Where did uranium come from? Most likely, it appears during supernova explosions. The fact is that for the nucleosynthesis of elements heavier than iron, there must be a powerful flow of neutrons, which occurs precisely during a supernova explosion. It would seem that then, during condensation from the cloud of new star systems formed by it, uranium, having collected in a protoplanetary cloud and being very heavy, should sink into the depths of the planets. But that's not true. Uranium is a radioactive element and when it decays it releases heat. Calculations show that if uranium were evenly distributed throughout the entire thickness of the planet, at least with the same concentration as on the surface, it would emit too much heat. Moreover, its flow should weaken as uranium is consumed. Since nothing like this has been observed, geologists believe that at least a third of uranium, and perhaps all of it, is concentrated in the earth’s crust, where its content is 2.5∙10 –4%. Why this happened is not discussed.
Where is uranium mined? There is not so little uranium on Earth - it is in 38th place in terms of abundance. And most of this element is found in sedimentary rocks - carbonaceous shales and phosphorites: up to 8∙10 –3 and 2.5∙10 –2%, respectively. In total, the earth's crust contains 10 14 tons of uranium, but the main problem is that it is very dispersed and does not form powerful deposits. Approximately 15 uranium minerals are of industrial importance. This is uranium tar - its basis is tetravalent uranium oxide, uranium mica - various silicates, phosphates and more complex compounds with vanadium or titanium based on hexavalent uranium.
What are Becquerel's rays? After the discovery of X-rays by Wolfgang Roentgen, French physicist Antoine-Henri Becquerel became interested in the glow of uranium salts, which occurs under the influence of sunlight. He wanted to understand if there were X-rays here too. Indeed, they were present - the salt illuminated the photographic plate through the black paper. In one of the experiments, however, the salt was not illuminated, but the photographic plate still darkened. When a metal object was placed between the salt and the photographic plate, the darkening underneath was less. Therefore, new rays did not arise due to the excitation of uranium by light and did not partially pass through the metal. They were initially called “Becquerel’s rays.” It was subsequently discovered that these are mainly alpha rays with a small addition of beta rays: the fact is that the main isotopes of uranium emit an alpha particle during decay, and the daughter products also experience beta decay.
How radioactive is uranium? Uranium has no stable isotopes; they are all radioactive. The longest-lived is uranium-238 with a half-life of 4.4 billion years. Next comes uranium-235 - 0.7 billion years. They both undergo alpha decay and become the corresponding isotopes of thorium. Uranium-238 makes up more than 99% of all natural uranium. Due to its huge half-life, the radioactivity of this element is low, and in addition, alpha particles are not able to penetrate the stratum corneum on the surface of the human body. They say that after working with uranium, I.V. Kurchatov simply wiped his hands with a handkerchief and did not suffer from any diseases associated with radioactivity.
Researchers have repeatedly turned to the statistics of diseases of workers in uranium mines and processing plants. Here, for example, is a recent article by Canadian and American specialists who analyzed health data of more than 17 thousand workers at the Eldorado mine in the Canadian province of Saskatchewan for the years 1950–1999 ( Environmental Research, 2014, 130, 43–50, DOI:10.1016/j.envres.2014.01.002). They proceeded from the fact that radiation has the strongest effect on rapidly multiplying blood cells, leading to the corresponding types of cancer. Statistics have shown that mine workers have a lower incidence of various types of blood cancer than the average Canadian population. In this case, the main source of radiation is not considered to be uranium itself, but the gaseous radon it generates and its decay products, which can enter the body through the lungs.
