrogen atom had been calculated. In addition, it became known that the mass of an atom of any chemical element is always divided without a remainder by the mass of a hydrogen atom. This fact led Rutherford to the idea that the nuclei of hydrogen atoms enter into any nucleus. And he managed to prove it experimentally in 1919.
In his experiment, Rutherford placed a source of α-particles in a chamber in which a vacuum was created. The thickness of the foil covering the chamber window was such that α-particles could not escape. Outside the chamber window was a screen coated with zinc sulfide.
When the chamber was filled with nitrogen, flashes of light were recorded on the screen. This meant that, under the influence of α-particles, some new particles were knocked out of nitrogen, which easily penetrated the foil, which was impenetrable for α-particles. It turned out that unknown particles have a positive charge equal in magnitude to the charge of an electron, and their mass is equal to the mass of the nucleus of a hydrogen atom. Rutherford called these particles protons.
But it soon became clear that the nuclei of atoms consist not only of protons. After all, if this were so, then the mass of an atom would be equal to the sum of the masses of protons in the nucleus, and the ratio of the charge of the nucleus to the mass would be a constant value. In fact, this is true only for the simplest hydrogen atom. In atoms of other elements, everything is different. For example, in the nucleus of a beryllium atom, the sum of the masses of protons is 4 units, and the mass of the nucleus itself is 9 units. This means that in this nucleus there are other particles that have a mass of 5 units, but do not have a charge.
Discovery of the neutron
In 1930, the German physicist Walter Bothe Bothe and Hans Becker discovered during an experiment that the radiation arising from the bombardment of beryllium atoms with α-particles has an enormous penetrating power. After 2 years, the English physicist James Chadwick, a student of Rutherford, found out that even a 20 cm thick lead plate placed in the path of this unknown radiation does not weaken or amplify it. It turned out that the electromagnetic field does not have any effect on the emitted particles. This meant that they had no charge. Thus, another particle was discovered, which is part of the nucleus. They called her neutron. The neutron mass turned out to be equal to the mass proton.
Proton-neutron theory of the nucleus
After the experimental discovery of the neutron, the Russian scientist D. D. Ivanenko and the German physicist W. Heisenberg independently proposed the proton-neutron theory of the nucleus, which provided a scientific justification for the composition of the nucleus. According to this theory, the nucleus of any chemical element consists of protons and neutrons. Their common name is nucleons.
The total number of nucleons in the nucleus is denoted by the letter A. If the number of protons in the nucleus is denoted by the letter Z, and the number of neutrons by the letter N, then we get the expression:
A=Z+N
This equation is called Ivanenko-Heisenberg equation.
Since the charge of the nucleus of an atom is equal to the number of protons in it, then Z also called charge number. The charge number, or atomic number, is the same as its serial number in periodic system elements of Mendeleev.
In nature, there are elements Chemical properties which are exactly the same, but the mass numbers are different. Such elements are called isotopes. Isotopes have the same number of protons and different numbers of neutrons.
For example, hydrogen has three isotopes. All of them have a serial number equal to 1, and the number of neutrons in the nucleus is different for them. So, the simplest isotope of hydrogen, protium, has a mass number of 1, in the nucleus there is 1 proton and not a single neutron. It is the simplest chemical element.
The questions “What is matter made of?”, “What is the nature of matter?” has always occupied mankind. Since ancient times, philosophers and scientists have been looking for answers to these questions, creating both realistic and completely amazing and fantastic theories and hypotheses. However, literally a century ago, humanity came as close as possible to unraveling this mystery by discovering the atomic structure of matter. But what is the composition of the nucleus of an atom? What does everything consist of?
From theory to reality
By the beginning of the twentieth century, atomic structure had ceased to be just a hypothesis, but had become an absolute fact. It turned out that the composition of the nucleus of an atom is a very complex concept. It consists of But the question arose: the composition of the atom and include a different number of these charges or not?
planetary model
Initially, they imagined that the atom was built very similar to ours. solar system. However, it quickly turned out that this view was not entirely correct. The problem of a purely mechanical transfer of the astronomical scale of the picture to an area that occupies millionths of a millimeter has led to a significant and abrupt change properties and qualities of phenomena. The main difference was in the much more stringent laws and rules by which the atom is built.
