In the 20s of the 20th century, physicists no longer had any doubts about the complexity of the structure of atomic nuclei discovered by Rutherford in 1911. This fact was indicated by a large number of different experiments carried out by that time, such as:
- discovery of the phenomenon of radioactivity,
- experimental proof of the nuclear model of the atom,
- measurement of the ratio e m for an electron, an α particle and for an H particle, which is the nucleus of a hydrogen atom,
- discovery of artificial radioactivity and nuclear reactions,
- measurement of charges of atomic nuclei and many others.
What particles do the nuclei of atoms consist of? Nowadays, it is a fact that the nuclei of atoms of various elements consist of two types of particles, that is, neutrons and protons. The second of these particles is a hydrogen atom that has lost its only electron. Such a particle was already noticed in the experiments of J. Thomson in 1907. The scientist was able to measure her e m ratio.
Definition 1
E. Rutherford in 1919 discovered hydrogen atomic nuclei in the products of fission of atomic nuclei of a significant number of elements. The physicist gave the found particle a name proton. He suggested that any atomic nucleus contains protons.
The scheme of Rutherford's experiments is illustrated in Figure 6. 5 . 1 .
Figure 6. 5 . 1 . Scheme of Rutherford's experiments on the detection of protons in nuclear fission products. K is a lead container with a radioactive source of α-particles, F is metal foil, E is a screen coated with zinc sulfide, M is a microscope.
Rutherford's device consisted of an evacuated chamber with a container located in it TO, where the source was located α -particles Metal foil, indicated in the figure as F, blocked the camera window. The thickness of the foil was selected in such a way as to prevent penetration through it α -particles Behind the window was a screen coated with zinc sulphide, shown in image 6. 5 . 1 marked E. Using a microscope M, it was possible to observe light flashes or, as they are also called, scintillations at points, at points on the screen where heavy charged particles hit.
While the chamber was being filled with low-pressure nitrogen, flashes of light were detected on the screen. This phenomenon pointed to the fact that under the experimental conditions there is a flow of unknown particles that have the ability to penetrate through an almost completely blocking flow α - foil particles F. By repeatedly moving the screen away from the camera window, E. Rutherford was able to measure the mean free path of the observed particles in the air. The obtained value turned out to be approximately equal to 28 cm, which coincided with the estimate of the path length of the H particles previously observed by J. Thomson.
By studying the effects of electric and magnetic fields on particles knocked out of nitrogen nuclei, data were obtained on the positivity of their elementary charge. It has also been proven that the mass of such particles is equivalent to the mass of the nuclei of hydrogen atoms.
Subsequently, the experiment was performed with a number of other gaseous substances. In all similar experiments carried out, it was discovered that from their nuclei α -particles knock out H-particles or protons.
According to modern measurements, the positive charge of a proton is absolutely equivalent to the elementary charge e = 1.60217733 · 10 – 19 K l. In other words, in modulus it is equal to the negative charge of the electron. Nowadays, the equality of the charges of a proton and an electron has been verified with an accuracy of 10 – 22. Such a coincidence of charges of two particles that differ significantly from each other causes sincere bewilderment and remains one of the fundamental mysteries of modern physics to this day.
Definition 2
Based on modern measurements, we can state that the mass of a proton is m p = 1.67262 10 – 27 kg. In nuclear physics, the mass belonging to particles is often expressed in atomic mass units (a.m.u.), equal to the mass of a carbon atom with mass number 12:
1 a. e.m. = 1.66057 · 10 - 27 kg.
Accordingly, m p = 1.007276 a. eat.
Quite often, expressing the mass of a particle is most convenient when using equivalent energy values in accordance with the following formula: E = m c 2. Due to the fact that 1 e V = 1.60218 · 10 – 19 J, in energy units the proton mass is equal to 938.272331 Me V.
Consequently, Rutherford’s experiment, which discovered the phenomenon of splitting of nitrogen nuclei and other elements of the periodic table under the conditions of impacts of fast α particles, also showed that atomic nuclei include protons.
As a result of the discovery of protons, some physicists began to assume that new particles are not just part of the nuclei of atoms, but are its only possible elements. However, due to the fact that the ratio of the charge of a nucleus to its mass does not remain constant for different nuclei, as would be the case if the nuclei contained only protons, this assumption was recognized as untenable. For heavier nuclei, this ratio turns out to be smaller than for light ones, from which it follows that when moving to heavier nuclei, the mass of the nucleus increases faster than the charge.