Why is uranium harmful?? It, like other heavy metals, is highly toxic and can cause kidney and liver failure. On the other hand, uranium, being a dispersed element, is inevitably present in water, soil and, concentrating in the food chain, enters the human body. It is reasonable to assume that in the process of evolution, living beings have learned to neutralize uranium in natural concentrations. Uranium is the most dangerous in water, so the WHO set a limit: initially it was 15 µg/l, but in 2011 the standard was increased to 30 µg/g. As a rule, there is much less uranium in water: in the USA on average 6.7 µg/l, in China and France - 2.2 µg/l. But there are also strong deviations. So in some areas of California it is a hundred times more than the standard - 2.5 mg/l, and in Southern Finland it reaches 7.8 mg/l. Researchers are trying to understand whether the WHO standard is too strict by studying the effect of uranium on animals. Here is a typical job ( BioMed Research International, 2014, ID 181989; DOI:10.1155/2014/181989). French scientists fed rats water for nine months with additives of depleted uranium, and in relatively high concentrations - from 0.2 to 120 mg/l. The lower value is water near the mine, while the upper value is not found anywhere - the maximum concentration of uranium, measured in Finland, is 20 mg/l. To the surprise of the authors - the article is called: “The unexpected absence of a noticeable effect of uranium on physiological systems ...” - uranium had practically no effect on the health of rats. The animals ate well, gained weight properly, did not complain of illness and did not die from cancer. Uranium, as it should be, was deposited primarily in the kidneys and bones and in a hundred times smaller quantities in the liver, and its accumulation expectedly depended on the content in the water. However, this did not lead to renal failure or even the noticeable appearance of any molecular markers of inflammation. The authors suggested that a review of the WHO's strict guidelines should begin. However, there is one caveat: the effect on the brain. There was less uranium in the rats' brains than in the liver, but its content did not depend on the amount in the water. But uranium affected the functioning of the brain’s antioxidant system: the activity of catalase increased by 20%, glutathione peroxidase by 68–90%, and the activity of superoxide dismutase decreased by 50%, regardless of the dose. This means that the uranium clearly caused oxidative stress in the brain and the body responded to it. This effect - the strong effect of uranium on the brain in the absence of its accumulation in it, by the way, as well as in the genitals - was noticed before. Moreover, water with uranium in a concentration of 75–150 mg/l, which researchers from the University of Nebraska fed rats for six months ( Neurotoxicology and Teratology, 2005, 27, 1, 135–144; DOI:10.1016/j.ntt.2004.09.001), affected the behavior of animals, mainly males, released into the field: they crossed lines, stood up on their hind legs and preened their fur differently than the control ones. There is evidence that uranium also leads to memory impairment in animals. Behavioral changes were correlated with levels of lipid oxidation in the brain. It turns out that the uranium water made the rats healthy, but rather stupid. These data will be useful to us in the analysis of the so-called Gulf War Syndrome.
Does uranium contaminate shale gas development sites? It depends on how much uranium is in the gas-containing rocks and how it is associated with them. For example, Associate Professor Tracy Bank of the University at Buffalo studied the Marcellus Shale, which stretches from western New York through Pennsylvania and Ohio to West Virginia. It turned out that uranium is chemically related precisely to the source of hydrocarbons (remember that related carbonaceous shales have the highest uranium content). Experiments have shown that the solution used during fracturing perfectly dissolves uranium. “When the uranium in these waters reaches the surface, it can cause contamination of the surrounding area. This does not pose a radiation risk, but uranium is a poisonous element,” notes Tracy Bank in a university press release dated October 25, 2010. No detailed articles have yet been prepared on the risk of environmental contamination with uranium or thorium during shale gas production.
Why is uranium needed? Previously, it was used as a pigment for making ceramics and colored glass. Now uranium is the basis of nuclear energy and atomic weapons. In this case, its unique property is used - the ability of the nucleus to divide.
What is nuclear fission? The decay of a nucleus into two unequal large pieces. It is because of this property that during nucleosynthesis due to neutron irradiation, nuclei heavier than uranium are formed with great difficulty. The essence of the phenomenon is as follows. If the ratio of the number of neutrons and protons in the nucleus is not optimal, it becomes unstable. Typically, such a nucleus emits either an alpha particle - two protons and two neutrons, or a beta particle - a positron, which is accompanied by the transformation of one of the neutrons into a proton. In the first case, an element of the periodic table is obtained, spaced two cells back, in the second - one cell forward. However, in addition to emitting alpha and beta particles, the uranium nucleus is capable of fission - decaying into the nuclei of two elements in the middle of the periodic table, for example barium and krypton, which it does, having received a new neutron. This phenomenon was discovered shortly after the discovery of radioactivity, when physicists exposed the newly discovered radiation to everything they could. Here is how Otto Frisch, a participant in the events, writes about this (“Advances in Physical Sciences,” 1968, 96, 4). After the discovery of beryllium rays - neutrons - Enrico Fermi irradiated uranium with them, in particular, to cause