Physical properties of atomic nuclei.
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Core charge. Kernel size. Moments of nuclei.
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Spin of the nucleus. magnetic and electric moments kernels. The mass of the nucleus and the mass of the atom. mass defect. Communication energy. The main features of the bond energy. Basic rule. nuclear forces: main characteristics, Coulomb and nuclear potentials of the nucleus. Exchange character of nuclear forces.

Moseley's law. The electric charge of the nucleus is formed by the protons that make up its composition. Number of protons Z called its charge, meaning that the absolute value of the charge of the nucleus is equal to Ze. The charge of the nucleus is the same as the serial number Z element in Mendel-Eev's periodic system of elements. For the first time, the charges of atomic nuclei were determined by the English physicist Moseley in 1913. By measuring the wavelength with a crystal λ characteristic X-ray radiation for the atoms of certain elements, Moseley discovered a regular change in wavelength λ for elements following one after another in the periodic system (Fig. 2.1). Moseley interpreted this observation as the dependence λ from some atomic constant Z, changing by one from element to element and equal to one for hydrogen:

where and are constants. From experiments on the scattering of X-ray quanta by atomic electrons and α -particles by atomic nuclei, it was already known that the charge of the nucleus is approximately equal to half the atomic mass and, therefore, is close to the ordinal number of the element. Since the emission of characteristic X-ray radiation is a consequence of electrical processes in the atom, Moseley concluded that the atomic constant found in his experiments, which determines the wavelength of the characteristic X-ray radiation and coincides with the serial number of the element, must be only the charge of the atomic nucleus (Moseley's law).

Rice. 2.1. X-ray spectra of atoms of neighboring elements obtained by Moseley

The measurement of X-ray wavelengths is carried out with great accuracy, so that, on the basis of Moseley's law, the belonging of an atom to a chemical element is established absolutely reliably. However, the fact that the constant Z in the last equation is the charge of the nucleus, although it is justified by indirect experiments, it ultimately rests on the postulate - Moseley's law. For this reason, after Moseley's discovery, the charges of nuclei were repeatedly measured in scattering experiments. α -particles based on Coulomb's law. In 1920, Chadwig improved the method for measuring the proportion of scattered α -particles and received the charges of the nuclei of atoms of copper, silver and platinum (see table 2.1). Chadwig's data leave no doubt about the validity of Moseley's law. In addition to these elements, the charges of the nuclei of magnesium, aluminum, argon and gold were also determined in the experiments.

Table 2.1. The results of Chadwick's experiments

Definitions. After Moseley's discovery, it became clear that the main characteristic of an atom is the charge of the nucleus, and not its atomic mass, as chemists of the 19th century assumed, because the charge of the nucleus determines the number of atomic electrons, which means that Chemical properties atoms. The reason for the difference between the atoms of chemical elements is precisely that their nuclei have a different number of protons in their composition. On the contrary, a different number of neutrons in the nuclei of atoms with the same number of protons does not change the chemical properties of atoms in any way. Atoms that differ only in the number of neutrons in their nuclei are called isotopes chemical element.

An atom with a certain number of protons and neutrons in the composition of the nucleus is usually called nuclide. The composition of the nucleus is given by numbers Z and A. An isotope is only spoken of when referring to belonging to a chemical element, for example, 235 U is an isotope of uranium, but 235 U is a fissile nuclide, not a fissile isotope.

Atoms whose nuclei contain the same number of neutrons but a different number of protons are called isotons. atoms with the same mass numbers, but different proton-neutron composition of nuclei, are called isobars.

CHARGE OF THE NUCLEAR - concept and types. Classification and features of the category "CHARGE OF THE NUCLEAR" 2014, 2015.

atomic nucleus

and elementary particles

Chapter 32

Elements of nuclear physics

§251. Size, composition and charge of the atomic nucleus. Mass and charge number

E. Rutherford, investigating the passage of -particles with an energy of several megaelectron-volts through thin films of gold (see § 208), came to the conclusion that an atom consists of a positively charged nucleus and electrons surrounding it. After analyzing these experiments, Rutherford also showed that atomic nuclei have dimensions of approximately 10 -1 4 -10 -1 5 m (the linear dimensions of an atom are approximately 10 - 10 m).

The atomic nucleus is made up of elementary particles - protons and neutrons(The proton-neutron model of the nucleus was proposed by the Soviet physicist D. D. Ivanenko (b. 1904), and subsequently developed by V. Heisenberg).

Proton (R) has a positive charge equal to the electron charge and rest mass m p =1.6726 10 -2 7 kg 1836m e , where m e - the mass of an electron. Neutron (n) - neutral particle with rest mass m n =1.6749 10 -2 7 kg 1839m e ,. Protons and neutrons are called nucleons(from lat. nucleus - core). Total number nucleons in an atomic nucleus is called mass numberBUT.

