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Molecule(novolat. molecula, shortened from lat. moles-mass), a microparticle formed from two or more atoms and capable of independent existence. It has a constant composition (qualitative and quantitative) of its atomic nuclei and a fixed number of electrons, and has a set of properties that make it possible to distinguish one molecule from others, including from molecules of the same composition. A molecule, as a system consisting of interacting electrons and nuclei, can be in different states and pass from one state to another forcedly (under the influence of external influences) or spontaneously. For all molecules of this type, a certain set of states is characteristic, which can serve to identify molecules. As an independent formation, a molecule has in each state a certain set physical properties, these properties are preserved to some extent during the transition from molecules to the substance consisting of them and determine the properties of this substance. During chemical transformations, the molecules of one substance exchange atoms with the molecules of another substance, break up into molecules with a smaller number of atoms, and also enter into chemical reactions of other types. Therefore, chemistry studies substances and their transformations in close connection with the structure and state of molecules.

Usually, a molecule is an electrically neutral particle; if the molecule carries electric charge(positive or negative), then they talk about molecular ions (cations or anions, respectively). In matter, positive ions always coexist with negative ones. Molecules that are in states with a multiplicity other than unity (as a rule, in doublet states) are called radicals. free radicals in normal conditions, as a rule, cannot exist long time. However, free radicals of a relatively complex structure are known, which are quite stable and can exist under normal conditions.

According to the number of atomic nuclei included in the molecule, diatomic, triatomic, etc. molecules are distinguished. If the number of atoms in a molecule exceeds hundreds or thousands, the molecule is called a macromolecule. The sum of the masses of all the atoms that make up the molecule is considered as the molecular weight (see also Molecular weight of the polymer. Molecular weight distribution). According to the molecular weight, all substances are conditionally divided into low and high molecular weight.

Atom(from other Greek ἄτομος - indivisible) - a particle of a substance of microscopic size and mass, the smallest part of a chemical element, which is the carrier of its properties.

The idea of ​​atoms as indivisible smallest particles of matter arose in ancient times, but only in the 18th century, the works of A. Lavoisier, M. V. Lomonosov and other scientists proved the reality of the existence of atoms.

General characteristics of the structure of the atom. An atom consists of a positively charged nucleus surrounded by a cloud of negatively charged electrons. The dimensions of an atom as a whole are determined by the dimensions of its electron cloud and are large in comparison with the dimensions of the _nucleus of an atom (the linear dimensions of an atom are ~ 10~8 cm, its nuclei are ~ 10" -10" 13 cm). The electron cloud of the atom does not have strictly defined boundaries, so the size of the atom in means. degrees are conditional and depend on the methods of their determination (see Atomic radii). The nucleus of an atom consists of Z protons and N neutrons held together by nuclear forces (see Atomic nucleus). Positive proton charge and negative. the electron charge is the same in abs. the value and are equal to e = 1.60 * 10 -19 C; the neutron does not have electricity. charge. Nuclear charge +Ze - main. characteristic of an atom that determines its belonging to a particular chemical. element. The ordinal number of the element in the period. to the Mendeleev system (atomic number) is equal to the number of protons in the nucleus.

In an electrically neutral atom, the number of electrons in the cloud is equal to the number of protons in the nucleus. However, under certain conditions, it can lose or gain electrons, turning resp. in posit. or deny. ion, for example. Li +, Li 2+ or O -, O 2-. Speaking of atoms of a certain element, they mean both neutral atoms and ions of this element.

Atomic structure and propertiessubstances. Chem. St. islands are determined by the structure of the external. electron shells of atoms, in which the electrons are relatively weakly bound (binding energies from a few eV to several tens of eV). The structure of the external shells of atoms chem. elements of one group (or subgroup) periodic. systems similarly, which determines the similarity of chemical. St. in these elements. (1) With an increase in the number of electrons in a filling shell, their binding energy, as a rule, increases; max. the binding energy is possessed by electrons in a closed shell. Therefore, atoms with one or several. electrons in a partially filled ext. shell give them in chem. districts. Atoms, to-the Crimea is missing one or several. electrons to form a closed ext. shells usually accept them. Atoms of noble gases with closed ext. shells, under normal conditions do not enter into chemical. districts.

