Definition of an atom and a molecule. Definition of the atom until 1932

From the period of antiquity to the middle of the 18th century, science was dominated by the notion that an atom is a particle of matter that can not be divided. English scientist, as well as naturalist D. Dalton gave the definition of the atom as the smallest part of the chemical element. MV Lomonosov in his atomic-molecular theory was able to define the atom and molecule. He was sure that the molecules he called "corpuscles" consist of "elements" - atoms - and are in constant motion.

DI Mendeleyev believed that this subunit of substances that make up the material world retains all its properties only if it is not subject to separation. In this article we will define the atom as an object of the microworld and study its properties.

The background of the theory of the structure of the atom

In the 19th century, the statement of the indivisibility of the atom was generally accepted. Most scientists believed that the particles of a single chemical element can not under any circumstances turn into atoms of another element. These representations served as the basis on which the definition of the atom was based until 1932. At the end of the 19th century, fundamental discoveries were made in science that changed this point of view. First of all, in 1897 the English physicist D. J. Thomson discovered an electron. This fact radically changed the ideas of scientists about the indivisibility of the constituent part of a chemical element.

How to prove that an atom is complicated

Even before the discovery of the electron, scientists unanimously agreed that atoms do not have charges. Then it was established that the electrons are easily extracted from any chemical element. They can be found in a flame, they are carriers of an electric current, they are released by substances during X-ray radiation.

But if the electrons are included in all the atoms without exception and are negatively charged, then there are still some particles in the atom that necessarily have a positive charge, otherwise the atoms would not be electrically neutral. Help to unravel the structure of the atom was helped by such a physical phenomenon as radioactivity. It gave the correct definition of an atom in physics, and then in chemistry.

Invisible rays

The French physicist A. Becquerel first described the phenomenon of the emission by atoms of certain chemical elements, visually invisible rays. They ionize the air, pass through substances, cause the blackening of the photographic plates. Later the Curie couple and E. Rutherford found that radioactive substances are converted into atoms of other chemical elements (for example, uranium to neptunium).

Radioactive radiation is heterogeneous in composition: alpha particles, beta particles, gamma rays. Thus, 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 introduced into the definition of the atom. What particles does the atom consist of, taking into account Rutherford's new scientific facts? The answer to this question was the nuclear model of the atom proposed by the scientist, according to which electrons around the positive-charged nucleus rotate.

Contradictions of the Rutherford model

The theory of the scientist, despite her outstanding character, could not objectively define the atom. Her conclusions ran counter to the fundamental laws of thermodynamics, according to which all the electrons that revolve around the nucleus lose their energy and, however that may be, sooner or later they must fall on it. The atom is destroyed in this case. This does not actually happen, because the chemical elements and particles from which they are composed exist in nature for a very long time. This definition of an atom, based on Rutherford's theory, is inexplicable, as does the phenomenon that occurs when the incandescent simple substances pass through a diffraction grating. After all, the resulting atomic spectra have a linear form. This was in conflict with the Rutherford model of the atom, according to which the spectra would have to be continuous. According to the concepts of quantum mechanics, at the present time electrons are characterized in the nucleus not as point objects, but as having the form of an electron cloud.

Its greatest density in a certain locus of space around the nucleus is considered to be the location of the particle at a given instant of time. It was also found out that in the atom the electrons are layered. The number of layers can be determined by knowing the number of the period in which the element is located in the periodic table of DI Mendeleyev. 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 principal quantum number.

It has been experimentally established that electrons of the energy level located closest to the nucleus have the lowest energy. Each energy envelope is divided into sublevels, and they, in turn, into orbitals. The electrons located on different orbitals have the same cloud shape (s, p, d, f).

Proceeding from the above, it follows that the shape of the electron cloud can not 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-the magnetic and spin quantum numbers. The first is based on the Schrodinger equation and characterizes the spatial orientation of the electron cloud based on the three-dimensionality of our world. The second parameter is the spin number, it determines the rotation of the electron about its axis along or counter-clockwise.

The discovery of a neutron

Thanks to the work of D. Chadwick, conducted by him in 1932, a new definition of the atom in chemistry and physics was given. In his experiments, the scientist has proved that the splitting of polonium generates radiation caused by particles that do not have a charge, with a mass of 1.008665. A new elementary particle was called a neutron. Its discovery and study of its properties allowed Soviet scientists V. Gapon and D. Ivanenko to create a new theory of the structure of an atomic nucleus containing protons and neutrons.

According to the new theory, the definition of the atom of matter had the following form: 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 ordinal number of the chemical element in the periodic system.

Later, Professor A. Zhdanov in his experiments confirmed that under the influence of hard cosmic radiation, atomic nuclei are split into protons and neutrons. In addition, it was proved that the forces holding these elementary particles in the core are extremely energy-intensive. They operate at very short distances (of the order of 10 ^-23 cm) and are called nuclear ones. As it was said before, MV Lomonosov was able to define the atom and the molecule on the basis of scientific facts known to him.

At present, the following model is widely recognized: the atom consists of a nucleus and electrons moving around it along strictly defined trajectories - orbitals. Electrons simultaneously manifest properties and particles, and waves, that is, have a dual nature. Almost all of its mass is concentrated in the nucleus of the atom. It consists of protons and neutrons bound by nuclear forces.