beta decay - he hoped to use it to obtain the next, 93rd element, now called neptunium. It was he who discovered a new type of radioactivity in irradiated uranium, which he associated with the appearance of transuranium elements. At the same time, slowing down the neutrons, for which the beryllium source was covered with a layer of paraffin, increased this induced radioactivity. American radiochemist Aristide von Grosse suggested that one of these elements was protactinium, but he was wrong. But Otto Hahn, who was then working at the University of Vienna and considered protactinium discovered in 1917 to be his brainchild, decided that he was obliged to find out what elements were obtained. Together with Lise Meitner, at the beginning of 1938, Hahn suggested, based on experimental results, that entire chains of radioactive elements are formed due to multiple beta decays of the neutron-absorbing nuclei of uranium-238 and its daughter elements. Soon Lise Meitner was forced to flee to Sweden, fearing possible reprisals from the Nazis after the Anschluss of Austria. Hahn, having continued his experiments with Fritz Strassmann, discovered that among the products there was also barium, element number 56, which in no way could be obtained from uranium: all chains of alpha decays of uranium end with much heavier lead. The researchers were so surprised by the result that they did not publish it; they only wrote letters to friends, in particular to Lise Meitner in Gothenburg. There, at Christmas 1938, her nephew, Otto Frisch, visited her, and, walking in the vicinity of the winter city - he on skis, the aunt on foot - they discussed the possibility of the appearance of barium during the irradiation of uranium as a result of nuclear fission (for more information about Lise Meitner, see “Chemistry and Life ", 2013, No. 4). Returning to Copenhagen, Frisch literally caught Niels Bohr on the gangway of a ship departing for the United States and told him about the idea of fission. Bohr, slapping himself on the forehead, said: “Oh, what fools we were! We should have noticed this earlier." In January 1939, Frisch and Meitner published an article on the fission of uranium nuclei under the influence of neutrons. By that time, Otto Frisch had already carried out a control experiment, as had many American groups who had received the message from Bohr. They say that physicists began to disperse to their laboratories right during his report on January 26, 1939 in Washington at the annual conference on theoretical physics, when they grasped the essence of the idea. After the discovery of fission, Hahn and Strassmann revised their experiments and found, just like their colleagues, that the radioactivity of irradiated uranium is associated not with transuraniums, but with the decay of radioactive elements formed during fission from the middle of the periodic table.
How does a chain reaction occur in uranium? Soon after the possibility of fission of uranium and thorium nuclei was experimentally proven (and there are no other fissile elements on Earth in any significant quantity), Niels Bohr and John Wheeler, who worked at Princeton, as well as, independently of them, the Soviet theoretical physicist Ya. I. Frenkel and the Germans Siegfried Flügge and Gottfried von Droste created the theory of nuclear fission. Two mechanisms followed from it. One is associated with the threshold absorption of fast neutrons. According to it, to initiate fission, a neutron must have a fairly high energy, more than 1 MeV for the nuclei of the main isotopes - uranium-238 and thorium-232. At lower energies, neutron absorption by uranium-238 has a resonant character. Thus, a neutron with an energy of 25 eV has a capture cross-sectional area that is thousands of times larger than with other energies. In this case, there will be no fission: uranium-238 will become uranium-239, which with a half-life of 23.54 minutes will turn into neptunium-239, which with a half-life of 2.33 days will turn into long-lived plutonium-239. Thorium-232 will become uranium-233.
The second mechanism is the non-threshold absorption of a neutron, it is followed by the third more or less common fissile isotope - uranium-235 (as well as plutonium-239 and uranium-233, which are not found in nature): by absorbing any neutron, even slow, so-called thermal, with energy as for molecules participating in thermal motion - 0.025 eV, such a nucleus will split. And this is very good: thermal neutrons have a capture cross-sectional area four times higher than fast, megaelectronvolt neutrons. This is the significance of uranium-235 for the entire subsequent history of nuclear energy: it is it that ensures the multiplication of neutrons in natural uranium. After being hit by a neutron, the uranium-235 nucleus becomes unstable and quickly splits into two unequal parts. Along the way, several (on average 2.75) new neutrons are emitted. If they hit the nuclei of the same uranium, they will cause neutrons to multiply exponentially - a chain reaction will occur, which will lead to an explosion due to the rapid release of a huge amount of heat. Neither uranium-238 nor thorium-232 can work like that: after all, during fission, neutrons are emitted with an average energy of 1–3 MeV, that is, if there is an energy threshold of 1 MeV, a significant part of the neutrons will certainly not be able to cause a reaction, and there will be no reproduction. This means that these isotopes should be forgotten and the neutrons will have to be slowed down to thermal energy so that they interact as efficiently as possible with the nuclei of uranium-235. At the same time, their resonant absorption by uranium-238 cannot be allowed: after all, in natural uranium this isotope is slightly less than 99.3% and neutrons more often collide with it, and not with the target uranium-235. And by acting as a moderator, it is possible to maintain the multiplication of neutrons at a constant level and prevent an explosion - control the chain reaction.