The structure of the internal shells of atoms, the electrons of which are much stronger (binding energy 10 2 -10 4 eV), appears only when the interaction. atoms with fast particles and high energy photons. Such interactions determine the nature of x-ray spectra and the scattering of particles (electrons, neutrons) on atoms (see Diffraction methods). The mass of an atom determines such its physical. St-va, as an impulse, kinetic. energy. From mechanical and related magn. and electric moments of the nucleus of an atom depend on some subtle physical. effects (NMR, NQR, hyperfine structure of spectral lines, see Spectroscopy).

1 footnote: Electron-volt(rarely electron volt; Russian designation: eV, international: eV) is an off-system unit of energy used in atomic and nuclear physics, in physics elementary particles and in close and related fields of science (biophysics, physical chemistry, astrophysics, etc.). In the Russian Federation, the electron volt is approved for use as an off-system unit without a time limit with a scope.

Nuclear model of the atom

At the beginning of the 20th century, as a result of the study of cathode rays, negative particles were discovered - electrons with a charge of 1.6. 10‾ 19 C, mass 9.11. 10‾ 31 kg, X-ray electromagnetic radiation discovered. Summarizing these discoveries, J. Thomson in 1897 proposed his own model of the atom - it is a positively charged sphere in which negative electrons are interspersed (like raisins in a pudding). If this model is correct, then the metal foil is a film of positive electricity containing electrons and the flow of α-particles should easily penetrate through it without changing direction.

In 1909, employees of the English. scientist E. Rutherford checked this. 1 out of 100,000 α - particles, when passing through the gold foil, scattered at large angles and even turned back. Analyzing the results of the experiment, Rutherford concluded that the mass and charge of an atom are concentrated in a small part of the volume, called the nucleus. Those α-particles that collide with nuclei are deflected. Most of the α - particles pass through the space between the nuclei. The model of the structure of the atom proposed by E. Rutherford resembled the solar system. It is called the planetary model. According to it, in the center of the atom there is a positive nucleus, in which the entire mass of the atom is concentrated. Electrons move around the nucleus in circular orbits. The charge of the nucleus and the number of electrons are the same, i.e. atom is a neutral particle.

In 1913 The English physicist Moseley measured the wavelengths of X-rays emitted by different metals in a cathode tube and plotted the reciprocal of the square root of the X-ray wavelength versus the element's atomic number. This graph (Figure 1) shows that serial number reflects some important characteristic of the element. Moseley suggested that this characteristic is the charge of the nucleus of an atom, and that it increases by one when moving from one element to the next one in order. He called the serial number the atomic number - Z.

Moseley's law:

The square root of the reciprocal of the wavelength of X-rays emitted by atoms of various elements is linearly dependent on the atomic number of the element.

This is a law that relates the frequency of the spectral lines of the characteristic x-ray radiation of an atom of a chemical element with its serial number.

where  is the wavelength, a- constant value, Z is the ordinal number of the element (nuclear charge).

Later it became known that the serial number is equal to the number of protons in the nucleus. Thus, the ordinal (atomic) number is equal to the charge of the nucleus and it also determines the presence of protons (positive particles) in it. And since atoms are neutral, the number of electrons in an atom must be equal to the number of protons. But the masses of atoms turned out to be greater than the total mass of protons. To explain the excess mass, the existence of neutrons was suggested. These particles had to have the same mass as the proton, but zero charge (1.675.10 - 27 kg). The neutron was discovered by Rutherford's employee Chadwig in 1932. It was finally established that the atom consists of a nucleus and electrons, and the nucleus consists of protons and neutrons. Their sum is called nucleon number or massive - BUT.

BUT= Z+ N,

Z- number of protons, N is the number of neutrons.

Atoms with different number protons ( Z) and neutrons ( N), but with the same number of nucleons BUT, called isobars . For example,

isotopes are atoms with the same number of protons ( Z), nose different number nucleons

isotons are atoms with the same number of neutrons ( N)

Thus, the fractional values ​​of atomic masses in the periodic system are explained by the presence of isotopes for the same element.

Atomic nucleus- the central part of the atom, in which its main mass is concentrated (more than 99.9%). The nucleus is positively charged, the charge of the nucleus determines the chemical element to which the atom is assigned. The dimensions of the nuclei of various atoms are several femtometers, which is more than 10 thousand times smaller than the size of the atom itself.

Spectral line- a feature of the spectrum section, expressed in a local increase (light, emission lines, spectral maxima) or decrease (dark lines, absorption lines, spectral minima) of the signal level.

Residual intensity called the amplification / attenuation of radiation in the spectral line compared to the continuous spectrum.

