Until the end of the XIX century, all chemical elements seemed to be permanent and indivisible. There was no question of how to convert unchangeable elements. But the discovery of radioactivity reversed the world known to us and paved the way for the discovery of new substances.
Discovery of radioactivity
The honor of the discovery of the transformation of elements belongs to the French physicist Antoine Becquerel. For one chemical experiment, he needed uranyl potassium sulfate crystals. He wrapped the substance in black paper and put the bag near the photographic plate. After the film was developed, the scientist saw in the picture the outlines of uranyl crystals. Despite the thick layer of paper, they were clearly distinguishable. Becquerel repeated this experiment several times, but the result was the same: the outlines of crystals containing uranium were clearly visible on the photographic plates.
The results of the discovery Becquerel announced at the next meeting, which was held by the Paris Academy of Sciences. His report began with the words "invisible radiation." So he described the results of his experiments. After that, the concept of radiation entered into use by physicists.
The results of Becquerel's observations interested French scientists Marie and Paul Curie. They rightly considered that not only uranium could have radioactive properties. Researchers have noticed that the ore residues from which this substance is extracted are still highly radioactive. Searches for elements other than the original ones led to the discovery of a substance with properties similar to uranium. A new radioactive element received the name polonium. This name Marie Curie gave the substance in honor of their homeland - Poland. Following this, radium was discovered. The radioactive element was the product of the decay of pure uranium. After that, an era of new, previously unnatural chemicals in nature began in chemistry.
Most of the currently known nuclei of chemical elements are unstable. Over time, such compounds spontaneously break up into other elements and various smallest particles. The heavier parent element in the community of physicists is called the source material. Products formed during the decomposition of a substance are referred to as children or decomposition products. The process itself is accompanied by the release of various radioactive particles.
The instability of chemical elements can be explained by the existence of different isotopes of the same substance. Isotopes are variations of some elements of the periodic system with the same properties, but with different numbers of neutrons in the nucleus. Very many common chemicals have at least one isotope. The fact that these elements are widely distributed and well studied confirms that they are in a stable state for an arbitrarily long time. But each of these “long-lived” elements contains isotopes. Their scientists obtain nuclei in the course of laboratory reactions. An artificial radioactive element produced by synthetic means cannot exist in a stable state for a long time and decays over time. This process can go in three ways. By the name of elementary particles, which are by-products of a thermonuclear reaction, all three types of decay got their names.
The radioactive chemical element can be transformed according to the first decay scheme. In this case, an alpha particle emitted from the nucleus, whose energy reaches 6 million eV. A detailed study of the results of the reaction, it was found that this particle is a helium atom. It takes two protons out of the nucleus, so the resulting radioactive element will have atomic number in the periodic system two positions lower than that of the parent substance.
The beta decay reaction is accompanied by the emission of one electron from the nucleus. The appearance of this particle in an atom is associated with the decay of a neuron into an electron, proton and neutrino. As the electron leaves the nucleus, the radioactive chemical element increases its atomic number by one unit and becomes heavier than its parent.
With gamma decay, the nucleus emits a photon beam with different energies. These rays are called gamma radiation. In this process, the radioactive element is not modified. He just loses his energy.
In itself, the instability that a radioactive element possesses does not at all mean that with a certain number of isotopes our substance suddenly disappears, releasing enormous energy. In reality, the disintegration of the kernel resembles popcorn cooking - the chaotic movement of corn kernels in a frying pan, and it is completely unknown which of them will be revealed first. The law of the reaction of radioactive decay can guarantee only that over a certain period of time the number of particles, proportional to the number of nucleons remaining in the nucleus, will fly out of the nucleus. In the language of mathematics, this process can be described by the following formula:
Here on the face is the proportional dependence of the number of nucleons dN leaving the nucleus over the period dt, on the number of all nucleons present in the nucleus N. The coefficient λ is the radioactivity constant of the decaying substance.
The number of nucleons remaining in the nucleus at time t is described by the formula:
in which N0 - the number of nucleons in the nucleus at the beginning of the observation.
For example, the radioactive element halogen with atomic number 85 was discovered only in 1940. Its half-life is quite long - 7.2 hours. The content of radioactive halogen (astatine) on the entire planet does not exceed one gram of pure substance. Thus, in 3.1 hours, its amount in nature should, in theory, be halved. But the constant processes of decay of uranium and thorium give rise to new and new astatine atoms, albeit in very small doses. Therefore, its quantity in nature remains stable.
The radioactivity constant serves to determine with its help how quickly the element under study will decay. But for practical problems, physicists more often use a quantity called the half-life. This indicator tells you how long a substance will lose exactly half of its nucleons. For different isotopes, this period varies from tiny fractions of a second to billions of years.
It is important to understand that time in this equation does not add up, but multiplies. For example, if during a period of time t a substance lost half of its nucleons, then over a period of 2t it will lose another half of the rest - that is, one fourth of the initial number of nucleons.
The emergence of radioactive elements
Naturally, radioactive substances are formed in the upper atmosphere of the Earth, in the ionosphere. Under the action of cosmic radiation, gas at high altitude undergoes various changes that turn a stable substance into a radioactive element. The most common gas in our atmosphere is N2. for example, from a stable isotope, nitrogen-14 is converted into a radioactive isotope of carbon-14.
Nowadays, much more often a radioactive element arises in the chain of man-made reactions of atomic fission. This is the name of the process in which the core of a parent substance breaks down into two subsidiaries, and then into four radioactive granddaughters. A classic example is the isotope of uranium 238. Its half-life is 4.5 billion years. Almost as many our planet exists. After ten stages of decay, radioactive uranium is transformed into stable lead 206. Artificially obtained radioactive element is no different in its properties from its natural counterpart.
The practical significance of radioactivity
After the Chernobyl disaster, many began to seriously talk about the curtailment of programs for the development of nuclear power plants. But in everyday life, radioactivity brings enormous benefits to humanity. The study of the possibilities of its practical application is engaged in the science of radiography. For example, radioactive phosphorus is injected into a patient to get a complete picture of bone fractures. Nuclear energy also serves to generate heat and electricity. Perhaps in the future we are waiting for new discoveries in this amazing field of science.