What is an atomic nucleus. The structure of the atom: nucleus, neutron, proton, electron
An atom consists of a positively charged nucleus and surrounding electrons. Atomic nuclei have dimensions of approximately 10 -14 ... 10 -15 m (the linear dimensions of an atom are 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 Russian physicist D. D. Ivanenko, and subsequently developed by V. Heisenberg.
Proton ( R) has a positive charge equal to that of an electron and a rest mass t p =
1.6726∙10 -27 kg 1836 m e, where m e is the mass of the electron. Neutron ( n)-neutral particle with rest mass m n= 1.6749∙10 -27 kg 1839t e ,.
The mass of protons and neutrons is often expressed in other units - in atomic mass units (a.m.u., a unit of mass equal to 1/12 of the mass of a carbon atom
). The masses of the proton and neutron are approximately equal to one atomic mass unit. Protons and neutrons are called nucleons(from lat. nucleus-kernel). Total number nucleons in an atomic nucleus is called the mass number BUT).
The radii of the nuclei increase with increasing mass number in accordance with the relation R= 1,4BUT 1/3 10 -13 cm.
Experiments show that nuclei do not have sharp boundaries. There is a certain density of nuclear matter in the center of the nucleus, and it gradually decreases to zero with increasing distance from the center. Due to the lack of a well-defined boundary of the nucleus, its "radius" is defined as the distance from the center at which the density of nuclear matter is halved. The average matter density distribution for most nuclei turns out to be not just spherical. Most of the nuclei are deformed. Often the nuclei are in the form of elongated or flattened ellipsoids.
The atomic nucleus is characterized chargeZe, where Zcharge 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.
The nucleus is denoted by the same symbol as the neutral atom:
, where X- symbol of a chemical element, Z atomic number (number of protons in the nucleus), BUT- mass number (number of nucleons in the nucleus). Mass number BUT approximately equal to the mass of the nucleus in atomic mass units.
Since the atom is neutral, the charge of the nucleus Z determines the number of electrons in an atom. The number of electrons depends on the distribution over states in the atom. The nuclear charge 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.
Nuclei with the same charge numbers Z, but with different mass numbers BUT(i.e. with different numbers neutrons N=A-Z) are called isotopes, and nuclei with the same BUT, but different Z- isobars. For example, hydrogen ( Z= l) has three isotopes: H - protium ( Z=l, N= 0), H - deuterium ( Z=l, N= 1), H - tritium ( Z=l, N\u003d 2), tin - ten isotopes, etc. In the vast majority of cases, isotopes of the same chemical element have the same chemical and almost the same physical properties.
E, MeV
Energy levels
and observed transitions for the boron atom nucleus
Quantum theory strictly limits the energy values that the constituent parts of nuclei can have. Sets of protons and neutrons in nuclei can only be in certain discrete energy states characteristic of a given isotope.When an electron changes from a higher to a lower energy state, the energy difference is emitted in the form of a photon. The energy of these photons is of the order of several electron volts. For nuclei, the level energies lie in the range from approximately 1 to 10 MeV. During transitions between these levels, photons of very high energies (γ-quanta) are emitted. To illustrate such transitions in Fig. 6.1 shows the first five energy levels of the nucleus
.Vertical lines indicate observed transitions. For example, a γ-quantum with an energy of 1.43 MeV is emitted during the transition of the nucleus from a state with an energy of 3.58 MeV to a state with an energy of 2.15 MeV.
The composition of the nucleus of an atom. Calculation of protons and neutrons
According to modern ideas An atom consists of a nucleus and electrons around it. The nucleus of an atom, in turn, consists of smaller elementary particles- from a certain amount protons and neutrons(the common name for which is nucleons), interconnected by nuclear forces.
Number of protons in the nucleus determines the structure of the electron shell of the atom. And the electron shell determines the physical Chemical properties substances. The number of protons corresponds to the atomic number in the periodic system chemical elements Mendeleev, also called the charge number, atomic number, atomic number. For example, the number of protons in a Helium atom is 2. In periodic table it stands at number 2 and is designated as He 2. The symbol for the number of protons is the Latin letter Z. When writing formulas, the number indicating the number of protons is often located below the element symbol, either to the right or to the left: He 2 / 2 He.
