From the period of antiquity to the middle of the 18th century, the notion that the atom was a particle of matter that could not be separated prevailed in science. The English scientist, as well as the natural scientist D. Dalton, defined the atom as the smallest component of a chemical element. MV Lomonosov in his atomic-molecular theory was able to give a definition of an atom and a molecule. He was convinced that the molecules he called “corpuscles” consist of “elements” - atoms - and are in constant motion.
D. I. Mendeleev believed that this subunit of the 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 creating a theory of the structure of the atom
In the 19th century, the assertion of the indivisibility of an atom was generally recognized. Most scientists believed that particles of one chemical element cannot under any circumstances turn into atoms of another element. These ideas served as the basis on which the definition of the atom until 1932 was based. At the end of the 19th century, fundamental discoveries were made in science that changed this point of view. First of all, in 1897, an electron was discovered by the English physicist D.J. Thomson. This fact radically changed the ideas of scientists about the indivisibility of a component of a chemical element.
How to prove that an 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 radiation.
But if electrons are part of all atoms without exception and are negatively charged, it means that there are some other particles in the atom that necessarily have a positive charge, otherwise the atoms would not be electrically neutral. A physical phenomenon such as radioactivity helped to unravel the structure of the atom. It gave the correct definition of an atom in physics, and then in chemistry.
Invisible rays
The French physicist A. Beckerel was the first to describe the phenomenon of the emission by atoms of certain chemical elements, visually invisible rays. They ionize the air, pass through substances, cause blackening of photographic plates. Later, the Curie and E. Rutherford spouses found that radioactive substances are converted into atoms of other chemical elements (for example, uranium - into 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 caused the changes made to the definition of an 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.
Rutherford Model Contradictions
The theory of the scientist, despite its outstanding character, could not objectively give a definition of the atom. Her findings were contrary 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, must fall on it sooner or later. The atom in this case is destroyed. This does not actually happen, since the chemical elements and particles of which they are composed have existed in nature for a very long time. Inexplicably, such a definition of an atom is based on Rutherford's theory, as well as the phenomenon that occurs when passing incandescent simple substances through a diffraction grating. After all, the atomic spectra formed in this case have a linear shape. This contradicted the Rutherford model of the atom, according to which the spectra would have to be continuous. According to the ideas of quantum mechanics, at present, electrons are characterized in the nucleus not as point objects, but as having the form of an electron cloud.
Its greatest density at a certain locus of space around the nucleus is considered to be the location of the particle at a given time. It was also found that electrons in an atom are arranged in layers. The number of layers can be determined by knowing the number of the period in which the element is 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 established that the electrons of the energy level located 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 foregoing, it follows that the shape of the electron cloud cannot be arbitrary. It is strictly defined according to the orbital quantum number. We add also that the state of an electron in a particle 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 the electron around its axis clockwise or counterclockwise.
Neutron discovery
Thanks to the work of D. Chadwick, conducted by him in 1932, a new definition of the atom was given in chemistry and physics. In his experiments, the scientist proved that the splitting of polonium produces radiation caused by particles having no 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 the atomic nucleus containing protons and neutrons.
According to the new theory, the definition of an 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 nucleus are extremely energy intensive. They act at very short distances (of the order of 10 -23 cm) and are called nuclear. As mentioned earlier, even MV Lomonosov was able to give a definition of an atom and a molecule based on the 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 consists of protons and neutrons bound by nuclear forces.
Is it possible to weigh an atom
It turns out that every atom has a mass. For example, in hydrogen it is 1.67x10 -24 g. It is even hard 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. For calculating the weight of an atom and a molecule, the relative mass is a more convenient quantity. It shows how many times the weight of a molecule or atom is more 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 do not have a dimension.
Scientists are well aware that the atomic mass of a chemical element is the average mass value of all its isotopes. It turns out that in nature units of one chemical element can have different masses. In this case, the charges of the nuclei of such structural particles are the same.
Scientists have found that isotopes differ among themselves by the number of neutrons in the nucleus, and the charge of the nuclei 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, such simple substances as potassium, argon, 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 able to exist at least for a short time separately, without striving to form a more complex particle - a molecule, then they say that such substances have an atomic structure. For example, a multi-stage methane chlorination reaction. It is widely used in organic chemistry to obtain the most important halogen-containing derivatives: dichloromethane, carbon tetrachloride. In it, the splitting of chlorine molecules into atoms with high reactivity occurs. They destroy 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 bleach. The determination of atomic oxygen as a product of the breakdown of hydrogen peroxide occurs both in living cells (under the action of the enzyme catalase) 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 is the atomic shell
We have already found out earlier that the structural unit of a chemical element has a complex structure. Around a positively charged nucleus, negative electrons rotate. The Nobel Laureate Niels Bohr, based on the quantum theory of light, created his own doctrine, in which the characteristic and definition of an atom are as follows: electrons move around a nucleus only along certain stationary paths and do not emit energy. Bohr's teachings proved that particles of the microworld, which include atoms and molecules, do not obey the laws that are valid for large bodies - objects of the macrocosm.
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 the electrons make rotational movements around the nucleus not randomly, but along certain stationary paths. Pauli found that within one energy level on each of its orbitals s, p, d, f in the electron cells can be no more than two negatively charged particles with the opposite spin value + ½ and - ½.
The Hund rule explained how orbits with the same energy level are correctly filled with electrons.
The Klechkovsky rule, also called the n + l rule, explained how the orbitals of many-electron atoms (elements of 5, 6, 7 periods) are filled. All of the above patterns served as the 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 degree of oxidation 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 oxidation state 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, in nitrogen it is -3, -2, 0, +1, +2, +3, +4, +5. But such a chemical element as fluorine in all its compounds has only one oxidation state equal to -1. If it is represented by a simple substance, then its oxidation state is zero. It is convenient to use this chemical quantity to classify substances and to describe their properties. Most often, the degree of oxidation of an atom is used in chemistry in the preparation of equations of redox reactions.
Atomic properties
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 passes into an orbital with a higher energy index, such an atom is called excited.
It should be noted that electrons cannot be in such orbitals unusual for them 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, and 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, which is possible due to the creation of various types of stable chemical bonds: ionic, covalently-polar and non-polar, donor-acceptor (as varieties of covalent bonds) and m metalic. The latter determines the most important physical and chemical properties of all metals.
It was experimentally established that the size of an atom can vary. Everything will depend on which molecule it enters. 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 structural unit of an element. Possessing the laws of changes in the radii of atoms included in a period or in a group of chemical elements, their physical and chemical properties can be predicted. 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 non-metallic ones increase.
Thus, knowledge of the structure of the atom allows you to accurately determine the physical and chemical properties of all elements that are part of the periodic system.