What does the atomic nucleus consist of? Nuclear Forces – Knowledge Hypermarket

DEFINITION

Atom consists of a positively charged nucleus, inside of which there are protons and neutrons, and electrons move in orbits around it. Atomic nucleus located in the center and almost all of its mass is concentrated in it.

The amount of charge on the nucleus of an atom determines the chemical element to which this atom belongs.

The existence of the atomic nucleus was proven in 1911 by E. Rutherford and described in a work entitled “The Scattering of α and β Rays and the Structure of the Atom.” After this, various scientists put forward numerous theories of the structure of the atomic nucleus (drop theory (N. Bohr), shell theory, cluster theory, optical theory, etc.).

Electronic structure of the atomic nucleus

According to modern concepts, the atomic nucleus consists of positively charged protons and neutral neutrons, which together are called nucleons. They are held in the core due to strong interactions.

The number of protons in the nucleus is called the charge number (Z). It can be determined using the Periodic Table of D.I. Mendeleev - it is equal to the serial number of the chemical element to which the atom belongs.

The number of neutrons in a nucleus is called the isotopic number (N). The total number of nucleons in the nucleus is called the mass number (M) and it is equal to the relative atomic mass of an atom of a chemical element, indicated in D. I. Mendeleev’s Periodic Table.

Nuclei with the same number of neutrons but different numbers of protons are called isotones. If the nucleus has the same number of protons, but different neutrons - isotopes. In the case when the mass numbers are equal, but the composition of the nucleons is different - isobars.

The nucleus of an atom can be in a stable (ground) state and in an excited state.

Let us consider the structure of the nucleus of an atom using the example of the chemical element oxygen. Oxygen has serial number 8 in D.I. Mendeleev’s Periodic Table and a relative atomic mass of 16 amu. This means that the nucleus of the oxygen atom has a charge equal to (+8). The nucleus contains 8 protons and 8 neutrons (Z=8, N=8, M=16), and 8 electrons move in 2 orbits around the nucleus (Fig. 1).

Rice. 1. Schematic representation of the structure of the oxygen atom.

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Atomic nucleus
Atomic nucleus

Atomic nucleus - the central and very compact part of the atom, in which almost all of its mass and all the positive electric charge are concentrated. The nucleus, holding electrons close to itself by Coulomb forces in an amount that compensates for its positive charge, forms a neutral atom. Most nuclei have a shape close to spherical and a diameter of ≈ 10 -12 cm, which is four orders of magnitude smaller than the diameter of an atom (10 -8 cm). The density of the substance in the core is about 230 million tons/cm 3 .
The atomic nucleus was discovered in 1911 as a result of a series of experiments on the scattering of alpha particles by thin gold and platinum foils, carried out in Cambridge (England) under the direction of E. Rutherford. In 1932, after the discovery of the neutron there by J. Chadwick, it became clear that the nucleus consists of protons and neutrons
(V. Heisenberg, D.D. Ivanenko, E. Majorana).
To designate an atomic nucleus, the symbol of the chemical element of the atom that contains the nucleus is used, and the upper left index of this symbol shows the number of nucleons (mass number) in this nucleus, and the lower left index shows the number of protons in it. For example, a nickel nucleus containing 58 nucleons, of which 28 are protons, is designated . This same core can also be designated 58 Ni, or nickel-58.

The nucleus is a system of densely packed protons and neutrons moving at a speed of 10 9 -10 10 cm/sec and held by powerful and short-range nuclear forces of mutual attraction (their area of ​​action is limited to distances of ≈ 10 -13 cm). Protons and neutrons are about 10 -13 cm in size and are considered two different states of a single particle called a nucleon. The radius of the nucleus can be approximately estimated by the formula R ≈ (1.0-1.1)·10 -13 A 1/3 cm, where A is the number of nucleons (the total number of protons and neutrons) in the nucleus. In Fig. Figure 1 shows how the density of matter changes (in units of 10 14 g/cm 3) inside a nickel nucleus, consisting of 28 protons and 30 neutrons, depending on the distance r (in units of 10 -13 cm) to the center of the nucleus.
Nuclear interaction (interaction between nucleons in a nucleus) occurs due to the fact that nucleons exchange mesons. This interaction is a manifestation of the more fundamental strong interaction between the quarks that make up nucleons and mesons (in the same way that chemical bonding forces in molecules are a manifestation of the more fundamental electromagnetic forces).
The world of nuclei is very diverse. About 3000 nuclei are known, differing from each other either in the number of protons, or in the number of neutrons, or both. Most of them are obtained artificially.
Only 264 cores are stable, i.e. do not experience any spontaneous transformations over time, called decays. The rest experience various forms of decay - alpha decay (the emission of an alpha particle, i.e. the nucleus of a helium atom); beta decay (simultaneous emission of an electron and an antineutrino or a positron and a neutrino, as well as the absorption of an atomic electron with the emission of a neutrino); gamma decay (photon emission) and others.
The different types of nuclei are often called nuclides. Nuclides with the same number of protons and different numbers of neutrons are called isotopes. Nuclides with the same number of nucleons, but different ratios of protons and neutrons are called isobars. Light nuclei contain approximately equal numbers of protons and neutrons. In heavy nuclei, the number of neutrons is approximately 1.5 times greater than the number of protons. The lightest nucleus is the nucleus of the hydrogen atom, consisting of one proton. The heaviest known nuclei (they are obtained artificially) have a number of nucleons of ≈290. Of these, 116-118 are protons.
Different combinations of the number of protons Z and neutrons correspond to different atomic nuclei. Atomic nuclei exist (i.e., their lifetime t > 10 -23 s) in a rather narrow range of changes in the numbers Z and N. Moreover, all atomic nuclei are divided into two large groups - stable and radioactive (unstable). Stable nuclei are grouped near the line of stability, which is determined by the equation