Number of neutrons corresponds to a particular isotope of an element. Isotopes are elements with the same atomic number (the same number of protons and electrons) but different mass numbers. Mass number- the total number of neutrons and protons in the nucleus of an atom (denoted by the Latin letter A). When writing formulas, the mass number is indicated at the top of the element symbol on one of the sides: He 4 2 / 4 2 He (Helium isotope - Helium - 4)
Thus, to find out the number of neutrons in a particular isotope, the number of protons should be subtracted from the total mass number. For example, we know that a Helium-4 He 4 2 atom contains 4 elementary particles, since the mass number of the isotope is 4. At the same time, we know that He 4 2 has 2 protons. Subtracting from 4 (total mass number) 2 (number of protons) we get 2 - the number of neutrons in the nucleus of Helium-4.
THE PROCESS OF CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN THE NUCLEAR OF THE ATOM. As an example, we deliberately considered Helium-4 (He 4 2), the nucleus of which consists of two protons and two neutrons. Since the Helium-4 nucleus, called the alpha particle (α particle), has the highest efficiency in nuclear reactions, it is often used for experiments in this direction. It should be noted that in the formulas of nuclear reactions, the symbol α is often used instead of He 4 2 .
It was with the participation of alpha particles that E. Rutherford carried out the first nuclear transformation reaction in the official history of physics. During the reaction, α-particles (He 4 2) “bombarded” the nuclei of the nitrogen isotope (N 14 7), resulting in the formation of an oxygen isotope (O 17 8) and one proton (p 1 1)
This nuclear reaction looks like this:
Let us calculate the number of phantom Po particles before and after this transformation.
TO CALCULATE THE NUMBER OF PHANTOM PARTICLES BY IT IS NECESSARY:
Step 1. Calculate the number of neutrons and protons in each nucleus:
- the number of protons is indicated in the lower indicator;
- we find out the number of neutrons by subtracting the number of protons (lower indicator) from the total mass number (upper indicator).
Step 2. Calculate the number of phantom Po particles in the atomic nucleus:
- multiply the number of protons by the number of phantom Po particles contained in 1 proton;
- multiply the number of neutrons by the number of phantom Po particles contained in 1 neutron;
Step 3. Add the number of phantom particles By:
- add the received amount of phantom Po particles in protons with the received amount in neutrons in nuclei before the reaction;
- add the received amount of phantom Po particles in protons with the received amount in neutrons in nuclei after the reaction;
- compare the number of phantom Po particles before the reaction with the number of phantom Po particles after the reaction.
EXAMPLE OF THE DETAILED CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN THE NUCLEI OF ATOMS.
(Nuclear reaction involving an α-particle (He 4 2), carried out by E. Rutherford in 1919)
BEFORE REACTION (N 14 7 + He 4 2)
N 14 7
Number of protons: 7
Number of neutrons: 14-7 = 7
in 1 proton - 12 Po, which means in 7 protons: (12 x 7) \u003d 84;
in 1 neutron - 33 Po, which means in 7 neutrons: (33 x 7) = 231;
Total number of phantom Po particles in the nucleus: 84+231 = 315
He 4 2
Number of protons - 2
Number of neutrons 4-2 = 2
Number of phantom particles By:
in 1 proton - 12 Po, which means in 2 protons: (12 x 2) \u003d 24
in 1 neutron - 33 Po, which means in 2 neutrons: (33 x 2) \u003d 66
Total number of phantom Po particles in the nucleus: 24+66 = 90
Total number of phantom Po particles before the reaction
N 14 7 + He 4 2
315 + 90 = 405
AFTER REACTION (O 17 8) and one proton (p 1 1):
O 17 8
Number of protons: 8
Number of neutrons: 17-8 = 9
Number of phantom particles By:
in 1 proton - 12 Po, which means in 8 protons: (12 x 8) \u003d 96
in 1 neutron - 33 Po, which means in 9 neutrons: (9 x 33) = 297
Total number of phantom Po particles in the nucleus: 96+297 = 393
p 1 1
Number of protons: 1
Number of neutrons: 1-1=0
Number of phantom particles By:
In 1 proton - 12 Po
There are no neutrons.
