What is the mass number of an atomic nucleus. The structure and composition of the atomic nucleus

(collectively called "nucleons") in the nucleus. Usually denoted by the letter A. The mass number is close to the atomic mass of the isotope, expressed in atomic mass units, but coincides with it only for carbon-12, since the atomic mass unit (a.m.u.) is now defined as 1 ⁄ 12 atom mass 12 C. In all other cases atomic mass is not an integer, unlike a mass number. Thus, the mass number of the chlorine isotope 35 Cl is 35, and its atomic mass is 34.96885 a.m.u.

The mass number in the designation of a particular nuclide (a type of atomic nucleus) is written in the upper left index, for example 232 Th. Nuclides with the same mass number are called isobars (for example, 14 C and 14 N nuclides are isobars).

Knowing the mass number allows you to estimate the mass of the nucleus and the atom. If the mass number is known, then the mass M an atom and its nucleus is estimated from the following relation M ≈ A m N, where m N ≈ 1.67 10 −27 kg is the mass of a nucleon, that is, a proton or a neutron. For example, an aluminum-27 atom and its nucleus contain 27 nucleons (13 protons and 14 neutrons). Its mass is approximately equal to 27 1.67 10 −27 kg ≈ 4.5 10 −26 kg. If it is necessary to obtain the mass of the nucleus with greater accuracy, then it must be taken into account that the nucleons in the nucleus are bound by the forces of nuclear attraction, and therefore, in accordance with the relation E=mc 2 the mass of the nucleus decreases. The total mass of electrons in orbits around the nucleus should also be added to the mass of an atom. However, all these corrections do not exceed 1%.

238 92 U

→

234 90 Th

4 2 He

on the left side, the mass number of the initial nucleus is 238; on the right side of the reaction, there are two nuclei with mass numbers 234 and 4, which gives a total of 238. Considering that the mass number of an alpha particle (helium-4 nucleus) is 4, alpha -decay reduces the mass number of the decaying nucleus by 4 units. Any types of beta decay (beta minus decay, positron decay, electron capture, all types of double beta decay) do not change the mass number, since in this process only the transformation of some nucleons of the nucleus from one type to another (protons into neutrons or back). The isomeric transition also does not change the mass number of the nucleus.

Write a review on the article "Mass number"

Notes

An excerpt characterizing the Mass number

- Tell me, did you know about the death of the countess when you stayed in Moscow? - said Princess Mary, and immediately blushed, noticing that, making this question after his words that he was free, she ascribed to his words such a meaning that they, perhaps, did not have.
“No,” answered Pierre, obviously not finding awkward the interpretation that Princess Mary gave to his mention of his freedom. - I learned this in Orel, and you can not imagine how it struck me. We were not exemplary spouses, ”he said quickly, looking at Natasha and noticing in her face the curiosity about how he would respond about his wife. “But this death shocked me terribly. When two people quarrel, both are always to blame. And one's own guilt suddenly becomes terribly heavy in front of a person who is no longer there. And then such a death ... without friends, without consolation. I’m very, very sorry for her, ”he finished, and with pleasure noticed the joyful approval on Natasha’s face.
“Yes, here you are again a bachelor and a groom,” said Princess Mary.
Pierre suddenly blushed crimson and for a long time tried not to look at Natasha. When he ventured to look at her, her face was cold, stern, and even contemptuous, as it seemed to him.
“But you definitely saw and spoke with Napoleon, as we were told?” - said Princess Mary.
Pierre laughed.
- Never, never. It always seems to everyone that being a prisoner means being visiting Napoleon. Not only have I not seen him, but I have not heard of him either. I was in much worse society.
Dinner was over, and Pierre, who at first refused to tell about his captivity, gradually became involved in this story.
“But is it true that you stayed behind to kill Napoleon?” Natasha asked him, smiling slightly. - I then guessed when we met you at the Sukharev Tower; remember?
Pierre admitted that this was true, and from this question, gradually guided by the questions of Princess Mary and especially Natasha, he became involved in detailed story about your adventures.
At first he spoke with that mocking, meek look that he now had on people, and especially on himself; but then, when he came to the story of the horrors and sufferings that he saw, he, without noticing it, got carried away and began to speak with the restrained excitement of a man who experiences strong impressions in his memory.