The function characterizing the dependence of the residual intensity on frequency is called the line profile.

X-ray radiation- electromagnetic waves, the photon energy of which lies on the scale of electromagnetic waves between ultraviolet radiation and gamma radiation, which corresponds to wavelengths from 10 −2 to 10 2 Å (from 10 −12 to 10 −8 m).

Photon(from other Greek φῶς, genus Pad. φωτός, "light") - an elementary particle, a quantum of electromagnetic radiation (in the narrow sense of light). It is a massless particle that can exist in vacuum only by moving at the speed of light.

An atom (from the Greek άτομοσ - indivisible) is the smallest particle of a chemical element that retains all of its Chemical properties. An atom consists of a dense nucleus of positively charged protons and electrically neutral neutrons, which is surrounded by a much larger cloud of negatively charged electrons. When the number of protons matches the number of electrons, the atom is electrically neutral, otherwise it is an ion, with a certain charge. Atoms are classified by the number of protons and neutrons: the number of protons determines the chemical element, and the number of neutrons determines the nuclide of the element.

Forming bonds with each other, atoms are combined into molecules and large solids.

About existence smallest particles Humanity has guessed substances since ancient times, but confirmation of the existence of atoms was received only at the end of the 19th century. But almost immediately it became clear that atoms, in turn, have a complex structure, which determines their properties.

The concept of an atom as the smallest indivisible particle of matter was first proposed by ancient Greek philosophers. In the 17th and 18th centuries, chemists established that chemical substances enter into reactions in certain proportions, which are expressed using small numbers. In addition, they identified certain simple substances, which they called chemical elements. These discoveries led to a revival of the idea of ​​indivisible particles. The development of thermodynamics and statistical physics showed that the thermal properties of bodies can be explained by the motion of such particles. In the end, the sizes of atoms were experimentally determined.

In the late 19th and early 20th centuries, physicists discovered the first of the subatomic particles, the electron, and somewhat later the atomic nucleus, thus showing that the atom is not indivisible. The development of quantum mechanics made it possible to explain not only the structure of atoms, but also their properties: optical spectra, the ability to enter into reactions and form molecules, i.e.

General characteristics of the structure of the atom

Modern ideas about the structure of the atom are based on quantum mechanics.

At the popular level, the structure of the atom can be described in terms of the wave model, which is based on the Bohr model, but also takes into account additional information on quantum mechanics.

For this model:

Atoms consist of elementary particles (protons, electrons and neutrons). The mass of an atom is mostly concentrated in the nucleus, so most of the volume is relatively empty. The nucleus is surrounded by electrons. The number of electrons is equal to the number of protons in the nucleus, the number of protons determines the ordinal number of the element in the periodic system. In a neutral atom, the total negative charge of the electrons is equal to the positive charge of the protons. Atoms of the same element with different numbers of neutrons are called isotopes.
At the center of an atom is a tiny, positively charged nucleus made up of protons and neutrons.
The nucleus of an atom is about 10,000 times smaller than the atom itself. Thus, if an atom is enlarged to the size of Boryspil airport, the size of the nucleus will be smaller size table tennis ball.
The nucleus is surrounded by an electron cloud that occupies most its volume. In an electron cloud, shells can be distinguished, for each of which there are several possible orbitals. The filled orbitals are electronic configuration characteristic of each chemical element.
Each orbital can contain up to two electrons, characterized by three quantum numbers: basic, orbital and magnetic.
Each electron in an orbital has a unique value for the fourth quantum number: spin.
Orbitals are defined by a specific probability distribution of where exactly an electron can be found. Examples of orbitals and their designations are shown in the figure on the right. The "boundary" of an orbital is the distance at which the probability that an electron can be outside of it is less than 90%.
Each shell can contain no more than a strictly defined number of electrons. For example, the shell closest to the nucleus can have a maximum of two electrons, the next - 8, the third from the nucleus - 18, and so on.
When electrons join an atom, they drop into a low-energy orbital. Only outer shell electrons can participate in the formation of interatomic bonds. Atoms can donate and gain electrons, becoming positively or negatively charged ions. The chemical properties of an element are determined by the ease with which the nucleus can donate or acquire electrons. It depends both on the number of electrons and on the degree of filling of the outer shell.
Atom size

The size of an atom is a quantity difficult to measure, because the central nucleus is surrounded by a blurry electron cloud. For atoms forming solid crystals, the distance between adjacent sites crystal lattice can serve as an approximate value of their size. For atoms, crystals do not form, other evaluation techniques are used, including theoretical calculations. For example, the size of a hydrogen atom is estimated as 1.2 × 10-10 m. This value can be compared with the size of a proton (which is the nucleus of a hydrogen atom): 0.87 × 10-15 m and make sure that the nucleus of a hydrogen atom is 100 000 times smaller than the atom itself. Atoms of other elements retain approximately the same ratio. The reason for this is that elements with a large positively charged nucleus attract electrons more strongly.