In Fig. Figure 2 shows the NZ diagram of atomic nuclei. Black dots indicate stable nuclei. The region where stable nuclei are located is usually called the valley of stability. On the left side of stable nuclei there are nuclei overloaded with protons (proton-rich nuclei), on the right - nuclei overloaded with neutrons (neutron-rich nuclei). Currently discovered atomic nuclei are highlighted in color. There are about 3.5 thousand of them. It is believed that there should be 7 – 7.5 thousand in total. Proton-rich nuclei (raspberry color) are radioactive and turn into stable ones mainly as a result of β + decays; the proton included in the nucleus is converted into a neutron. Neutron-rich nuclei (blue color) are also radioactive and become stable as a result of - - decays, with the transformation of a neutron of the nucleus into a proton.
The heaviest stable isotopes are those of lead (Z = 82) and bismuth (Z = 83). Heavy nuclei, along with the processes of β + and β - decay, are also subject to α-decay (yellow) and spontaneous fission, which become their main decay channels. The dotted line in Fig. 2 outlines the region of possible existence of atomic nuclei. The line B p = 0 (B p is the energy of proton separation) limits the region of existence of atomic nuclei on the left (proton drip-line). Line B n = 0 (B n – neutron separation energy) – on the right (neutron drip-line). Outside these boundaries, atomic nuclei cannot exist, since they decay during the characteristic nuclear time (~10 -23 – 10 -22 s) with the emission of nucleons.
When two light nuclei combine (synthesis) and divide a heavy nucleus into two lighter fragments, large amounts of energy are released. These two methods of obtaining energy are the most effective of all known. So 1 gram of nuclear fuel is equivalent to 10 tons of chemical fuel. Nuclear fusion (thermonuclear reactions) is the source of energy for stars. Uncontrolled (explosive) fusion occurs when a thermonuclear (or so-called “hydrogen”) bomb is detonated. Controlled (slow) fusion underlies a promising energy source under development - a thermonuclear reactor.
Uncontrolled (explosive) fission occurs when an atomic bomb explodes. Controlled fission is carried out in nuclear reactors, which are the energy sources in nuclear power plants.
Quantum mechanics and various models are used to theoretically describe atomic nuclei.
The nucleus can behave both as a gas (quantum gas) and as a liquid (quantum liquid). Cold nuclear liquid has superfluid properties. In a highly heated nucleus, nucleons decay into their constituent quarks. These quarks interact by exchanging gluons. As a result of this decay, the collection of nucleons inside the nucleus turns into a new state of matter - quark-gluon plasma

At the end of the 19th and beginning of the 20th centuries, physicists proved that the atom is a complex particle and consists of simpler (elementary) particles. Were discovered:

· cathode rays (English physicist J. J. Thomson, 1897), the particles of which are called electrons e - (carry a single negative charge);

· natural radioactivity of elements (French scientists - radiochemists A. Becquerel and M. Sklodowska-Curie, physicist Pierre Curie, 1896) and the existence of α-particles (helium nuclei 4 He 2 +);

· the presence of a positively charged nucleus at the center of the atom (English physicist and radiochemist E. Rutherford, 1911);

· artificial transformation of one element into another, for example nitrogen into oxygen (E. Rutherford, 1919). From the nucleus of an atom of one element (nitrogen - in Rutherford’s experiment), upon collision with an α-particle, the nucleus of an atom of another element (oxygen) and a new particle were formed, carrying a unit positive charge and called a proton (p +, 1H nucleus)

· presence in the nucleus of an atom of electrically neutral particles - neutrons n 0 (English physicist J. Chadwick, 1932). As a result of the research, it was found that the atom of each element (except 1H) contains protons, neutrons and electrons, with protons and neutrons concentrated in the nucleus of the atom, and electrons on its periphery (in the electron shell).