Disadvantages of the planetary model
First, since atoms of the same kind and element must be exactly the same in terms of parameters and properties, the orbits of the electrons of these atoms must also be the same. However, the laws of motion of astronomical bodies could not provide answers to these questions. The second contradiction lies in the fact that the movement of an electron along the orbit, if well-studied physical laws are applied to it, must necessarily be accompanied by a permanent release of energy. As a result, this process would lead to the depletion of the electron, which would eventually die out and even fall into the nucleus.
Wave structure of the mother and
In 1924, a young aristocrat, Louis de Broglie, came up with an idea that turned the scientific community around such questions as the composition of atomic nuclei. The idea was that an electron is not just a moving ball that revolves around the nucleus. This is a blurry substance that moves according to laws resembling the propagation of waves in space. Quite quickly, this idea was extended to the movement of any body as a whole, explaining that we notice only one side of this very movement, but the second is not actually manifested. We can see the propagation of waves and not notice the movement of the particle, or vice versa. In fact, both of these sides of motion always exist, and the rotation of an electron in orbit is not only the movement of the charge itself, but also the propagation of waves. This approach is fundamentally different from the previously accepted planetary model.
Elementary basis
The nucleus of an atom is the center. Electrons revolve around it. Everything else is determined by the properties of the core. It is necessary to talk about such a concept as the composition of the nucleus of an atom from the most important point - from the charge. In the composition of the atom, there is a certain one that carries a negative charge. The nucleus itself has a positive charge. From this we can draw certain conclusions:
- The nucleus is a positively charged particle.
- Around the nucleus is a pulsating atmosphere created by charges.
- It is the nucleus and its characteristics that determine the number of electrons in an atom.
Kernel properties
Copper, glass, iron, wood have the same electrons. An atom can lose a couple of electrons or even all. If the nucleus remains positively charged, then it is able to attract the right amount of negatively charged particles from other bodies, which will allow it to survive. If an atom loses a certain number of electrons, then the positive charge on the nucleus will be greater than the remainder of the negative charges. In this case, the entire atom will acquire an excess charge, and it can be called a positive ion. In some cases, an atom can attract large quantity electrons, and then it becomes negatively charged. Therefore, it can be called a negative ion.
How much does an atom weigh ?
The mass of an atom is mainly determined by the nucleus. The electrons that make up the atom and the atomic nucleus weigh less than one thousandth of the total mass. Since mass is considered a measure of the energy reserve that a substance possesses, this fact is considered incredibly important when studying such a question as the composition of the atomic nucleus.
Radioactivity
The most difficult questions arose after the discovery that radioactive elements emit alpha, beta, and gamma waves. But such radiation must have a source. Rutherford in 1902 showed that such a source is the atom itself, or rather, the nucleus. On the other hand, radioactivity is not only the emission of rays, but also the conversion of one element into another, with completely new chemical and physical properties. That is, radioactivity is a change in the nucleus.
What do we know about nuclear structure?
Almost a hundred years ago, the physicist Prout put forward the idea that the elements in the periodic system are not incoherent forms, but are combinations. Therefore, one could expect that both the charges and the masses of the nuclei would be expressed in terms of integer and multiple charges of hydrogen itself. However, this is not quite true. Studying the properties of atomic nuclei using electromagnetic fields, the physicist Aston found that elements whose atomic weights were not integers and multiples were in fact a combination of different atoms, and not one substance. In all cases where the atomic weight is not an integer, we observe a mixture of different isotopes. What it is? If we talk about the composition of the nucleus of an atom, isotopes are atoms with the same charges, but with different masses.