In 1920, E. Rutherford put forward a hypothesis about the presence in the nuclei of a certain compact tightly bound pair consisting of an electron and a proton. In the scientist’s understanding, this bundle was an electrically neutral formation as a particle with a mass almost equivalent to the mass of a proton. He also came up with a name for this hypothetical particle; Rutherford wanted to call it a neutron. Unfortunately, the idea given, despite its beauty, was wrong. It was found that the electron cannot be part of the nucleus. Quantum mechanical calculations based on the uncertainty relation show that an electron localized in the nucleus, i.e., a region of size R ≈ 10 – 13 cm, must have incredible kinetic energy, which is many orders of magnitude greater than the binding energy of nuclei per particle.
The idea of the existence of some heavy neutrally charged particle in the nucleus was extremely attractive to Rutherford. The scientist immediately turned to a group of his students led by J. Chadwick with a proposal to search for her. After 12 years, in 1932, Chadwick conducted an experimental study of the radiation produced when beryllium was irradiated with alpha particles. In the process, he discovered that this radiation is a stream of neutral particles with a mass almost equivalent to the mass of a proton. Thus the neutron was discovered. In Figure 6. 5 . Figure 2 illustrates a simplified diagram of a setup for detecting neutrons.
Figure 6. 5 . 2. Diagram of a setup for detecting neutrons.
In the process of bombarding beryllium with alpha particles emitted by radioactive polonium, powerful penetrating radiation appears, capable of passing through a barrier in the form of a 10 - 20 centimeter layer of lead. This radiation was discovered almost at the same time as Chadwick by the spouses, daughter of Marie and Pierre Curie, Irene and Frederic Joliot-Curie, but they suggested that these were high-energy γ-rays. They noticed that if a paraffin plate is installed in the path of beryllium radiation, the ionizing ability of this radiation increases abruptly. The couple proved that beryllium radiation knocks out large quantities of protons present in the given hydrogen-containing substance from paraffin. Using the mean free path of protons in air, scientists estimated the energy of γ quanta, which have the ability to impart the required speed to protons under collision conditions. The energy value obtained as a result of the assessment turned out to be huge - about 50 MeV.
In 1932, J. Chadwick carried out a whole series of experiments aimed at a comprehensive study of the properties of radiation that occurs when beryllium is irradiated with α particles. In his experiments, Chadwick used various methods for studying ionizing radiation.
Definition 3
In Figure 6. 5 . 2 illustrated Geiger counter, a device used to detect charged particles.
This device consists of a glass tube coated on the inside with a metal layer (cathode) and a thin thread running along the axis of the tube (anode). The tube is filled with an inert gas, usually argon, at low pressure. When a charged particle moves through a gas, it causes ionization of molecules.
Definition 4
Free electrons resulting from ionization are accelerated by the electric field between the anode and cathode to energies at which the phenomenon of impact ionization begins. An avalanche of ions appears and a short discharge current pulse passes through the counter.
Definition 5
Another instrument of extreme importance for particle research is cloud chamber, in which a fast charged particle leaves a trace or, as it is also called, a track.
The particle's trajectory can be photographed or observed directly. The basis of the action of the Wilson chamber, created in 1912, is the phenomenon of condensation of supersaturated vapor on ions that are formed in the working volume of the chamber along the trajectory of a charged particle. Using a cloud chamber, it becomes possible to observe the curvature of the trajectory of a charged particle in electric and magnetic fields.
Evidence 1
In his experiments, J. Chadwick observed in a cloud chamber traces of nitrogen nuclei that had collided with beryllium radiation. Based on these experiments, the scientist estimated the energy of the γ-quantum capable of imparting the speed observed in the experiment to nitrogen nuclei. The resulting value was 100 – 150 Me V. The γ quanta emitted by beryllium could not have such enormous energy. Based on this fact, Chadwick concluded that when beryllium is exposed to α particles, it is not massless γ quanta that are emitted, but rather heavy particles. These particles had considerable penetrating power and did not directly ionize the gas in the Geiger counter; accordingly, they were electrically neutral. Thus, the existence of the neutron was proven, a particle that Rutherford predicted more than 10 years before Chadwick’s experiments.