Electrons are usually denoted as follows: e − .

Electrons e are very light, almost weightless, but have a negative electrical charge. It is equal to -1. The electric current we all use is a stream of electrons running in wires.

Neutrons are designated as follows: n 0, and protons as follows: p +.

Neutrons and protons are almost identical in mass.

The number of protons in the nucleus is equal to the number of electrons in the shell of the atom and corresponds to the serial number of this element in the Periodic Table.

Atomic nucleus

The central part of an atom, in which the bulk of its mass is concentrated and the structure of which determines the chemical element to which the atom belongs.

The atomic nucleus consists of nucleons - positively charged protons p + and neutral neutrons n 0, which are interconnected through strong interaction. The atomic nucleus, considered as a class of particles with a certain number of protons and neutrons, is often called a nuclide.

The number of protons in a nucleus is called its charge number Z - this number is equal to the atomic number of the element to which the atom belongs in the periodic table.

The number of neutrons in the nucleus is denoted by the letter N, and the number of protons by the letter Z. These numbers are related to each other by a simple ratio:

The total number of nucleons in a nucleus is called its mass number A = N + Z and is approximately equal to the average mass of an atom shown in the periodic table.

Atomic nuclei with the same number of protons and different numbers of neutrons are called isotopes.

Many elements have one natural isotope, for example, Be, F, Na, Al, P, Mn, Co, I, Au and some others. But most elements have two or three most stable isotopes.

For example:

Atomic nuclei with the same number of neutrons, but different numbers of protons are called isotones.

Atoms of different elements with the same atomic mass-A are called isobars.

« Physics - 11th grade"

The structure of the atomic nucleus. Nuclear forces

Immediately after the neutron was discovered in Chadwick's experiments, the Soviet physicist D. D. Ivanenko and the German scientist W. Heisenberg in 1932 proposed a proton-neutron model of the nucleus.
It was confirmed by subsequent studies of nuclear transformations and is now generally accepted.

Proton-neutron model of the nucleus

According to the proton-neutron model, nuclei consist of two types of elementary particles - protons and neutrons.

Since the atom as a whole is electrically neutral, and the charge of a proton is equal to the modulus of the charge of an electron, the number of protons in the nucleus is equal to the number of electrons in the atomic shell.
Therefore, the number of protons in the nucleus is equal to the atomic number of the element Z in the periodic system of elements by D.I. Mendeleev.

The sum of the number of protons Z and number of neutrons N in the kernel is called mass number and denoted by the letter A:

A = Z + N

The masses of a proton and a neutron are close to each other and each of them is approximately equal to an atomic mass unit.
The mass of electrons in an atom is much less than the mass of its nucleus.
Therefore, the mass number of the nucleus is equal to the relative atomic mass of the element rounded to a whole number.
Mass numbers can be determined by approximately measuring the mass of nuclei using instruments that are not highly accurate.

Isotopes are nuclei with the same value Z, but with different mass numbers A, i.e. with different numbers of neutrons N.

Nuclear forces

Since nuclei are very stable, protons and neutrons must be held inside the nucleus by some forces, and very strong ones at that.
It is not gravitational forces that are too weak.
The stability of the nucleus cannot be explained by electromagnetic forces either, since electric repulsion operates between like-charged protons.
And neutrons have no electrical charge.

This means that between nuclear particles - protons and neutrons, they are called nucleons- there are special forces called nuclear forces.

What are the main properties of nuclear forces? Nuclear forces are approximately 100 times greater than electrical (Coulomb) forces.
These are the most powerful forces of all existing in nature.
Therefore, the interactions of nuclear particles are often called strong interactions.

Strong interactions manifest themselves not only in the interactions of nucleons in the nucleus.
This is a special type of interaction inherent in most elementary particles along with electromagnetic interactions.

Another important feature of nuclear forces is their short duration.
Electromagnetic forces weaken relatively slowly with increasing distance.
Nuclear forces noticeably manifest themselves only at distances equal to the size of the nucleus (10 -12 -10 -13 cm), which was already shown by Rutherford's experiments on the scattering of α-particles by atomic nuclei.
A complete quantitative theory of nuclear forces has not yet been developed.
Significant progress in its development has been achieved quite recently - in the last 10-15 years.

The nuclei of atoms consist of protons and neutrons. These particles are held in the nucleus by nuclear forces.

Isotopes

The study of the phenomenon of radioactivity led to an important discovery: the nature of atomic nuclei was clarified.