Einstein and the nucleus of the atom
The theory of relativity says that mass is not a measure by which the amount of matter is determined, but a measure of the energy that matter possesses. Accordingly, matter can be measured not by mass, but by the charge that makes up this matter, and the energy of the charge. When the same charge approaches another of the same, the energy will increase, otherwise it will decrease. This, of course, does not mean a change in matter. Accordingly, from this position, the nucleus of an atom is not a source of energy, but rather, a residue after its release. So there is some contradiction.
Neutrons
The Curies, when bombarded with alpha particles of beryllium, discovered some incomprehensible rays, which, colliding with the nucleus of an atom, repel it with great force. However, they are able to pass through a large thickness of matter. This contradiction was resolved by the fact that the given particle turned out to have a neutral electric charge. Accordingly, it was called the neutron. Thanks to further research, it turned out that it is almost the same as that of the proton. Generally speaking, the neutron and the proton are incredibly similar. Taking into account this discovery, it was definitely possible to establish that both protons and neutrons are included in the composition of the nucleus of an atom, and in equal quantities. Everything gradually fell into place. The number of protons is the atomic number. Atomic weight is the sum of the masses of neutrons and protons. An isotope can also be called an element in which the number of neutrons and protons will not be equal to each other. As mentioned above, in such a case, although the element remains essentially the same, its properties may change significantly.
The composition of the nucleus of an atom
In 1932 after the discovery of the proton and neutron by scientists D.D. Ivanenko (USSR) and W. Heisenberg (Germany) proposed proton-neutronmodelatomic nucleus.
According to this model, the core consists of protons and neutrons. The total number of nucleons (i.e., protons and neutrons) is called mass number
A: A = Z + N
. The nuclei of chemical elements are denoted by the symbol:
X is the chemical symbol of the element.
For example, hydrogen
A number of notations are introduced to characterize atomic nuclei. The number of protons that make up the atomic nucleus is denoted by the symbol Z and call charge number (this is the serial number in the periodic table of Mendeleev). The nuclear charge is Ze , where e is the elementary charge. The number of neutrons is denoted by the symbol N .
nuclear forces
In order for atomic nuclei to be stable, protons and neutrons must be held inside the nuclei by huge forces, many times greater than the Coulomb repulsive forces of protons. The forces that hold nucleons in the nucleus are called nuclear . They are a manifestation of the most intense of all types of interaction known in physics - the so-called strong interaction. The nuclear forces are about 100 times greater than the electrostatic forces, and by tens of orders of magnitude greater than the forces of the gravitational interaction of nucleons.
Nuclear forces have the following properties:
- have attractive forces
- is the forces short-range(appear at small distances between nucleons);
- nuclear forces do not depend on the presence or absence of an electric charge on the particles.
Mass Defect and Binding Energy of the Nucleus of an Atom
The most important role in nuclear physics is played by the concept nuclear binding energy .
The binding energy of the nucleus is equal to the minimum energy that must be expended for the complete splitting of the nucleus into individual particles. It follows from the law of conservation of energy that the binding energy is equal to the energy that is released during the formation of a nucleus from individual particles.
The binding energy of any nucleus can be determined by accurately measuring its mass. At present, physicists have learned to measure the masses of particles - electrons, protons, neutrons, nuclei, etc. - with very high accuracy. These measurements show that the mass of any nucleus M i is always less than the sum of the masses of its constituent protons and neutrons: 
The mass difference is called mass defect. Based on the mass defect using the Einstein formula E = mc 2 it is possible to determine the energy released during the formation of a given nucleus, i.e., the binding energy of the nucleus E St:
This energy is released during the formation of the nucleus in the form of radiation of γ-quanta.
Nuclear energy
In our country, the world's first nuclear power plant was built and launched in 1954 in the USSR, in the city of Obninsk. The construction of powerful nuclear power plants. There are currently 10 operating nuclear power plants in Russia. After the accident at Chernobyl nuclear power plant additional measures have been taken to ensure the safety of nuclear reactors.