A calculation carried out by Ya. B. Zeldovich and Yu. B. Khariton in the same fateful year of 1939 showed that for this it is necessary to use a neutron moderator in the form of heavy water or graphite and enrich natural uranium with uranium-235 at least 1.83 times. Then this idea seemed to them pure fantasy: “It should be noted that approximately double the enrichment of those rather significant quantities of uranium that are necessary to carry out a chain explosion,<...>is an extremely cumbersome task, close to practical impossibility.” Now this problem has been solved, and the nuclear industry is mass-producing uranium enriched with uranium-235 to 3.5% for power plants.
What is spontaneous nuclear fission? In 1940, G. N. Flerov and K. A. Petrzhak discovered that fission of uranium can occur spontaneously, without any external influence, although the half-life is much longer than with ordinary alpha decay. Since such fission also produces neutrons, if they are not allowed to escape from the reaction zone, they will serve as the initiators of the chain reaction. It is this phenomenon that is used in the creation of nuclear reactors.
Why is nuclear energy needed? Zeldovich and Khariton were among the first to calculate the economic effect of nuclear energy (Uspekhi Fizicheskikh Nauk, 1940, 23, 4). “...At the moment, it is still impossible to make final conclusions about the possibility or impossibility of carrying out a nuclear fission reaction with infinitely branching chains in uranium. If such a reaction is feasible, then the reaction rate is automatically adjusted to ensure its smooth progress, despite the enormous amount of energy at the experimenter’s disposal. This circumstance is extremely favorable for the energy use of the reaction. Let us therefore present - although this is a division of the skin of an unkilled bear - some numbers characterizing the possibilities of the energy use of uranium. If the fission process proceeds with fast neutrons, therefore, the reaction captures the main isotope of uranium (U238), then<исходя из соотношения теплотворных способностей и цен на уголь и уран>the cost of a calorie from the main isotope of uranium turns out to be approximately 4000 times cheaper than from coal (unless, of course, the processes of “combustion” and heat removal turn out to be much more expensive in the case of uranium than in the case of coal). In the case of slow neutrons, the cost of a “uranium” calorie (based on the above figures) will be, taking into account that the abundance of the U235 isotope is 0.007, already only 30 times cheaper than a “coal” calorie, all other things being equal.”
The first controlled chain reaction was carried out in 1942 by Enrico Fermi at the University of Chicago, and the reactor was controlled manually - pushing graphite rods in and out as the neutron flux changed. The first power plant was built in Obninsk in 1954. In addition to generating energy, the first reactors also worked to produce weapons-grade plutonium.
How does a nuclear power plant operate? Nowadays, most reactors operate on slow neutrons. Enriched uranium in the form of a metal, an alloy such as aluminum, or an oxide is placed in long cylinders called fuel elements. They are installed in a certain way in the reactor, and moderator rods are inserted between them, which control the chain reaction. Over time, reactor poisons accumulate in the fuel element - uranium fission products, which are also capable of absorbing neutrons. When the concentration of uranium-235 falls below a critical level, the element is taken out of service. However, it contains many fission fragments with strong radioactivity, which decreases over the years, causing the elements to emit a significant amount of heat for a long time. They are kept in cooling pools, and then either buried or tried to be processed - to extract unburned uranium-235, produced plutonium (it was used to make atomic bombs) and other isotopes that can be used. The unused part is sent to burial grounds.
In so-called fast reactors, or breeder reactors, reflectors made of uranium-238 or thorium-232 are installed around the elements. They slow down and send back into the reaction zone neutrons that are too fast. Neutrons slowed down to resonant speeds absorb these isotopes, turning into plutonium-239 or uranium-233, respectively, which can serve as fuel for a nuclear power plant. Since fast neutrons react poorly with uranium-235, its concentration must be significantly increased, but this pays off with a stronger neutron flux. Despite the fact that breeder reactors are considered the future of nuclear energy, since they produce more nuclear fuel than they consume, experiments have shown that they are difficult to manage. Now there is only one such reactor left in the world - at the fourth power unit of the Beloyarsk NPP.