As a result of observing a huge number of radioactive transformations, it was gradually discovered that there are substances that are identical in their chemical properties, but have completely different radioactive properties (that is, they decay differently).
They could not be separated by any of the known chemical methods.
On this basis, Soddy in 1911 suggested the possibility of the existence of elements with the same chemical properties, but differing, in particular, in their radioactivity.
These elements must be placed in the same cell of D.I. Mendeleev’s periodic system.
Soddy called them isotopes(i.e. occupying the same places).

Soddy's assumption received brilliant confirmation and profound interpretation a year later, when J. J. Thomson made precise measurements of the mass of neon ions by deflecting them in electric and magnetic fields.
He discovered that neon is a mixture of two types of atoms.
Most of them have a relative mass of 20.
But there is a small fraction of atoms with a relative atomic mass of 22.
As a result, the relative atomic mass of the mixture was taken to be 20.2.
Atoms that have the same chemical properties differ in mass.

Both types of neon atoms, naturally, occupy the same place in D.I. Mendeleev’s table and, therefore, are isotopes.
Thus, isotopes can differ not only in their radioactive properties, but also in mass.
That is why isotopes have the same charges of atomic nuclei, which means the number of electrons in the shells of atoms and, consequently, the chemical properties of isotopes are the same.
But the masses of the nuclei are different.
Moreover, nuclei can be both radioactive and stable.
The difference in the properties of radioactive isotopes is due to the fact that their nuclei have different masses.

The existence of isotopes for most chemical elements has now been established.
Some elements have only unstable (i.e. radioactive) isotopes.
The heaviest element existing in nature - uranium (relative atomic masses 238, 235, etc.) and the lightest - hydrogen (relative atomic masses 1, 2, 3) have isotopes.

Hydrogen isotopes are especially interesting, since they differ in mass by 2 and 3 times.
An isotope with a relative atomic mass of 2 is called deuterium.
It is stable (i.e., not radioactive) and appears as a small impurity (1:4500) in ordinary hydrogen.
When deuterium combines with oxygen, so-called heavy water is formed.
Its physical properties differ markedly from those of ordinary water.
At normal atmospheric pressure, it boils at 101.2 °C and freezes at 3.8 °C.

An isotope of hydrogen with atomic mass 3 is called tritium.
It is β-radioactive and has a half-life of about 12 years.

The existence of isotopes proves that the charge of the atomic nucleus does not determine all the properties of the atom, but only its chemical properties and those physical properties that depend on the periphery of the electron shell, for example, the size of the atom.
The mass of an atom and its radioactive properties are not determined by the serial number in D.I. Mendeleev’s table.

It is noteworthy that when accurately measuring the relative atomic masses of isotopes, it turned out that they were close to whole numbers.
But the atomic masses of chemical elements sometimes differ greatly from whole numbers.
Thus, the relative atomic mass of chlorine is 35.5.
This means that in its natural state, a chemically pure substance is a mixture of isotopes in various proportions.
The (approximate) integrity of the relative atomic masses of isotopes is very important for elucidating the structure of the atomic nucleus.

Most chemical elements have isotopes.
The charges of the atomic nuclei of isotopes are the same, but the masses of the nuclei are different.

As already noted, an atom consists of three types of elementary particles: protons, neutrons and electrons. The atomic nucleus is the central part of an atom, consisting of protons and neutrons. Protons and neutrons have the common name nucleon; they can transform into each other in the nucleus. 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 the atom. However, almost the entire mass of the 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, which is 1836 times greater than the mass of an electron. The electric charge is positive and equal to 1.66x10 -19 C. A coulomb is a unit of electric charge equal to the amount of electricity passing through the cross-section of a conductor in a time of 1 s at a constant current of 1A (ampere).

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 determines what chemical element we are dealing with. For example, if there is one proton in the nucleus, it is hydrogen, if there are 26 protons, it is iron. The number of protons in the atomic nucleus determines the charge of the nucleus (charge number Z) and the atomic number of the element in the periodic table of elements D.I. Mendeleev (atomic number of the element).

Nneutron– 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 the original number of neutrons decay) 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 included in 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 in relation to the electron. The mass of the electron and positron are equal, and the electric charges are equal in absolute value, but opposite in sign.

The different types of nuclei are called nuclides. Nuclide is a type of atom with given numbers 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 different numbers 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 designated 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, radioactive isotopes of phosphorus are designated 32 P, 33 P or 15 32 P and 15 33 P, respectively. When designating an isotope without indicating the element symbol, 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 1H-protium, heavy hydrogen 2H-deuterium and superheavy hydrogen 3H-tritium are known. Uranium has 11 isotopes; in natural compounds there are three (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 (about 280 of them) and natural isotopes that are part of radioactive families (46 of them). The rest are classified as 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, the isotopes of carbon 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.