Another characteristic of the size of an atom is the van der Waals radius - the distance that another atom can approach a given atom. Interatomic distances in molecules are characterized by the length of chemical bonds or covalent radius.

Nucleus

The main mass of an atom is concentrated in the nucleus, which consists of nucleons: protons and neutrons, interconnected by the forces of nuclear interaction.

The number of protons in an atom's nucleus determines its atomic number and the element to which the atom belongs. For example, carbon atoms contain 6 protons. All atoms with a particular atomic number have the same physical characteristics and exhibit the same chemical properties. The elements are listed in the periodic table in ascending order of atomic number.

The total number of protons and neutrons in an element's atom determines its atomic mass, since a proton and a neutron have a mass of approximately 1 amu. Neutrons in a nucleus do not affect which element an atom belongs to, but a chemical element can have atoms with the same number of protons and a different number of neutrons. Such atoms have the same atomic number but different atomic mass and are called isotopes of the element. When writing the name of an isotope, the atomic mass is written after it. For example, the isotope carbon-14 contains 6 protons and 8 neutrons, for a total atomic mass of 14. Another popular notation method is to superscript the atomic mass before the element symbol. For example, carbon-14 is referred to as 14C.

The atomic mass of an element given in the periodic table is an average of the masses of naturally occurring isotopes. Averaging is carried out according to the abundance of the isotope in nature.

With an increase in the atomic number, the positive charge of the nucleus increases, and, consequently, the Coulomb repulsion between protons. More and more neutrons are needed to hold protons together. However a large number of neutrons is unstable, and this circumstance imposes a limitation on the possible charge of the nucleus and the number of chemical elements that exist in nature. Chemical elements with high atomic numbers have a very short lifetime, can only be created by bombarding the nuclei of light elements with ions, and are observed only during experiments using accelerators. As of February 2008, ununoctium is the heaviest synthesized chemical element.

Many isotopes of chemical elements are unstable and decay over time. This phenomenon is used by the radioelement test to determine the age of objects and is of great importance for archeology and paleontology.

Bohr model

The Bohr model is the first physical model that was able to correctly describe the optical spectra of the hydrogen atom. After the development of the exact methods of quantum mechanics, the Bohr model has only historical significance, but due to its simplicity, it is still widely taught and used for a qualitative understanding of the structure of the atom.

Bohr's model is based on Rutherford's planetary model, which describes the atom as a small positively charged nucleus with negatively charged electrons in orbits at different levels, which resembles the structure solar system. Rutherford proposed a planetary model to explain the results of his experiments on the scattering of alpha particles by metal foil. According to the planetary model, an atom consists of a heavy nucleus around which electrons revolve. But the fact that the electrons rotating around the nucleus do not fall in a spiral onto it was incomprehensible to the physicists of that time. Indeed, according to classical theory electromagnetism, an electron that revolves around the nucleus must radiate electromagnetic waves(light), which would lead to a gradual loss of energy and fall to the core. So how can an atom exist at all? Moreover, the study of the electromagnetic spectrum of atoms showed that the electrons in an atom can only emit light of a certain frequency.

These difficulties were overcome in the model proposed by Niels Bohr in 1913, which postulates that:

Electrons can only be in orbits that have discrete quantized energies. That is, not all orbits are possible, but only some specific ones. The exact values ​​of the energies of admissible orbits depend on the atom.
Laws classical mechanics do not operate when electrons move from one allowable orbit to another.
When an electron moves from one orbit to another, the difference in energy is emitted (or absorbed) by a single quantum of light (photon), whose frequency is directly related to the energy difference between the two orbits.

where ν is the frequency of the photon, E is the energy difference, and h is a constant of proportionality, also known as Planck's constant.
Determine what can be written

where ω is the angular frequency of the photon.
Permissible orbits depend on the quantized values ​​of the orbital angular momentum L, described by the equation

where n = 1,2,3,...
and is called the quantum number of angular momentum.
These assumptions made it possible to explain the results of the then observations, for example, why the spectrum consists of discrete lines. Assumption (4) states that smallest value n is 1. Accordingly, the smallest allowable atomic radius is 0.526 Å (0.0529 nm = 5.28 10-11 m). This value is known as the Bohr radius.