"") in the kernel. Usually denoted by the letter A. The mass number is close to the atomic mass of the isotope, expressed in atomic mass units, but coincides with it only for carbon-12, since the atomic mass unit (amu) is now defined as 1 ⁄ 12 of the mass of an atom 12 C. In all other cases, the atomic mass is not an integer, unlike the mass number. Thus, the mass number of the chlorine isotope 35 Cl is 35, and its atomic mass is 34.96885 a.m.u.

238 92 U

→

234 90 Th

4 2 He

Notes

MASS NUMBER- (number of nucleons, symbol A), total number NUCLEONS (NEUTRONS AND PROTONS) IN THE NUCLEAR OF THE ATOM. It is usually written as a superscript before the element's chemical symbol. Thus, the lightest element, hydrogen, has only one proton in the nucleus, and ... ... Scientific and technical encyclopedic Dictionary

MASS NUMBER- total number of nucleons (neutrons and protons) in at. core. Different for isotopes of the same chem. element. Physical Encyclopedic Dictionary. Moscow: Soviet Encyclopedia. Chief Editor A. M. Prokhorov. 1983. MASS NUMBER ... Physical Encyclopedia

MASS NUMBER is the number of nucleons in an atomic nucleus. Usually indicated at the top left of the symbol chemical element(e.g. 10V) … Big Encyclopedic Dictionary

MASS NUMBER- the total number of nucleons (protons and neutrons) in the atomic nucleus, denoted by A and indicated by the index at the top left of the symbol of the corresponding element, for example. 32S means a sulfur isotope with a mass number of 32 (A = 32). The mass of the isotope is equal to the whole ... ... Great Polytechnic Encyclopedia

mass number- — [Ya.N. Luginsky, M.S. Fezi Zhilinskaya, Yu.S. Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow] Electrical engineering topics, basic concepts EN mass number ... Technical Translator's Handbook

mass number is the number of nucleons in an atomic nucleus. Usually indicated at the top left of the symbol of a chemical element (for example, 10V). * * * MASS NUMBER MASS NUMBER, the number of nucleons in an atomic nucleus. Usually indicated at the top left of the chemical element symbol ... ... encyclopedic Dictionary

mass number- masės skaičius statusas T sritis Standartizacija ir metrologija apibrėžtis Atomo branduolio nukleonų skaičius. atitikmenys: engl. mass number; nuclear number; nucleon number vok. Massenzahl, f; Nukleonenzahl, f rus. mass number, n; number… … Penkiakalbis aiskinamasis metrologijos terminų žodynas

mass number- masės skaičius statusas T sritis chemija apibrėžtis Atomo branduolio nukleonų skaičius. atitikmenys: engl. mass number; nuclear number; nucleon number eng. mass number... Chemijos terminų aiskinamasis žodynas

mass number- masės skaičius statusas T sritis fizika atitikmenys: angl. mass number; nuclear number; nucleon number vok. Massenzahl, f; Massezahl, f; Nukleonenzahl, f rus. mass number, n pranc. nombre de masse, m; nombre de nucléons, m … Fizikos terminų žodynas

Mass number- the number of nucleons (protons and neutrons) in the atomic nucleus; denoted by the letter A and is usually indicated at the top left next to the symbol of the element, for example, 32S means a sulfur isotope with A \u003d 32. M. h. And the nuclear charge Z, expressed in units of elementary ... Great Soviet Encyclopedia

Books

Incompetent manager. Incompetence as Mass Madness, Adrian Farnham. A surprisingly large number of people work under managers who are obviously incompetent. Some experts believe that in some sectors of the economy the number of incompetent ...

The study of atomic nuclei began after the establishment of the following experimental facts: 1) the discovery in 1896 by the French scientist Henri Becquerel of natural radioactivity; 2) the discovery in 1910 by the English scientist Soddy of the isotopy of chemical elements; 3) the nuclear model of the atom, proposed in 1911 by the great English physicist Ernest Rutherford.