Is it possible to weigh an atom

It turns out that each atom has a mass. For example, it has a hydrogen content of 1.67x10 ^-24 g. It is even difficult to imagine how small this value is. To find the weight of such an object, not a balance is used, but an oscillator that is a carbon nanotube. To calculate the weight of an atom and a molecule, the more convenient is the relative mass. It shows how many times the weight of a molecule or atom is greater than 1/12 of the carbon atom, which is 1.66x10 ^-27 kg. Relative atomic masses are indicated in the periodic system of chemical elements, and they have no dimension.

Scientists well know that the atomic mass of a chemical element is the average value of the mass numbers of all its isotopes. It turns out that in nature the units of one chemical element can have different masses. The charges of the nuclei of such structural particles are the same.

Scientists have established that the isotopes differ in the number of neutrons in the nucleus, and the charge of the nuclei in them is the same. For example, a chlorine atom having 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, simple substances such as potassium, argon, and oxygen contain atoms representing 3 different isotopes.

Definition of atomicity

It has several interpretations. Consider what is meant by this term in chemistry. If the atoms of a chemical element are capable of at least a short time to exist separately, without seeking to form a more complex particle - a molecule, then they say that such substances have an atomic structure. For example, multi-stage reaction of chlorination of methane. It is widely used in the chemistry of organic synthesis to produce the most important halogen-containing derivatives: dichloromethane, carbon tetrachloride. In it, the chlorine molecules split into atoms that have a high reactivity. They destroy the sigma bonds in the methane molecule, providing a chain reaction of substitution.

Another example of a chemical process of great importance in industry is the use of hydrogen peroxide as a disinfectant and bleaching agent. The determination of atomic oxygen as a product of hydrogen peroxide cleavage 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 the ability to destroy pathogenic agents: bacteria, fungi and their spores.

How is the atomic shell

We have already clarified earlier that the structural unit of a chemical element has a complex structure. Around the positive-charged nucleus the negative particles are rotating electrons. Nobel Prize winner Niels Bohr, based on the quantum theory of light, created his own doctrine, in which the characterization and definition of an atom have the following form: electrons move around the nucleus only along certain stationary trajectories, without emitting energy. Bohr's teaching proved that the particles of the microworld, to which atoms and molecules belong, do not obey laws that are valid for large bodies-objects of the macrocosm.

The structure of the electron shells of particulates was studied in papers on quantum physics of such scientists as Hund, Pauli, Klechkovsky. So it became known that electrons make rotational motions around the nucleus not chaotically, but on certain stationary trajectories. Pauli established that within one energy level on each of its orbitals s, p, d, f in electronic cells there can be no more than two negatively charged particles with the opposite spin + 1 and -1.

Hund's rule explained how the orbitals with the same level of energy fill up properly with electrons.

The Klechkovsky rule, also called the rule n + 1, explained how the orbitals of many-electron atoms (elements of 5, 6, 7 periods) are being filled. All of the above laws served as a theoretical basis for the system of chemical elements created by Dmitry Mendeleyev.

Degree of oxidation

It is a fundamental concept in chemistry and characterizes the state of an atom in a molecule. The modern definition of the degree of oxidation of atoms is as follows: it is a conditional charge of an atom in a molecule, which is calculated from the notion that a molecule has only an ionic composition.

The degree of oxidation can be expressed as a whole or fractional number, with a positive, negative or zero value. Most often, atoms of chemical elements have several degrees of oxidation. For example, in nitrogen this is -3, -2, 0, +1, +2, +3, +4, +5. But such a chemical element as fluorine in all its compounds has only one degree of oxidation, equal to -1. If it is represented by a simple substance, then its degree of oxidation is zero. This chemical value is convenient to use for the classification of substances and for describing their properties. Most often, the degree of oxidation of an atom is used in chemistry when composing the equations of oxidation-reduction 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 atomic nucleus does not change during chemical reactions. Only stationary electronic 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 orbital with a higher energy index, such an atom is called excited.

It should be noted that electrons can not stay on such orbitals that are unusual for them for a long time. Returning to its stationary orbit, the electron emits an energy quantum. The study of such characteristics of 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 microworld, but also allowed them to explain the ability of atoms to form a stable and energetically more favorable molecular state of matter, Creation of different types of stable chemical bond: ionic, covalent-polar and nonpolar, donor-acceptor (as a kind of covalent bond) and m 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 vary. Everything will depend on which molecule it enters. Thanks to X-ray analysis, it is possible to calculate the distance between atoms in a chemical compound, as well as determine the radius of the structural unit of the element. Possessing the laws of variation in the radii of atoms entering the period or in a group of chemical elements, one can predict their physical and chemical properties. For example, in periods with an increase in the charge of the nucleus of atoms, their radii decrease ("atom compression"), therefore, the metallic properties of the compounds weaken, and the nonmetallic properties are strengthened.

Thus, knowledge of the structure of the atom makes it possible to accurately determine the physical and chemical properties of all elements that make up the periodic system of Mendeleev.