Bohr's model is sometimes referred to as Semiclassical because although it includes some ideas from quantum mechanics, it is not a complete quantum mechanical description of the hydrogen atom. However, Bohr's model was a significant step towards such a description.

With a strict quantum mechanical description of the hydrogen atom, the energy levels are found from the solution of the stationary Schrödinger equation. These levels are characterized by the three quantum numbers mentioned above, the formula for quantizing the angular momentum is different, the quantum number of the angular momentum is zero for spherical s-orbitals, one for prolate dumbbell-shaped p-orbitals, etc. (see picture above).

The energy of the atom and its quantization

The energy values ​​that an atom can have are calculated and interpreted based on the provisions of quantum mechanics. This takes into account such factors as the electrostatic interaction of electrons with the nucleus and electrons among themselves, the spins of electrons, the principle of identical particles. In quantum mechanics, the state in which an atom is located is described by a wave function, which can be found from the solution of the Schrödinger equation. There is a certain set of states, each of which has a certain energy value. The state with the lowest energy is called the ground state. Other states are called excited. An atom is in an excited state for a finite time, emitting sooner or later a quantum of an electromagnetic field (photon) and passing into the ground state. An atom can stay in the ground state for a long time. In order to be excited, he needs external energy, which can only come to him from the external environment. An atom emits or absorbs light only at certain frequencies, corresponding to the difference in the energies of its states.

The possible states of an atom are indexed by quantum numbers such as spin, quantum number of orbital momentum, quantum number of total momentum. You can read more about their classification in the article electronic term

Electronic shells of complex atoms

Complex atoms have dozens, and for very heavy elements, even hundreds of electrons. According to the principle of identical particles, the electronic states of atoms are formed by all electrons, and it is impossible to determine where each of them is located. However, in the so-called one-electron approximation, one can speak of certain energy states of individual electrons.

According to these ideas, there is a certain set of orbitals that are filled with the electrons of the atom. These orbitals form a certain electronic configuration. Each orbital can contain no more than two electrons (Pauli exclusion principle). Orbitals are grouped into shells, each of which can only have a certain fixed number of orbitals (1, 4, 10, etc.). Orbitals are divided into internal and external. In the ground state of an atom, the inner shells are completely filled with electrons.

In inner orbitals, electrons are very close to the nucleus and are strongly attached to it. To pull an electron out of the inner orbit, you need to provide it with a lot of energy, up to several thousand electron volts. An electron on the inner shell can obtain such energy only by absorbing an X-ray quantum. The energies of the inner shells of atoms are individual for each chemical element, and therefore an atom can be identified by the X-ray absorption spectrum. This circumstance is used in x-ray analysis.

In the outer shell, the electrons are far from the nucleus. It is these electrons that are involved in the formation of chemical bonds, so the outer shell is called valence, and the outer shell electrons are called valence electrons.

Quantum transitions in the atom

Transitions between different states of atoms are possible, caused by an external perturbation, more often electromagnetic field. Due to the quantization of atomic states, the optical spectra of atoms consist of individual lines if the energy of a light quantum does not exceed the ionization energy. At higher frequencies, the optical spectra of atoms become continuous. The probability of excitation of an atom by light decreases with a further increase in frequency, but increases sharply at certain frequencies characteristic of each chemical element in the X-ray range.

Excited atoms emit light quanta with the same frequencies at which absorption occurs.

Transitions between different states of atoms can also be caused by interactions with fast charged particles.

Chemical and physical properties of the atom

The chemical properties of an atom are determined mainly by valence electrons - electrons in the outer shell. The number of electrons in the outer shell determines the valency of the atom.

The atoms of the last column of the periodic table of elements have a completely filled outer shell, and for the transition of an electron to the next shell, a very large amount of energy must be provided to the atom. Therefore, these atoms are inert, not inclined to enter into chemical reactions. Inert gases thin out and crystallize only at very low temperatures.

Atoms of the first column periodic table elements have one electron on the outer shell, and are chemically active. Their valency is 1. A characteristic type of chemical bond for these atoms in the crystallized state is a metallic bond.

