From the period of antiquity to the middle of the 18th century, science was dominated by the idea that an atom is a particle of matter that cannot be divided. The English scientist, as well as the naturalist D. D alton, defined the atom as the smallest component of a chemical element. M. V. Lomonosov in his atomic and molecular theory was able to define the atom and the molecule. He was convinced that the molecules, which he called "corpuscles", were made up of "elements" - atoms - and were in constant motion.
D. I. Mendeleev believed that this subunit of substances that make up the material world retains all its properties only if it is not subjected to separation. In this article, we will define the atom as an object of the microworld and study its properties.
Prerequisites for the creation of the theory of the structure of the atom
In the 19th century, the statement about the indivisibility of the atom was generally accepted. Most scientists believed that particles of one chemical element under no circumstances could turn into atoms of another element. These ideas served as the basis on which the definition of the atom was based until 1932. At the end of the 19th century, science madefundamental discoveries that changed this point of view. First of all, in 1897, the English physicist J. J. Thomson discovered the electron. This fact radically changed the ideas of scientists about the indivisibility of the constituent part of a chemical element.
How to prove that the atom is complex
Even before the discovery of the electron, scientists unanimously agreed that atoms have no charges. Then it was found that electrons are easily released from any chemical element. They can be found in a flame, they are carriers of electric current, they are released by substances during X-ray emission.
But if electrons are part of all atoms without exception and are negatively charged, then there are some other particles in the atom that necessarily have a positive charge, otherwise the atoms would not be electrically neutral. To help unravel the structure of the atom, such a physical phenomenon as radioactivity helped. It gave the correct definition of the atom in physics and then in chemistry.
Invisible rays
French physicist A. Becquerel was the first to describe the phenomenon of emission by atoms of certain chemical elements, visually invisible rays. They ionize the air, pass through substances, cause blackening of photographic plates. Later, the Curies and E. Rutherford found that radioactive substances are converted into atoms of other chemical elements (for example, uranium into neptunium).
Radioactive radiation is inhomogeneous in composition: alpha particles, beta particles, gamma rays. SoThus, the phenomenon of radioactivity confirmed that the particles of the elements of the periodic table have a complex structure. This fact was the reason for the changes made to the definition of the atom. What particles does an atom consist of, given the new scientific facts obtained by Rutherford? The answer to this question was the nuclear model of the atom proposed by the scientist, according to which electrons revolve around a positively charged nucleus.
Contradictions of the Rutherford model
The theory of the scientist, despite its outstanding character, could not objectively define the atom. Her conclusions went against the fundamental laws of thermodynamics, according to which all electrons revolving around the nucleus lose their energy and, be that as it may, sooner or later must fall into it. The atom is destroyed in this case. This does not actually happen, since the chemical elements and the particles of which they are composed exist in nature for a very long time. Such a definition of the atom, based on Rutherford's theory, is inexplicable, as well as the phenomenon that occurs when hot simple substances are passed through a diffraction grating. After all, the resulting atomic spectra have a linear shape. This was in conflict with Rutherford's model of the atom, according to which the spectra should have been continuous. According to the concepts of quantum mechanics, at present, electrons in the nucleus are characterized not as point objects, but as having the form of an electron cloud.
Its highest density in a certain locus of space around the nucleus andconsidered to be the location of the particle at a given point in time. It was also found that the electrons in the atom are arranged in layers. The number of layers can be determined by knowing the number of the period in which the element is located in the periodic system of D. I. Mendeleev. For example, a phosphorus atom contains 15 electrons and has 3 energy levels. The indicator that determines the number of energy levels is called the main quantum number.
It was experimentally found that the electrons of the energy level closest to the nucleus have the lowest energy. Each energy shell is divided into sublevels, and they, in turn, into orbitals. Electrons located in different orbitals have the same cloud shape (s, p, d, f).
Based on the above, it follows that the shape of the electron cloud cannot be arbitrary. It is strictly defined according to the orbital quantum number. We also add that the state of an electron in a macroparticle is determined by two more values - magnetic and spin quantum numbers. The first is based on the Schrödinger equation and characterizes the spatial orientation of the electron cloud based on the three-dimensionality of our world. The second indicator is the spin number, it determines the rotation of an electron around its axis clockwise or counterclockwise.
Discovery of the neutron
Thanks to the work of D. Chadwick, carried out by him in 1932, a new definition of the atom in chemistry and physics was given. In his experiments, the scientist proved that during the splitting of polonium, radiation occurs, caused byparticles that do not have a charge, with a mass of 1.008665. The new elementary particle was called the neutron. Its discovery and study of its properties allowed the Soviet scientists V. Gapon and D. Ivanenko to create a new theory of the structure of the atomic nucleus containing protons and neutrons.
According to the new theory, the definition of an atom of matter was as follows: it is a structural unit of a chemical element, consisting of a nucleus containing protons and neutrons and electrons moving around it. The number of positive particles in the nucleus is always equal to the atomic number of the chemical element in the periodic system.
Later, Professor A. Zhdanov confirmed in his experiments that under the influence of hard cosmic radiation, atomic nuclei split into protons and neutrons. In addition, it was proved that the forces holding these elementary particles in the nucleus are extremely energy intensive. They operate at very short distances (about 10-23 cm) and are called nuclear. As mentioned earlier, even M. V. Lomonosov was able to give a definition of an atom and a molecule based on scientific facts known to him.
At present, the following model is generally recognized: an atom consists of a nucleus and electrons moving around it along strictly defined trajectories - orbitals. Electrons simultaneously exhibit the properties of both particles and waves, that is, they have a dual nature. Almost all of its mass is concentrated in the nucleus of an atom. It is made up of protons and neutrons bound by nuclear forces.