How is nuclear energy criticized? If we do not talk about accidents, then the main point in the arguments of opponents of nuclear energy today is the proposal to add to the calculation of its efficiency the costs of protecting the environment after decommissioning the station and when working with fuel. In both cases, the challenges of reliable disposal of radioactive waste arise, and these are costs borne by the state. There is an opinion that if you transfer them to the cost of energy, then its economic attractiveness will disappear.
There is also opposition among supporters of nuclear energy. Its representatives point to the uniqueness of uranium-235, which has no replacement, because alternative isotopes fissile by thermal neutrons - plutonium-239 and uranium-233 - due to their half-lives of thousands of years, are not found in nature. And they are obtained precisely as a result of the fission of uranium-235. If it runs out, a wonderful natural source of neutrons for a nuclear chain reaction will disappear. As a result of such wastefulness, humanity will lose the opportunity in the future to involve thorium-232, the reserves of which are several times greater than uranium, into the energy cycle.
Theoretically, particle accelerators can be used to produce a flux of fast neutrons with megaelectronvolt energies. However, if we are talking, for example, about interplanetary flights on a nuclear engine, then implementing a scheme with a bulky accelerator will be very difficult. The depletion of uranium-235 puts an end to such projects.
What is weapons-grade uranium? This is highly enriched uranium-235. Its critical mass - it corresponds to the size of a piece of substance in which a chain reaction occurs spontaneously - is small enough to produce ammunition. Such uranium can be used to make an atomic bomb, and also as a fuse for a thermonuclear bomb.
What disasters are associated with the use of uranium? The energy stored in the nuclei of fissile elements is enormous. If it gets out of control due to oversight or intentionally, this energy can cause a lot of trouble. The two worst nuclear disasters occurred on August 6 and 8, 1945, when the US Air Force dropped atomic bombs on Hiroshima and Nagasaki, killing and injuring hundreds of thousands of civilians. Smaller scale disasters are associated with accidents at nuclear power plants and nuclear cycle enterprises. The first major accident occurred in 1949 in the USSR at the Mayak plant near Chelyabinsk, where plutonium was produced; Liquid radioactive waste ended up in the Techa River. In September 1957, an explosion occurred on it, releasing a large amount of radioactive material. Eleven days later, the British plutonium production reactor at Windscale burned down, and the cloud with the explosion products dispersed over Western Europe. In 1979, a reactor at the Three Mail Island Nuclear Power Plant in Pennsylvania burned down. The most widespread consequences were caused by the accidents at the Chernobyl nuclear power plant (1986) and the Fukushima nuclear power plant (2011), when millions of people were exposed to radiation. The first littered vast areas, releasing 8 tons of uranium fuel and decay products as a result of the explosion, which spread across Europe. The second polluted and, three years after the accident, continues to pollute the Pacific Ocean in fishing areas. Eliminating the consequences of these accidents was very expensive, and if these costs were broken down into the cost of electricity, it would increase significantly.
A separate issue is the consequences for human health. According to official statistics, many people who survived the bombing or living in contaminated areas benefited from radiation - the former have a higher life expectancy, the latter have less cancer, and experts attribute some increase in mortality to social stress. The number of people who died precisely from the consequences of accidents or as a result of their liquidation amounts to hundreds of people. Opponents of nuclear power plants point out that the accidents have led to several million premature deaths on the European continent, but they are simply invisible in the statistical context.
Removing lands from human use in accident zones leads to an interesting result: they become a kind of nature reserves where biodiversity grows. True, some animals suffer from radiation-related diseases. The question of how quickly they will adapt to the increased background remains open. There is also an opinion that the consequence of chronic irradiation is “selection for fools” (see “Chemistry and Life”, 2010, No. 5): even at the embryonic stage, more primitive organisms survive. In particular, in relation to people, this should lead to a decrease in mental abilities in the generation born in contaminated areas shortly after the accident.
What is depleted uranium? This is uranium-238, remaining after the separation of uranium-235 from it. The volumes of waste from the production of weapons-grade uranium and fuel elements are large - in the United States alone, 600 thousand tons of such uranium hexafluoride have accumulated (for problems with it, see Chemistry and Life, 2008, No. 5). The content of uranium-235 in it is 0.2%. This waste must either be stored until better times, when fast neutron reactors will be created and it will be possible to process uranium-238 into plutonium, or used somehow.