The atoms of the second column of the periodic table in the ground state have 2 s-electrons on the outer shell. Their outer shell is filled, so they must be inert. But the transition from the ground state with the s2 electron shell configuration to the state with the s1p1 configuration requires very little energy, so these atoms have a valence of 2, but they show less activity.

The atoms of the third column of the periodic table of elements have the electronic configuration s2p1 in the ground state. They can show different valencies: 1, 3, 5. The last possibility arises when the electron shell of the atom is completed to 8 electrons and becomes closed.

Atoms The fourth column of the periodic table of elements has a valence of 4 (for example, carbon dioxide CO2), although a valence of 2 is possible (for example, carbon monoxide CO). Before this column belongs carbon - an element that forms a wide variety of chemical compounds. A special branch of chemistry is devoted to carbon compounds - organic chemistry. Other elements of this column - silicon, germanium under normal conditions are solid-state semiconductors.

The elements of the fifth column have a valence of 3 or 5.

The elements of the sixth column of the periodic table in the ground state have an s2p4 configuration and a common spin of 1. Therefore, they are divalent. There is also the possibility of an atom transitioning to an excited state s2p3s" with spin 2, in which the valency is 4 or 6.

The elements of the seventh column of the periodic table lack one electron in the outer shell in order to fill it. They are mostly monovalent. However, they can enter into chemical compounds in excited states, showing valences of 3,5,7.

Transition elements are characterized by the filling of the outer s-shell before the d-shell is completely filled. Therefore, they mostly have a valence of 1 or 2, but in some cases one of the d-electrons is involved in the formation of chemical bonds, and the valence becomes equal to three.

At education chemical compounds atomic orbitals mutate, deform and become molecular orbitals. In this case, the process of hybridization of orbitals takes place - the formation of new orbitals, as a specific sum of the base ones.

History of the concept of atom

Read more in the article atomistics
The concept of atom, like the word itself, is of ancient Greek origin, although the truth of the hypothesis of the existence of atoms was confirmed only in the 20th century. The main idea behind this concept for all centuries was the idea of ​​the world as a set of huge amount indivisible elements that are very simple in structure and have existed since the beginning of time.

The first preachers of the atomistic doctrine

The philosopher Leucippus was the first to preach atomistic teachings in the 5th century BC. Then the baton was picked up by his student Democritus. Only fragments of their works have survived, from which it becomes clear that they proceeded from a small number of rather abstract physical hypotheses:

"Sweetness and bitterness, heat and cold are the meaning of the definition, in fact [only] atoms and emptiness."

According to Democritus, all nature consists of atoms, the smallest particles of matter that rest or move in a completely empty space. All atoms have a simple form, and atoms of the same kind are identical; the variety of nature reflects the variety of forms of atoms and the variety of ways in which atoms can interlock with each other. Both Democritus and Leucippus believed that atoms, having begun to move, continue to move according to the laws of nature.

The most difficult for the ancient Greeks was the question of the physical reality of the basic concepts of atomism. In what sense could one speak of the reality of emptiness if, having no matter, it cannot have any physical properties? The ideas of Leucippus and Democritus could not serve as a satisfactory basis for the theory of matter in physical plane, because they did not explain what atoms are not made of, nor why atoms are indivisible.

A generation after Democritus, Plato proposed his solution to this problem: “the smallest particles do not belong to the realm of matter, but to the realm of geometry; they are different bodily geometric figures bounded by flat triangles.

The concept of the atom in Indian philosophy

A thousand years later, the abstract reasoning of the ancient Greeks penetrated into India and was adopted by some schools of Indian philosophy. But if Western philosophy believed that the atomistic theory should become a concrete and objective basis for the theory material world, Indian philosophy has always perceived the material world as an illusion. When atomism appeared in India, it took the form of a theory according to which reality in the world has a process, not a substance, and that we are present in the world as links in a process, and not as clots of matter.

That is, both Plato and Indian philosophers thought something like this: if nature consists of small, but finite in size, shares, then why can’t they be divided, at least in the imagination, into even smaller particles, which became the subject of further consideration?

Atomistic theory in Roman science

The Roman poet Lucretius (96 - 55 BC) was one of the few Romans who showed an interest in pure science. In his poem On the Nature of Things (De rerum natura), he built up in detail the facts that testify in favor of the atomistic theory. For example, a wind that blows with great force, although no one can see it, is probably composed of particles, leaking to see them. We can feel things from a distance by smell, sound and heat that spread without being seen.