Definition 6
Neutron represents an elementary particle. It would be wrong to represent it as a compact proton-electron pair, as Rutherford originally assumed.
Based on the results of modern measurements, we can say that the neutron mass m n = 1.67493 · 10 – 27 k g = 1.008665 a. eat.
In energy units, the mass of a neutron is equivalent to 939.56563 MeV. The mass of a neutron is approximately two electron masses greater than the mass of a proton.
Immediately after the discovery of the neutron, the Russian scientist D. D. Ivanenko, together with the German physicist W. Heisenberg, put forward a hypothesis about the proton-neutron structure of atomic nuclei, which was fully confirmed by subsequent research.
Definition 7
Protons and neutrons are commonly called nucleons.
A number of notations are introduced to characterize atomic nuclei.
Definition 8
The number of protons that make up the atomic nucleus is denoted by the symbol Z and is called charge number or atomic number(this is the serial number in the periodic table of Mendeleev).
The charge of the nucleus is equal to Z e, where e is the elementary charge. The number of neutrons is denoted by the symbol N.
Definition 9
The total number of nucleons (i.e. protons and neutrons) is called the mass number of the nucleus A:
Definition of isotope
The nuclei of chemical elements are designated by the symbol X Z A, where X is the chemical symbol of the element. For example,
H 1 1 – hydrogen, He 2 4 – helium, C 6 12 – carbon, O 8 16 – oxygen, U 92 238 – uranium.
Definition 10
The number of neutrons in the nuclei of the same chemical element can be different. Such kernels are called isotopes.
Most chemical elements have several isotopes. For example, hydrogen has three of them: H 1 1 - ordinary hydrogen, H 1 2 - deuterium and H 1 3 - tritium. Carbon has 6 isotopes, oxygen has 3.
Chemical elements in natural conditions most often represent a mixture of isotopes. The existence of isotopes determines the value of the atomic mass of a natural element in the periodic table of Mendeleev. So, for example, the relative atomic mass of natural carbon is 12.011.
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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
|
Rice. 2. NZ diagram of atomic nuclei. |
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
An atom consists of a positively charged nucleus and electrons surrounding it. Atomic nuclei have dimensions of approximately 10 -14 ... 10 -15 m (the linear dimensions of an atom are 10 -10 m).
The atomic nucleus consists 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 W. Heisenberg.
Proton ( R) has a positive charge equal to the electron charge and a rest mass T p =
1.6726∙10 -27 kg 1836 m e, Where m eelectron mass. 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 another unit - atomic mass units (amu, a unit of mass equal to 1/12 the mass of a carbon atom 
). The masses of a proton and a neutron are approximately one atomic mass unit. Protons and neutrons are called nucleons(from lat. nucleuscore). The total number of nucleons in an atomic nucleus is called the mass number A).
The radii of nuclei increase with increasing mass number in accordance with the relation R= 1,4A 1/3 10 -13 cm.
Experiments show that nuclei do not have sharp boundaries. At the center of the nucleus there is a certain density of nuclear matter, and it gradually decreases to zero with increasing distance from the center. Due to the lack of a clearly 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 more than just spherical. Most of the nuclei are deformed. Often the nuclei have the shape 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 Mendeleev’s Periodic Table of Elements.
The nucleus is denoted by the same symbol as the neutral atom: 
, Where X- symbol of a chemical element, Zatomic number (number of protons in the nucleus), Amass number (number of nucleons in the nucleus). Mass number A approximately equal to the mass of the nucleus in atomic mass units.
Since the atom is neutral, the charge on the nucleus Z determines the number of electrons in an atom. Their distribution among states in an atom depends on the number of electrons. The nuclear charge determines the specifics of a given chemical element, that is, it determines the number of electrons in an atom, the configuration of their electron shells, the magnitude and nature of the intra-atomic electric field.
Nuclei with the same charge numbers Z, but with different mass numbers A(i.e. with different numbers of neutrons N = A – Z), are called isotopes, and nuclei with the same A, but different Z – isobars. For example, hydrogen ( Z= l) has three isotopes: N – protium ( Z= l, N= 0),
N – deuterium ( Z= l, N= 1),
N – tritium ( Z= l, N= 2), tin - ten isotopes, etc. In the vast majority of cases, isotopes of the same chemical element have the same chemical and almost identical physical properties.