Can an atom be weighed
It turns out that every atom hasmass. For example, for hydrogen it is 1.67x10-24g. It is even difficult to imagine how small this value is. To find the weight of such an object, they do not use scales, but an oscillator, which is a carbon nanotube. To calculate the weight of an atom and a molecule, a more convenient value is the relative mass. It shows how many times the weight of a molecule or atom is greater than 1/12 of a carbon atom, which is 1.66x10-27 kg. Relative atomic masses are given in the periodic system of chemical elements, and they have no units.
Scientists are well aware that the atomic mass of a chemical element is the average of the mass numbers of all its isotopes. It turns out that in nature, units of one chemical element can have different masses. At the same time, the charges of the nuclei of such structural particles are the same.
Scientists have found that isotopes differ in the number of neutrons in the nucleus, and the charge of their nuclei is the same. For example, a chlorine atom with a mass of 35 contains 18 neutrons and 17 protons, and with a mass of 37 - 20 neutrons and 17 protons. Many chemical elements are mixtures of isotopes. For example, such simple substances as potassium, argon, oxygen contain atoms representing 3 different isotopes.
Defining atomicity
It has several interpretations. Consider what is meant by this term in chemistry. If the atoms of any chemical element are able to exist separately at least for a short time, without striving to form a more complex particle - a molecule, then they say that such substances haveatomic structure. For example, a multi-stage methane chlorination reaction. It is widely used in the chemistry of organic synthesis to obtain the most important halogen-containing derivatives: dichloromethane, carbon tetrachloride. It splits chlorine molecules into highly reactive atoms. They break the sigma bonds in the methane molecule, providing a substitution chain reaction.
Another example of a chemical process of great importance in industry is the use of hydrogen peroxide as a disinfectant and bleach. Determination of atomic oxygen, as a product of the breakdown of hydrogen peroxide, occurs both in living cells (under the action of the catalase enzyme) and in laboratory conditions. Atomic oxygen is qualitatively determined by its high antioxidant properties, as well as by its ability to destroy pathogenic agents: bacteria, fungi and their spores.
How the atomic shell works
We have already found out earlier that the structural unit of a chemical element has a complex structure. Electrons revolve around a positively charged nucleus. The Nobel Prize winner Niels Bohr, based on the quantum theory of light, created his doctrine, in which the characteristics and definition of an atom are as follows: electrons move around the nucleus only along certain stationary trajectories, while they do not radiate energy. Bohr's doctrine proved that the particles of the microcosm, which include atoms and molecules, do not obey laws that are fairfor large bodies - macrocosmic objects.
The structure of the electron shells of macroparticles was studied in works on quantum physics by such scientists as Hund, Pauli, Klechkovsky. So it became known that electrons make rotational movements around the nucleus not randomly, but along certain stationary trajectories. Pauli found that within one energy level on each of its s, p, d, f orbitals, no more than two negatively charged particles with opposite spins + ½ and - ½ can be found in electronic cells.
Hund's rule explained how orbitals with the same energy level are correctly filled with electrons.
Klechkovsky's rule, also called the n+l rule, explained how the orbitals of multielectron atoms (elements of 5, 6, 7 periods) are filled. All of the above patterns served as a theoretical justification for the system of chemical elements created by Dmitry Mendeleev.
Oxidation state
It is a fundamental concept in chemistry and characterizes the state of an atom in a molecule. The modern definition of the oxidation state of atoms is as follows: this is the conditional charge of an atom in a molecule, which is calculated based on the notion that the molecule has only an ionic composition.
The degree of oxidation can be expressed as an integer or fractional number, with positive, negative or zero values. Most often, atoms of chemical elements have several oxidation states. For example, nitrogen has -3, -2, 0, +1, +2, +3, +4, +5. But such a chemical element as fluorine, in all itscompounds has only one oxidation state, equal to -1. If it is represented by a simple substance, then its oxidation state is zero. This chemical quantity is convenient to use for the classification of substances and for describing their properties. Most often, the oxidation state of an atom is used in chemistry when compiling equations for redox reactions.
Properties of atoms
Thanks to the discoveries of quantum physics, the modern definition of the atom, based on the theory of D. Ivanenko and E. Gapon, is supplemented by the following scientific facts. The structure of the nucleus of an atom does not change during chemical reactions. Only stationary electron orbitals are subject to change. Their structure can explain a lot of physical and chemical properties of substances. If an electron leaves a stationary orbit and goes to an orbit with a higher energy index, such an atom is called excited.
It should be noted that electrons cannot stay in such unusual orbitals for a long time. Returning to its stationary orbit, the electron emits a quantum of energy. The study of such characteristics of the structural units of chemical elements as electron affinity, electronegativity, ionization energy, allowed scientists not only to define the atom as the most important particle of the microcosm, but also allowed them to explain the ability of atoms to form a stable and energetically more favorable molecular state of matter, possible due to creation of various types of stable chemical bonds: ionic, covalentpolar and non-polar, donor-acceptor (as a kind of covalent bond) and metallic. The latter determines the most important physical and chemical properties of all metals.
It has been experimentally established that the size of an atom can change. Everything will depend on which molecule it is included in. Thanks to X-ray diffraction analysis, it is possible to calculate the distance between atoms in a chemical compound, as well as to find out the radius of the element's structural unit. Knowing the patterns of change in the radii of atoms included in a period or group of chemical elements, it is possible to predict their physical and chemical properties. For example, in periods with an increase in the charge of the nucleus of atoms, their radii decrease (“compression of the atom”), so the metallic properties of the compounds weaken, and the non-metallic ones increase.
Thus, knowledge of the structure of the atom allows us to accurately determine the physical and chemical properties of all elements included in the periodic system of Mendeleev.