They found a use for it. Uranium, like other transition elements, is used as a catalyst. For example, the authors of the article in ACS Nano dated June 30, 2014, they write that a catalyst made of uranium or thorium with graphene for the reduction of oxygen and hydrogen peroxide “has enormous potential for use in the energy sector.” Because uranium has a high density, it serves as ballast for ships and counterweights for aircraft. This metal is also suitable for radiation protection in medical devices with radiation sources.
What weapons can be made from depleted uranium? Bullets and cores for armor-piercing projectiles. The calculation here is as follows. The heavier the projectile, the higher its kinetic energy. But the larger the projectile, the less concentrated its impact. This means that heavy metals with high density are needed. Bullets are made of lead (Ural hunters at one time also used native platinum, until they realized that it was a precious metal), while the shell cores are made of tungsten alloy. Environmentalists point out that lead contaminates the soil in places of military operations or hunting and it would be better to replace it with something less harmful, for example, tungsten. But tungsten is not cheap, and uranium, similar in density, is a harmful waste. At the same time, the permissible contamination of soil and water with uranium is approximately twice as high as for lead. This happens because the weak radioactivity of depleted uranium (and it is also 40% less than that of natural uranium) is neglected and a truly dangerous chemical factor is taken into account: uranium, as we remember, is poisonous. At the same time, its density is 1.7 times greater than that of lead, which means that the size of uranium bullets can be reduced by half; uranium is much more refractory and hard than lead - it evaporates less when fired, and when it hits a target it produces fewer microparticles. In general, a uranium bullet is less polluting than a lead bullet, although such use of uranium is not known for certain.
But it is known that plates made of depleted uranium are used to strengthen the armor of American tanks (this is facilitated by its high density and melting point), and also instead of tungsten alloy in cores for armor-piercing projectiles. The uranium core is also good because uranium is pyrophoric: its hot small particles formed upon impact with the armor flare up and set fire to everything around. Both applications are considered radiation safe. Thus, the calculation showed that even after sitting for a year in a tank with uranium armor loaded with uranium ammunition, the crew would receive only a quarter of the permissible dose. And to get the annual permissible dose, you need to screw such ammunition to the surface of the skin for 250 hours.
Shells with uranium cores - for 30-mm aircraft cannons or artillery sub-calibers - have been used by the Americans in recent wars, starting with the Iraq campaign of 1991. That year they rained down on Iraqi armored units in Kuwait and during their retreat, 300 tons of depleted uranium, of which 250 tons, or 780 thousand rounds, were fired at aircraft guns. In Bosnia and Herzegovina, during the bombing of the army of the unrecognized Republika Srpska, 2.75 tons of uranium were spent, and during the shelling of the Yugoslav army in the region of Kosovo and Metohija - 8.5 tons, or 31 thousand rounds. Since WHO was by that time concerned about the consequences of the use of uranium, monitoring was carried out. He showed that one salvo consisted of approximately 300 rounds, of which 80% contained depleted uranium. 10% hit targets, and 82% fell within 100 meters of them. The rest dispersed within 1.85 km. A shell that hit a tank burned up and turned into an aerosol; the uranium shell pierced through light targets like armored personnel carriers. Thus, at most one and a half tons of shells could turn into uranium dust in Iraq. According to experts from the American strategic research center RAND Corporation, more, from 10 to 35% of the used uranium, turned into aerosol. Croatian anti-uranium munitions activist Asaf Durakovic, who has worked in a variety of organizations from Riyadh's King Faisal Hospital to the Washington Uranium Medical Research Center, estimates that in southern Iraq alone in 1991, 3-6 tons of submicron uranium particles were formed, which were scattered over a wide area , that is, uranium contamination there is comparable to Chernobyl.
All materials are formed from three elementary particles: electrons, protons and neutrons.
But since protons and neutrons easily transform into each other, and both are called nucleons, we might as well say that matter is made up of building blocks: electrons and nucleons. Matter is built from these particles in two stages: first, nucleons are organized into atomic kernels, and only then these atomic nuclei combine with electrons, forming atoms.
The atomic nucleus consists of a certain number of united nucleons. This number varies from one to two hundred or more. The simplest atomic nucleus is the nucleus of the hydrogen atom, consisting of one free proton; The most complex of the normal atomic nuclei is the nucleus of the uranium atom, containing 238 nucleons. All numbers in the range from 1 to 238 also correspond to different atomic nuclei.