E, MeV
Energy levels
and observed transitions for the boron atomic nucleus
Quantum theory strictly limits the energies that the constituent parts of nuclei can possess. Collections of protons and neutrons in nuclei can only be in certain discrete energy states characteristic of a given isotope.When an electron goes from a higher to a lower energy state, the energy difference is emitted as a photon. The energy of these photons is on 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 levels of nuclear energy 
.Vertical lines indicate observed transitions. For example, a γ-quantum with an energy of 1.43 MeV is emitted when a nucleus transitions from a state with an energy of 3.58 MeV to a state with an energy of 2.15 MeV.
Long before the appearance of reliable data about the internal structure of all things, Greek thinkers imagined matter in the form of tiny 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 the absolute lack of evidence of this statement, it turned out to be true. Although scientists were able to confirm this bold guess only twenty-three centuries later.
Atomic structure
At the end of the 19th century, the properties of a discharge tube through which current was passed were investigated. Observations have shown that in this case two streams of particles are emitted:
The negative particles of cathode rays were called electrons. Subsequently, particles with the same charge-to-mass ratio were discovered in many processes. Electrons seemed to be universal components of various atoms, quite easily separated when bombarded by ions and atoms.
Particles carrying a positive charge were represented as fragments of atoms after they had lost one or more electrons. In fact, the positive rays were groups of atoms devoid of negative particles and, as a result, having a positive charge.
Thompson model
Based on experiments, it was found that positive and negative particles represented the essence of the atom and were its components. 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 into a cupcake. Charge compensation made the “cupcake” electrically neutral.
Rutherford model
The young American scientist Rutherford, analyzing the tracks left behind by alpha particles, came to the conclusion that Thompson’s model was imperfect. Some alpha particles were deflected at 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 the difference.
Rutherford proposes a new model that explains the structure of the atom and the atomic nucleus. The physics of the process states that an atom should be 99% empty, with a tiny nucleus and electrons rotating around it, moving in orbits.
He explains deviations during impacts by the fact that the particles of an atom have their own electrical 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 those with opposite charges attract.
State of atoms
At the beginning of the last century, when the first particle accelerators were launched, all theories that explained the structure of the atomic nucleus and the atom itself were awaiting experimental verification. By that time, the interactions of alpha and beta rays with atoms had already been thoroughly studied. Up until 1917, it was believed that atoms were either stable or radioactive. Stable atoms cannot be split, and 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. The scientist was prompted to this idea by an important conclusion from previous studies of the structure of matter: the masses of all chemical elements are divided without a remainder 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 were supposed to confirm or refute 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 an 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 and the physics of existing processes make it possible to carry out other nuclear transformations.
The scientist used the scintillation flash method in his experiments. Based on the frequency of flares, he drew conclusions about the composition and structure of the atomic nucleus, the characteristics of the generated particles, their atomic mass and atomic number. The unknown particle was named proton by Rutherford. It had all the characteristics of a hydrogen atom stripped of its single electron - a single positive charge and corresponding mass. Thus, it was proven that the proton and the hydrogen nucleus are the same particles.
In 1930, when the first large accelerators were built and launched, Rutherford's model of the atom was tested and proven: 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 during bombardment, scientists thought that these were the components of any atomic nucleus. But such a model of the atom of the nucleus 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 momentum and mass during a 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 conducted experiments aimed at discovering new components of atomic nuclei. In the 1930s, German physicists Becker and Bothe bombarded beryllium atoms with alpha particles. At the same time, unknown radiation was recorded, which it was decided to call G-rays. Detailed studies revealed some of the features of the new rays: they could propagate strictly in a straight line, did not interact with electric and magnetic fields, and had high penetrating ability. Later, the particles that form this type of radiation were found during the interaction of alpha particles with other elements - boron, chromium and others.
Chadwick's conjecture
Then James Chadwick, a colleague and student of Rutherford, gave a short message in the journal Nature, which later became generally known. Chadwick drew attention to the fact that contradictions in conservation laws can be easily resolved if we assume that the new radiation is a stream of neutral particles, each of which has a mass approximately equal to the mass of a proton. Considering this assumption, physicists significantly expanded the hypothesis that explains 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 atomic nuclei, a neutron is able to knock the nucleus out of 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 served as an excellent replacement for alpha particles. Currently, they are used to study the structure of the atomic nucleus. It is impossible to briefly describe their significance for science, but it was thanks to the bombardment of atomic nuclei by neutrons that physicists were able to obtain isotopes of almost all known elements.