The total number of phantom Po particles in the nucleus: 12
Total number of phantom particles Po after the reaction
(O 17 8 + p 1 1):
393 + 12 = 405
Let's compare the number of phantom Po particles before and after the reaction:
EXAMPLE OF A REDUCED FORM OF CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN A NUCLEAR REACTION.
A well-known nuclear reaction is the reaction of the interaction of α-particles with a beryllium isotope, in which the neutron was first discovered, which manifested itself as an independent particle as a result of nuclear transformation. This reaction was carried out in 1932 English physicist James Chadwick. Reaction formula:
213 + 90 → 270 + 33 - the number of phantom Po particles in each of the nuclei
303 = 303 - total sum of phantom Po particles before and after the reaction
The numbers of phantom Po particles before and after the reaction are equal.
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As already noted, an atom consists of three types of elementary particles: protons, neutrons and electrons. Atomic nucleus - central part an atom made up 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. Coulomb - a unit of electric charge, equal to the amount of electricity passing through the cross section of the conductor in a time of 1 s 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. The total number of 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.
Long before the emergence of reliable data on the internal structure of all things, Greek thinkers imagined matter in the form of the smallest fiery particles that were in constant motion. Probably, this vision of the world order of things was derived from purely logical conclusions. Despite some naivety and absolute lack of evidence for this statement, it turned out to be true. Although scientists were able to confirm a bold guess only twenty-three centuries later.
The structure of atoms
At the end of the 19th century, the properties of a discharge tube through which a current was passed were investigated. Observations have shown that two streams of particles are emitted:
The negative particles of the cathode rays were called electrons. Subsequently, particles with the same charge-to-mass ratio were found in many processes. Electrons seemed to be universal constituents of various atoms, quite easily separated by the bombardment of ions and atoms.
Particles carrying a positive charge were represented by fragments of atoms after they lost one or more electrons. In fact, the positive rays were groups of atoms devoid of negative particles, and therefore having a positive charge.
Thompson model
On the basis of experiments, it was found that positive and negative particles represented the essence of the atom, were its constituents. The English scientist J. Thomson proposed his theory. In his opinion, the structure of the atom and the atomic nucleus was a kind of mass in which negative charges were squeezed into a positively charged ball, like raisins in a cupcake. Charge compensation made the cake electrically neutral.
Rutherford model
The young American scientist Rutherford, analyzing the tracks left after alpha particles, came to the conclusion that the Thompson model is imperfect. Some alpha particles were deflected by small angles - 5-10 o . In rare cases, alpha particles were deflected at large angles of 60-80 o , and in exceptional cases, the angles were very large - 120-150 o . Thompson's model of the atom could not explain such a difference.
Rutherford proposes a new model that explains the structure of the atom and the atomic nucleus. The physics of processes states that an atom must be 99% empty, with a tiny nucleus and electrons revolving around it, which move in orbits.
He explains the deviations during impacts by the fact that the particles of the atom have their own electric charges. Under the influence of bombarding charged particles, atomic elements behave like ordinary charged bodies in the macrocosm: particles with the same charges repel each other, and with opposite charges they attract.
State of atoms
At the beginning of the last century, when the first particle accelerators were launched, all theories explaining the structure of the atomic nucleus and the atom itself were waiting for experimental verification. By that time, the interactions of alpha and beta rays with atoms had already been thoroughly studied. Until 1917, it was believed that atoms were either stable or radioactive. Stable atoms cannot be split, the decay of radioactive nuclei cannot be controlled. But Rutherford managed to refute this opinion.
First proton
In 1911, E. Rutherford put forward the idea that all nuclei consist of identical elements, the basis for which is the hydrogen atom. This idea was prompted by an important conclusion of previous studies of the structure of matter: the masses of all chemical elements are divided without a trace by the mass of hydrogen. The new assumption opened up unprecedented possibilities, allowing us to see the structure of the atomic nucleus in a new way. Nuclear reactions had to confirm or disprove the new hypothesis.
Experiments were carried out in 1919 with nitrogen atoms. By bombarding them with alpha particles, Rutherford achieved an amazing result.