Rutherford, investigating radioactivity, came to the conclusion in 1908 that during radioactive decay, the transformation of atoms of some chemical elements into atoms of other elements occurs. Later, investigating the passage of a-particles with an energy of several megaelectron-volts through thin films gold, Rutherford discovered nuclear model atom, after which it became clear that during radioactivity, the transformation of the nuclei of some elements into the nuclei of other elements takes place.

The discovery of isotopy played next role. Atomic weights, i.e. the masses of atoms of chemically pure elements, as a rule, are expressed in a.m.u. numbers that are not very close to integers. For example, the atomic weight of boron (B) is 10.82; Ne - 20.183; Cl - 35.457; Fe -56.85 ;… . With the discovery of isotopy, the opinion was established that a chemically pure element is a mixture of isotopes that differ from each other in atomic weights. The atomic weights of isotopes turned out to be closer to integers than the atomic weights of elements, and the closer, the lighter the isotope, i.e. the lower its atomic weight. This led scientists to the idea that the nucleus consists of particles whose atomic weights are close to unity. This condition is well satisfied by the nucleus of the hydrogen atom - a proton, the atomic weight of which is close to unity (1.008). In addition, since the charge of the proton is positive, the idea arose that the composition of the nucleus must necessarily include protons. Other constituent particles of the nucleus took a long time to figure out. The phenomenon of natural β-activity seemed to indicate that electrons were included in the composition of the nucleus. Therefore, a proton-electron model of the nucleus was proposed. However, the proton-electron model turned out to be untenable. According to this model, the spin of a nucleus composed of even number protons and electrons must be integer (the spin of a proton, like the spin of an electron, is ½ ħ), and in practice, half-integer numbers are also observed. The model did not explain why the magnetic moment of the nucleus is 2000 times less than the magnetic moment of the electron. Finally, the proton-electron model turned out to be in conflict with the Heisenberg principle. Knowing the size of the nucleus, it is possible to estimate the magnitude of the momentum of the electron, which is part of the nucleus, and, consequently, the magnitude of its energy. Such estimates give that the energy of an electron in a nucleus is about 200 MeV. According to the experiment, the binding energy of one particle in the nucleus is 7 - 8 MeV. In addition, the energy of 200 MeV is many times greater than the energy of the electrons emitted by the nucleus during β-decay.

A way out of the difficulty was found after in 1932 Rutherford's employee, Chadwick, discovered a new elementary particle- neutron. The mass of a neutron is approximately equal to the mass of a proton, slightly exceeding it, and electric charge is equal to 0. Shortly after the discovery of the neutron, in 1934, the Soviet physicist D.D. Ivanenko put forward a hypothesis about the proton-neutron structure of the nucleus. This same hypothesis was independently proposed by Heisenberg.

Currently proton-neutron structure of the nucleus is universally recognized and underlies modern ideas about the nucleus and all nuclear physics.

According to modern data, the proton (p) has a positive charge equal to the charge of the electrode qp= 1.6. 10 -19 C and rest mass m p= (1.0075957 ±0.000001) amu = (1836.09±0.01) me.

Neutron (n) - neutral particle with rest mass m n= (1.008982 ±0.000003)a.m.u. = (1838.63 ± 0.01) me, where 1amu = 1.667 . 10 -27 kg - 1/12 of the mass of the C 12 atom;

me= 9.106. 10 -31 kg – the rest mass of an electron.

In modern physics, it is believed that the proton and neutron are two charge states of the same particle, which are called nucleon(from lat. nucleus - core). So, the proton is the proton state of the nucleon, the neutron is the neutron state of the nucleon. The total number of nucleons in an atomic nucleus is called mass number A.

The atomic nucleus is characterized by the charge Ze , where Z is the charge number of the 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. The currently known 107 elements of the periodic table have charge numbers of nuclei from Z = 1 to Z = 107. Since Z is equal to the number of protons in the nucleus, the number of neutrons in the nucleus is: N = A - Z. In nuclear physics it is customary to designate the nucleus with the same symbol as the neutral atom: , where X- symbol

chemical element, Z- atomic number (number of protons in the nucleus), BUT- mass number (number of nucleons in the nucleus).