Lucretius connects the properties of things with the properties of their constituents, i.e. atoms: the atoms of a liquid are small and round in shape, which is why the liquid flows so easily and seeps through porous matter, while the atoms solids have hooks with which they are linked to each other. In the same way, various taste sensations and sounds of different loudness are composed of atoms of appropriate shapes - from simple and harmonious to sinuous and irregular.

But the teachings of Lucretius were condemned by the church, since he gave a rather materialistic interpretation of them: for example, the idea that God, having launched the atomic mechanism once, no longer interferes with its work, or that the soul dies with the body.

The first theories about the structure of the atom

One of the first theories about the structure of the atom, which already has modern outlines, was described by Galileo (1564-1642). According to his theory, matter consists of particles that are not at rest, but move in all directions under the influence of heat; heat is nothing but the movement of particles. The structure of the particles is complex, and if you deprive any part of its material shell, then light will spurt from within. Galileo was the first to present, albeit in fantastic form, the structure of the atom.

Scientific Foundations

In the 19th century, John Dalton obtained evidence for the existence of atoms, but assumed that they were indivisible. Ernest Rutherford showed experimentally that an atom consists of a nucleus surrounded by negatively charged particles - electrons.

The simple fact that everything around us is made up of the smallest particles of matter - molecules and atoms - actually has tremendous scientific power. From this statement alone, one can deduce big number consequences that provide a qualitative explanation for many physical phenomena. If suddenly humanity "forgot" all the natural science knowledge accumulated over many centuries, then, clinging only to this fact and using the scientific method, it could very quickly restore the basics of many sections of physics and chemistry.

Children will learn about the atomic structure of matter in primary school. But atoms are not visible either with the eye or with an optical microscope. Moreover, in ordinary experiments with matter, when we measure various characteristics of matter ( density, heat capacity, specific heat of fusion and vaporization, viscosity, liquid surface tension and so on), we may not think at all that it consists of individual particles. modern physics, of course, allows you to "see" individual atoms with the help of complex instruments. But the question arises: is there any simple way to determine the typical size of molecules without resorting to such a technique? It turns out yes.

A task

Armed with the simple fact that everything is made of atoms, estimate the size of a water molecule based on (some of) the above macroscopic characteristics. The numerical values ​​of these parameters for water can be easily found in reference books or on the Internet.

Clue

It is worth emphasizing right away that solutions that rely on the Avogadro number or on the properties of individual molecules are “deceptive”, since they implicitly already use the size of the molecules. For example, the required estimate is easily obtained from the density and molar mass of water and Avogadro's number. However, the Avogadro number, which connects the microcosm with the macrocosm and "knows" about the size of atoms, does not manifest itself in a purely macroscopic experiment and itself requires experimental measurement.

The size of atoms is proposed to be estimated (of course, not exactly, but only in order of magnitude) on the basis of precisely the macroscopic characteristics of the substance.

Solution

Molecular size can be derived from density, surface tension, and specific heat of vaporization. Let's do this in two ways.

Method 1. A liquid is made up of molecules, but it retains its volume, rather than breaking up into individual particles like a gas. This means, firstly, that the molecules in a liquid are kept relative to each other at a certain distance, in order of magnitude equal to the diameter of the molecule itself ( d), and secondly, that each pair interaction between molecules is characterized by some binding energy ( U). Quantities d and U- microscopic, numerical values we don't know in advance.

During evaporation, the liquid turns into a rarefied gas, in which all bonds between all molecules can be considered broken. Specific heat of vaporization E, measured in J/kg, is simply the sum of all the intermolecular binding energies that were originally in a kilogram of water. Multiplying the specific heat of vaporization by the density ρ and by the (still unknown) volume occupied by one molecule (on the order of d 3), we get the bond energy per molecule. This value is 2-3 times higher U- after all, each molecule is usually associated with several (4–6) neighbors: Eρ d 3 = 2U.

On the other hand, the phenomenon of surface tension is that any free surface of a liquid is characterized by an "extra" energy proportional to the surface area: E sov = σ S. This energy can be easily measured experimentally and the coefficient of surface tension σ can be derived from this. Microscopically, this energy arises from the fact that in the very surface layer of the liquid there are molecules with "broken bonds", that is, with bonds that stick out into the void, and are not closed to neighboring molecules. There are few such bonds, say one for each molecule, and the energy of this “broken bond” is approximately equal to U. Since each surface molecule occupies an area of ​​approximately d 2 , the same value U can be written as σ d 2 .