The N atom absorbed the alpha particle, then turned into an oxygen atom O 17 and emitted a hydrogen nucleus. This was the first artificial transformation of an atom of one element into another. Such an experience gave hope that the structure of the atomic nucleus, the physics of existing processes make it possible to carry out other nuclear transformations.
The scientist used in his experiments the method of scintillation - flashes. Based on the frequency of flashes, he drew conclusions about the composition and structure of the atomic nucleus, about the characteristics of the particles born, about their atomic mass and serial number. The unknown particle was named by Rutherford the proton. It had all the characteristics of a hydrogen atom stripped of its single electron - a single positive charge and a corresponding mass. Thus it was proved that the proton and the nucleus of hydrogen are the same particles.
In 1930, when the first large accelerators were built and launched, Rutherford's model of the atom was tested and proved: each hydrogen atom consists of a lone electron, the position of which cannot be determined, and a loose atom with a lone positive proton inside. Since protons, electrons, and alpha particles can fly out of an atom when bombarded, scientists thought that they were the constituents of any atom's nucleus. But such a model of the nucleus atom seemed unstable - the electrons were too large to fit in the nucleus, in addition, there were serious difficulties associated with the violation of the law of momentum and conservation of energy. These two laws, like strict accountants, said that the momentum and mass during the bombardment disappear in an unknown direction. Since these laws were generally accepted, it was necessary to find explanations for such a leak.
Neutrons
Scientists around the world set up experiments aimed at discovering new constituents of the nuclei of atoms. In the 1930s, German physicists Becker and Bothe bombarded beryllium atoms with alpha particles. In this case, an unknown radiation was registered, which it was decided to call G-rays. Detailed studies revealed some features of the new beams: they could propagate strictly in a straight line, did not interact with electric and magnetic fields, had a high penetrating power. Later, the particles that form this type of radiation were found in the interaction of alpha particles with other elements - boron, chromium and others.
Chadwick's hypothesis
Then James Chadwick, a colleague and student of Rutherford, gave a short report in Nature magazine, which later became well known. Chadwick drew attention to the fact that the contradictions in the conservation laws are easily resolved if we assume that the new radiation is a stream of neutral particles, each of which has a mass of approximately equal to the mass proton. Considering this assumption, physicists significantly supplemented the hypothesis explaining the structure of the atomic nucleus. Briefly, the essence of the additions was reduced to a new particle and its role in the structure of the atom.
Properties of the neutron
The discovered particle was given the name "neutron". The newly discovered particles did not form electromagnetic fields around themselves and easily passed through matter without losing energy. In rare collisions with light nuclei of atoms, the neutron is able to knock out the nucleus from the atom, losing a significant part of its energy. The structure of the atomic nucleus assumed the presence of a different number of neutrons in each substance. Atoms with the same nuclear charge but different numbers of neutrons are called isotopes.
Neutrons have served as an excellent replacement for alpha particles. Currently, they are used to study the structure of the atomic nucleus. Briefly, their significance for science cannot be described, but it was thanks to the bombardment of atomic nuclei by neutrons that physicists were able to obtain isotopes of almost all known elements.
The composition of the nucleus of an atom
At present, the structure of the atomic nucleus is a collection of protons and neutrons held together by nuclear forces. For example, a helium nucleus is a lump of two neutrons and two protons. Light elements have an almost equal number of protons and neutrons, while heavy elements have a much larger number of neutrons.
This picture of the structure of the nucleus is confirmed by experiments at modern large accelerators with fast protons. The electric forces of repulsion of protons are balanced by vigorous forces that act only in the nucleus itself. Although the nature of nuclear forces is not yet fully understood, their existence is practically proven and fully explains the structure of the atomic nucleus.
Relationship between mass and energy
In 1932, a cloud chamber captured an amazing photograph proving the existence of positive charged particles, with the mass of an electron.
Prior to this, positive electrons were theoretically predicted by P. Dirac. A real positive electron was also discovered in cosmic radiation. The new particle was called the positron. When colliding with its twin - an electron, annihilation occurs - the mutual annihilation of two particles. This releases a certain amount of energy.
Thus, the theory developed for the macrocosm was fully suitable for describing the behavior of the smallest elements of matter.