Each atom is made up of nuclei and atomic shell, which include various elementary particles - nucleons and electrons(Fig. 5.1). The nucleus is the central part of the atom, containing almost the entire mass of the atom and having a positive charge. The core is made up of protons and neutrons, which are doubly charged states of one elementary particle - the nucleon. Proton charge +1; neutron 0.
Core charge atom is Z . ē , where Z– serial number of elements (atomic number) in the periodic system of Mendeleev, equal to the number of protons in the nucleus; ē is the charge of an electron.
The number of nucleons in a nucleus is called the mass number of the element(A):
A = Z + N,
where Z is the number of protons; N is the number of neutrons in the atomic nucleus.
For protons and neutrons, the mass number is taken equal to 1, for electrons it is equal to 0.
Rice. 5.1. The structure of the atom
The following designations are generally accepted for any chemical element X: , here A- mass number, Z is the atomic number of the element.
Atomic nuclei of the same element can contain a different number of neutrons. N. These types of atomic nuclei are called isotopes this element. Thus, isotopes have: the same atomic number, but different mass numbers A. Most chemical elements are a mixture of different isotopes, for example, isotopes of uranium:
.
Atomic nuclei of different chemical elements can have the same mass number BUT(with different number of protons Z). These types of atomic nuclei are called isobars. For example:
– – – ; –
Atomic mass
To characterize the mass of atoms and molecules, the concept is used atomic mass M is a relative value, which is determined by the ratio
to the mass of the carbon atom and is taken equal to m a = 12,000,000. For
absolute definition of atomic mass was introduced atomic unit
masses(a.m.u.), which is defined in relation to the mass of a carbon atom in the following form:
.
Then the atomic mass of an element can be defined as:
where M is the atomic mass of the isotopes of the element under consideration. This expression makes it easier to determine the mass of the nuclei of elements, elementary particles, particles - products of radioactive transformations, etc.
Nuclear mass defect and nuclear binding energy
Binding energy of the nucleon – physical quantity, numerically equal to the work that must be done to remove the nucleon from the nucleus without imparting kinetic energy to it.
The nucleons are bound in the nucleus by nuclear forces, which are much greater than the electrostatic repulsion forces acting between protons. To split the nucleus, it is necessary to overcome these forces, i.e., to expend energy. The union of nucleons to form a nucleus, on the contrary, is accompanied by the release of energy, which is called nuclear binding energyΔ W St:
,
where is the so-called nuclear mass defect; With ≈ 3 . 10 8 m/s is the speed of light in vacuum.
Core binding energy- a physical quantity equal to the work that needs to be done to split the nucleus into individual nucleons without imparting kinetic energy to them.
When a nucleus is formed, its mass decreases, i.e., the mass of the nucleus is less than the sum of the masses of its constituent nucleons, this difference is called mass defectΔ m:
where m p is the proton mass; m n is the neutron mass; m nucleus is the mass of the nucleus.
In the transition from the mass of the nucleus m nucleus to atomic masses of an element m a, this expression can be written in the following form:
where m H is the mass of hydrogen; m n is the mass of the neutron and m a is the atomic mass of the element, determined through atomic mass unit(a.u.m.).
The criterion for the stability of the nucleus is the strict correspondence between the number of protons and neutrons in it. For the stability of nuclei, the following relation is true:
,
where Z is the number of protons; A is the mass number of the element.
Of the approximately 1700 types of nuclei known so far, only about 270 are stable. Moreover, even-even nuclei (that is, with an even number of protons and neutrons), which are especially stable, predominate in nature.
Radioactivity
Radioactivity- transformation of unstable isotopes of one chemical element into isotopes of another chemical element with the release of some elementary particles. Distinguish: natural and artificial radioactivity.
The main types include:
– α-radiation (decay);
– β-radiation (decay);
- spontaneous nuclear fission.
The nucleus of a decaying element is called maternal, and the nucleus of the resulting element is child. The spontaneous decay of atomic nuclei obeys the following law of radioactive decay:
where N 0 is the number of cores in chemical element at the initial moment of time; N is the number of cores at a time t; - the so-called "constant" of decay, which is the fraction of nuclei that decayed per unit time.