In trying to explain how a number of nucleons can be held together to create an atomic nucleus, we must assume that when the nucleons are very close to each other, a very strong attraction arises between them. The nature of this attraction is different from the electrical attraction that, for example, occurs between a positively charged proton and a negatively charged electron. The force of attraction between nucleons is called the nuclear force, and we recognize that a deeper study of its properties is perhaps the single most important task facing nuclear physics.
To visualize the structure of the atomic nucleus, let us imagine nucleons in the form of small balls attracted to each other when they come close together; in other words, nuclear forces hold them together in the form of a small, almost round lump - the atomic nucleus.
The mass of an atomic nucleus is approximately equal to the total mass of the nucleons that form it. For example, the nucleus of an iron atom containing 56 nucleons is said to have an “atomic weight” of 56, and its mass is approximately 56 times the mass of one nucleon. In fact, its total mass is somewhat less than 56 nucleon masses, because when these particles combine into a nucleus, a certain amount of energy, the so-called binding energy, is released and lost, and since all energy has mass, some of the mass is lost as a result of the nucleons combining into the nucleus. In all nuclei, however, the amount of mass lost is less than one percent of the total mass.
With the exception of atomic weight, the most important characteristic of a nucleus is its electrical charge, which determines the chemical and most physical properties of the atom. The charge of atomic nuclei ranges from 1 to about 100. Among all substances found in the natural state, the uranium nucleus has the greatest electrical charge. Its charge number (“atomic number”) is 92. Nuclei with even higher atomic numbers, such as plutonium, have been produced artificially. The most common uranium nuclei have an atomic weight of 238, i.e., they consist of 238 nucleons. Since protons have an electrical charge, and neutrons do not, we can say that of the nucleons that make up the uranium nucleus, 92 are protons, the rest are neutrons (238-92 = 146).
The atomic nuclei of two atoms having the same charge but different masses are called isotopes. One of the nuclei, for example, has a charge of 92 and a mass of 235; this atom is therefore an isotope of uranium-238. Since it is the charge that determines the chemical properties of the atom of which the nucleus is an integral part, both atoms having these isotopic nuclei have essentially the same chemical properties and they are both uranium atoms (Uranium-235 isotope is used to make atomic bombs).
Many nuclear reactions are accompanied by the release of huge amounts of energy. The radioactive decay of a substance releases a large amount of energy, but since all the radioactive substances we have in large quantities decay slowly, the release of it lasts so long that it does not cause much alarm. It was only after we managed to split the uranium and plutonium nuclei that we were able to achieve such a rapid and intense release of energy necessary for the explosion of an atomic bomb. Another and incomparably more important atomic reaction occurs inside the Sun and other stars, providing them with energy, which they then send into space. This reaction is much more complex, but its result is this: four protons combine into a helium nucleus and emit two positrons. Thus, the Sun's hydrogen gradually “burns” into helium. Without such a “fire,” the Earth’s temperature would soon drop to “absolute zero” (273°C below zero). Man is not yet able to produce such an atomic reaction on a large scale, which is much more efficient than the fission of uranium to release energy, but it is likely that we will soon be able to successfully harness it or some similar process (fusion energy).
Atomic nuclei, which together with electrons form the world in which we live, were probably formed several billion years ago as a result of the combination of free protons and neutrons. It is likely that this process is still happening inside stars.
Currently, enormous nuclear reactions are taking place inside the Sun and stars. The temperature at the center of the Sun is approximately 20 million degrees, which is just enough to “ignite” hydrogen, causing it to burn into helium. The product of these reactions is a large number of neutrons, which, when added to protons, form heavier elements. In some very hot stars, nuclear processes are very efficient; in exploding stars, "novae" or "supernovae", in particular, heavier elements can be expected to be produced in significant quantities. Thus, it is possible that elements are formed in the interiors of stars and then ejected into the space surrounding them.
These are some of the burning aspects of nuclear physics; but along with them there are other problems, less sensational, however, no less important and of no less interest.
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 cm3 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 1.6721 x 10-27 kg, which is 1836 times the mass of an electron. The electric charge is positive and equal to 1.66 x 10-19 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).
Neutron- an electrically neutral particle with a mass of 1.6749 x 10-27 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):
, 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.
They 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 32P, 33P, or P and P, respectively. When designating an isotope without indicating the element symbol, the mass number is given after the element designation, 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, carbon 12C and 14C. If atoms of different chemical elements are meant, it is recommended to use the term “nuclides”, for example, radionuclides 90Sr, 131J, 137Cs.