Composition of the nucleus of an atom
Currently, 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 electrical repulsive forces of protons are balanced by nuclear forces, which act only in the nucleus itself. Although the nature of nuclear forces has not yet been fully studied, their existence is practically proven and completely explains the structure of the atomic nucleus.
Relationship between mass and energy
In 1932, the Wilson's camera captured an amazing photograph proving the existence of positively charged particles with the mass of an electron.
Prior to this, positive electrons were predicted theoretically by P. Dirac. A real positive electron has also been discovered in cosmic rays. The new particle was called a positron. When colliding with its double - an electron, annihilation occurs - the mutual destruction 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.
Questions “What does matter consist of?”, “What is the nature of matter?” have always occupied humanity. 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 solving this mystery, 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 was no longer just a hypothesis, but an absolute fact. It turned out that the composition of the nucleus of an atom is a very complex concept. Its composition includes But the question arose: does the composition of the atom include different numbers of these charges or not?
Planetary model
Initially, it was imagined that the atom was built very similar to our solar system. However, it quickly turned out that this idea was not entirely true. The problem of a purely mechanical transfer of an astronomical scale of a picture into an area that occupies millionths of a millimeter entailed a significant and dramatic change in the properties and qualities of the phenomena. The main difference was the much stricter laws and rules by which the atom was built.
Disadvantages of the planetary model
Firstly, since atoms of the same kind and element must be completely identical in parameters and properties, then 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 is that the motion of an electron in its orbit, if we apply well-studied physical laws 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 decay and even fall into the nucleus.
Mother's wave structure And
In 1924, the young aristocrat Louis de Broglie put forward an idea that revolutionized the scientific community's understanding of such issues as the composition of atomic nuclei. The idea was that the electron is not just a moving ball that rotates around the nucleus. This is a blurry substance that moves according to laws reminiscent of 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 does not actually appear. We can see the propagation of waves and not notice the movement of a 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 radically different from the previously accepted planetary model.
Elementary basis
The nucleus of an atom is the center. Electrons revolve around it. The properties of the nucleus determine everything else. 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 are certain elements that carry 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 core there is a pulsating atmosphere created by the 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 of them. If the nucleus remains positively charged, then it is able to attract the required 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 more electrons, causing it to become 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 an issue as the composition of the nucleus of an atom.
Radioactivity
The most difficult questions arose after the discovery of Radioactive elements emit alpha, beta and gamma waves. But such radiation must have a source. Rutherford showed in 1902 that such a source is the atom itself, or more precisely, the nucleus. On the other hand, radioactivity is not only the emission of rays, but also the transformation 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 table are not incoherent forms, but are combinations. Therefore, one could expect that both the charges and masses of nuclei would be expressed in terms of whole and multiple charges of hydrogen itself. However, this is not quite true. By studying the properties of atomic nuclei using electromagnetic fields, physicist Aston found that elements whose atomic weights were not whole and multiples were actually a combination of different atoms, and not one substance. In all cases where the atomic weight is not a whole number, 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 has. Accordingly, matter can be measured not by mass, but by the charge that makes up this matter and the energy of the charge. When an identical charge approaches another similar charge, the energy will increase, otherwise it will decrease. This certainly 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. This means there is some kind of contradiction.
Neutrons
The Curies, when bombarding beryllium with alpha particles, discovered some strange rays that, when colliding with the nucleus of an atom, repel it with enormous force. However, they are able to pass through a large thickness of matter. This contradiction was resolved by the fact that this particle turned out to have a neutral electric charge. Accordingly, it was called a neutron. Thanks to further research, it turned out that it is almost the same as that of the proton. Generally speaking, the neutron and proton are incredibly similar. Taking into account this discovery, it was definitely possible to establish that the nucleus of an atom contains both protons and neutrons, 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 be called an element in which the number of neutrons and protons is not equal to each other. As discussed above, in such a case, although the element remains essentially the same, its properties may change significantly.