The atomic nucleus is characterized charge Ze where e- proton charge, Z - charge number nucleus, equal to the number of protons in the nucleus and coinciding with the serial number of the chemical element in the Periodic system of elements of Mendeleev. Currently known 107 elements of the periodic table have charge numbers of nuclei from Z=1 to Z=107.

The nucleus is denoted by the same symbol as the neutral atom: A Z X, where X is the symbol of the chemical element, Z is the atomic number (the number of protons in the nucleus), BUT - mass number (number of nucleons in the nucleus).

Now the proton-neutron model of the nucleus is beyond doubt. The hypothesis of the proton-electronic structure of the nucleus was also considered, but it did not stand up to experimental verification. So, if we adhere to this hypothesis, then the mass number BUT should be the number of protons in the nucleus, and the difference between the mass number and the number of electrons should be equal to the nuclear charge. This model was consistent with the values ​​of isotopic masses and charges, but contradicted the values ​​of the spins and magnetic moments of nuclei, the binding energy of the nucleus, etc. In addition, it turned out to be incompatible with the uncertainty relation (see §215). As a result, the hypothesis of the proton-electronic structure of the nucleus was rejected.

Since the atom is neutral, the charge of the nucleus determines the number of electrons in the atom. The number of electrons determines their distribution over states in the atom, which, in turn, determines the chemical properties of the atom. Consequently, the charge of the nucleus determines the specifics of a given chemical element, i.e., determines the number of electrons in an atom, the configuration of their electron shells, the magnitude and nature of the intraatomic electric field.

Kernels with the same Z but different BUT(i.e. with different numbers of neutrons N=

BUT - Z) are called isotopes, and nuclei with the same A but different Z - isobars. For example, hydrogen (Z=1) has three isotopes: 1 1 H - protium (Z=1, N=0), 2 1 H - deuterium (Z=1, N= 1), 3 1 H - tritium (Z \u003d 1, N \u003d 2), tin - ten, etc. In the vast majority of cases, isotopes of the same chemical element have the same chemical and almost the same physical properties (the exceptions are, for example, isotopes of hydrogen), determined mainly by the structure of the electron shells, which is the same for all isotopes of a given element. An example of isobar nuclei is the 10 4 Be, 10 5 B, 10 6 C nuclei. Currently, more than 2000 nuclei are known that differ either in Z, or A, or both.

core radius is given by the empirical formula

R \u003d R 0 A 1 / 3, (251.1)

where R 0 \u003d (1.3-1.7) 10 -1 5 m. However, when using this term, care must be taken (due to its ambiguity, for example, due to blurring of the core boundary). From formula (251.1) it follows that the volume of the nucleus is proportional to the number of nucleons in the nucleus. Consequently, the density of nuclear matter is approximately the same for all nuclei (10 17 kg / m 3).

From the planetary model of the structure of atoms, we know that an atom is a nucleus, and a cloud of electrons rotating around it. Moreover, the distance between the electrons and the nucleus is tens and hundreds of thousands of times greater than the size of the nucleus itself.

What is the core itself? Is it a small hard indivisible ball or is it made up of smaller particles? Not a single microscope that exists in the world is able to clearly show us what is happening at this level. Everything is too small. Then how to be? Is it even possible to study the physics of the atomic nucleus? How to find out the composition and characteristics of the atomic nucleus, if it is not possible to study it?

The charge of the nucleus of an atom

With a wide variety of indirect experiments, expressing hypotheses and testing them in practice, through trial and error, scientists managed to investigate atomic structure kernels. It turned out that the nucleus consists of even smaller particles. The size of the nucleus, its charge and the chemical properties of the substance depend on the number of these particles. Moreover, these particles have positive charge, which compensates negative charge electrons of an atom. These particles are called protons. Their number in the normal state is always equal to the number of electrons. The question of how to determine the charge of the nucleus no longer stood. The charge of the nucleus of an atom in a neutral state is always equal to the number of electrons revolving around it and is opposite in sign to the charge of the electrons. And physicists have already learned how to determine the number and charge of electrons.

The structure of the atomic nucleus: protons and neutrons

However, in the process of further research, a new problem arose. It turned out that protons, having the same charge, in some cases they differ twice in mass. This caused a lot of questions and inconsistencies. In the end, it was possible to establish that the composition of the atomic nucleus, in addition to protons, also includes some particles that are almost equal in mass to protons, but do not have any charge. These particles are called neutrons. The detection of neutrons resolved all inconsistencies in the calculations. As a result, protons and neutrons, as the constituent elements of the nucleus, were called nucleons. The calculation of any values ​​relating to the characteristics of the core has become much easier to understand. Neutrons do not take part in the formation of the nuclear charge, therefore, their influence on the chemical properties of matter is practically not manifested, however, neutrons participate in the formation of the mass of nuclei, respectively, affect the gravitational properties of the atomic nucleus. Thus, there is some indirect influence of neutrons on the properties of matter, but it is extremely insignificant.