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 Chemical properties atom. Consequently, the nuclear charge determines the specifics of a given chemical element, i.e., determines the number of electrons in an atom, their configuration electron shells, magnitude and nature of the intraatomic electric field.

Kernels with the same Z but different BUT(t. s. with different numbers neutrons) are called isotopes, and nuclei with the same BUT, but different Z isobars. For example, hydrogen ( Z= 1) has three isotopes; - protium ( Z =1, N= 0) ; - deuterium, ( Z =1, N= 1); - tritium ( Z =1, N= 2). In the vast majority of cases, isotopes of the same chemical element have the same chemical and almost identical physical properties(the exception is hydrogen isotopes), which are determined mainly by the structure of the electron shells, which is the same for all isotopes of a given element.

The following nuclei can serve as an example of isobar nuclei: , , . Currently, more than 2500 nuclei are known that differ either Z, or A, or both.

Rutherford showed that atomic nuclei have dimensions of approximately 10 -14 - 10 -15 m (for comparison, linear dimensions atom about 10-10 m). Core radius - given by empirical formula R = R 0 A 1/3 where R0= (1.3 ÷ 1.7) 10 -15 m. However, when using this concept, care must be taken because of its ambiguity, for example, because of the blurring of the core boundary. 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: ρ » 10 17 kg/m 3 .

The center of an atom contains most of its mass and all of its positive charge. This region of the atom is called the nucleus.

The dimensions of an atom are m, and the dimensions of the nucleus
m the mass of the nucleus is 99.95% of the mass of the atom. In a neutral atom Z electrons. The charge of the nucleus is positive and a multiple of the elementary charge
Cl. The nuclear charge can be represented as
, where Z- charge number, it coincides with the chemical number of the periodic table and is equal to the number of protons entering the nucleus.

The second most important characteristic of the nucleus is its mass. The mass of the nucleus turned out to be greater than the sum of the masses of the protons entering the nucleus.

It was suggested that the nucleus contains neutral particles. In 1932 Chadwig discovered neutrons. Ivanenko and Heisenberg proposed the proton-neutron theory of the nucleus. The nucleus splits into protons and neutrons. They are called nucleons. The total number of nucleons in the nucleus called the mass numberA . The total number of neutrons is N=A-Z. The rest mass of a proton is
kg, the neutron mass is
kg.

The nucleus of a chemical element is denoted by the same symbol as the neutral atom.
, where Z- atomic number (nuclear charge), A- mass number (number of nucleons in the nucleus). Nuclei with the same charge number but different masses are called isotopes (isotopes differ in the number of neutrons). Nuclei with the same mass number but different charges are called isobars.

28. Properties of nuclear forces.

Features of nuclear forces:

29. Radioactivity. Alpha and beta decay. Displacement rules.

Radioactivity is called the transformation of unstable isotopes of one chemical element into isotopes of another element, accompanied by the emission of certain particles. Natural radioactivity refers to the radioactivity observed in naturally occurring unstable isotopes. Artificial radioactivity is called the radioactivity of isotopes obtained as a result of nuclear reactions.

Radioactive radiation has a complex composition. In a magnetic field, a narrow beam of radioactive radiation is split into three components:

-particles - flow of helium nuclei with a charge Z=2 e and mass number A=4 (
). Speed -particles is equal to
m/s. Getting into the substance -particles actively interact with atoms and molecules, ionize and excite it. When the energy -particles are reduced to thermal motion, it captures two electrons and turns into a helium atom ( He). Before that, she goes through a path called a run. Because of the strong interaction with matter, the range is short. A sheet of paper or clothes hold up -particles. The 0.05 mm thick aluminum sheet also delays -particles. Ionizing ability -particles is large and equal
steam over the run.

-particles is a stream of electrons escaping from nuclei at a speed
m/s. The nucleus emits an electron when a neutron becomes a proton:

where - electron symbol, - electron antineutrino.

Ionizing ability particles hundreds of times smaller than -particles, and the penetrating power is greater. - radiation is delayed by a layer of aluminum 2 mm thick.