The reciprocal of the decay "constant" characterizes the average lifespan of the isotope. A characteristic of the stability of nuclei with respect to decay is half life, i.e., the time during which the initial number of nuclei is halved:
Relationship between and :
, 
.
During radioactive decay, charge conservation law:
,
where is the charge of the decayed or resulting (formed) "fragments"; and mass conservation rule:
where is the mass number of formed (decayed) “fragments”.
5.4.1. α and β decay
α-decay is the radiation from helium nuclei. Characteristic for "heavy" nuclei with large mass numbers A> 200 and charge z > 82.
The displacement rule for α-decay has the following form (a new element is formed):
.
; 
.
Note that α-decay (radiation) has the highest ionizing ability, but the lowest permeability.
There are the following types β-decay:
– electronic β-decay (β – decay);
– positron β-decay (β + -decay);
– electronic capture (k-capture).
β - -decay occurs with an excess of neutrons with the release of electrons and antineutrinos:
.
β + -decay occurs with an excess of protons with the release of positrons and neutrinos:
.
For electronic capture ( k-capture) characterized by the following transformation:
.
The displacement rule for β-decay has the following form (a new element is formed):
for β - -decay: 
;
for β + -decay: 
.
β-decay (radiation) has the lowest ionizing ability, but the highest permeability.
α and β radiation are accompanied γ-radiation, which is the radiation of photons and is not an independent type of radioactive radiation.
γ-photons are released with a decrease in the energy of excited atoms and do not cause a change in the mass number A and charge change Z. γ-radiation has the highest penetrating power.
Activity of radionuclides
Activity of radionuclides is a measure of radioactivity that characterizes the number of nuclear decays per unit time. For a certain amount of radionuclides in a certain energy state in this moment time activity BUT is given in the form:
where is the expected number of spontaneous nuclear transformations (the number of nuclear decays) occurring in the source of ionizing radiation over the time interval .
Spontaneous nuclear transformation is called radioactive decay.
The unit of measure for the activity of radionuclides is the reciprocal second (), which has a special name becquerel (Bq).
Becquerel is equal to the activity of the radionuclide in the source, in which for 1 sec. one spontaneous nuclear transformation occurs.
Off-system unit of activity - curie (Ku).
Curie - the activity of the radionuclide in the source, in which for a time of 1 sec. happening 3.7 . 10 10 spontaneous nuclear transformations, i.e. 1 Ku = 3.7 . 10 10 Bq.
For example, approximately 1 g of pure radium gives an activity of 3.7 . 10 10 nuclear disintegrations per second.
Not all nuclei of a radionuclide decay simultaneously. In each unit of time, spontaneous nuclear transformation occurs with a certain fraction of nuclei. The share of nuclear transformations for different radionuclides is different. For example, out of the total number of radium nuclei, 1.38 decays every second . part, and from the total number of radon nuclei - 2.1 . part. The fraction of nuclei decaying per unit time is called the decay constant λ .
From the above definitions it follows that the activity BUT related to the number of radioactive atoms N in the source at a given time by the ratio:
Over time, the number of radioactive atoms decreases according to the law:
, (3) – 30 years, surface radon or linear activity.
The choice of units of specific activity is determined by a specific task. For example, activity in the air is expressed in becquerels per cubic meter (Bq / m 3) - volumetric activity. Activity in water, milk and other liquids is also expressed as volumetric activity, since the amount of water and milk is measured in liters (Bq/l). Activity in bread, potatoes, meat and other products is expressed as specific activity (Bq/kg).
Obviously, the biological effect of exposure to radionuclides on the human body will depend on their activity, i.e., on the amount of the radionuclide. Therefore, the volume and specific activity of radionuclides in air, water, food, building and other materials are standardized.
Since for a certain time a person can be irradiated in various ways (from the entry of radionuclides into the body to external exposure), all exposure factors are associated with a certain value, which is called the radiation dose.