Equating the magnitude U, obtained by these two methods, we find the typical size: d= 2σ/ Eρ.

Method 2. Take a spherical drop of liquid and divide it into two drops. The total volume did not change, but the surface area increased, which means that the surface tension energy also increased. Therefore, for such a separation, we need to expend energy equal to the difference between the surface energies at the beginning and at the end. We will crush the drop further and further until we reach “drops” the size of a molecule. Strictly speaking, with such dimensions, it is no longer possible to speak about surface tension, but for the most rough estimates, one can nevertheless calculate the resulting “total surface area”, multiply it by σ and find how much energy must be spent on such a separation. But the division of a liquid into separate "drops" the size of a molecule is the process of vaporization. Thus, it is also possible to obtain a formula similar to the one above, but only with a slightly different numerical coefficient.

It remains to substitute the numbers. The density of water is 1000 kg / m 3, the coefficient of surface tension is 0.07 J / m 2, specific heat vaporization 2.3 MJ/kg. The size of the molecule is obtained from this 0.6 10 -10 m. This is about 3 times less than the actual size of the molecule, which is not bad at all for such a rough estimate.

Afterword

This, of course, is not the only way to find out the size of molecules from macroscopic data, but all such methods give only a very rough estimate in order of magnitude. A much more accurate measurement of dimensions can be obtained by scattering X-rays (and also electrons or neutrons) with a wavelength of less than a nanometer on crystals. The diffraction pattern shows not only the dimensions of the crystal cell, but also tells how the atoms in it are located relative to each other.

It is interesting to note that even at the beginning of the 20th century, not all scientists adhered to the atomistic picture of the structure of matter. Key Points that proved the reality of molecules was Einstein's description of Brownian motion and the law of diffusion, as well as Perrin's discovery of sedimentation equilibrium (Nobel Prize in Physics for 1926). In both experiments, microscopically particles of matter, the size of which could be determined through observation through a microscope, behaved somewhat like individual molecules of matter, which made it possible to “build bridges” between the microworld and the world of everyday phenomena.

Atoms do not have a distinct outer boundary, so their size is determined by the distance between the nuclei of neighboring atoms that have formed a chemical bond. The radius depends on the position of the atom, its type, the type of chemical bond, the number of nearby atoms (coordination number), and a quantum mechanical property known as spin. In the Periodic Table of Elements, the size of an atom increases as it moves from top to bottom in a column and decreases as it moves across a row from left to right. Accordingly, the smallest atom is a helium atom with a radius of 32 pm, and the largest is a cesium atom (225 pm).

An atom is a unique particle of the universe. This article will try to convey to the reader information about this element of matter. Here we will consider such questions: what is the diameter of an atom and its dimensions, what are its qualitative parameters, what is its role in the Universe.

Introduction to the atom

Atom - compound particle substances having microscopic size and mass. This is the smallest part of the elements of a chemical nature with an incredibly small size and mass.

Atoms are built from two basic structural elements, namely from electrons and the atomic nucleus, which, in turn, is formed by protons and neutrons. The number of protons may differ from the number of neutrons. In both chemistry and physics, atoms in which the size of protons is commensurate with the number of electrons are called electrically neutral. If the number of electrons is higher or lower than the number of protons, then the atom, acquiring a positive or negative charge, becomes an ion.

Historical data

Thanks to the achievements of science in the field of physics and chemistry, many discoveries have been made regarding the nature of the atom, its structure and capabilities. Numerous experiments and calculations were made, during which a person was able to answer such questions: what is the diameter of an atom, its size, and much more.

For the first time the concept of an atom was discovered and formulated by philosophers ancient greece and Rome. In the 17th and 18th centuries, chemists were able, through experiments, to prove the idea of ​​the atom as the smallest particle of matter. They showed that many substances can be broken down repeatedly using chemical methods. However, in the future, subatomic particles discovered by physicists showed that even an atom can be divided, and it is built from subatomic components.

The International Congress of Chemistry Scientists in Karlsruhe, Germany, in 1860 decided on the concept of atoms and molecules, where the atom is considered as the smallest part of the chemical elements. Therefore, it is also included in the composition of substances of a simple and complex type.

The diameter of the hydrogen atom was one of the very first to be studied. However, its calculations have been made many times and the last of them, published in 2010, showed that it is 4% less than previously thought (10 -8). The index of the total value of the size of the atomic nucleus corresponds to the number 10 -13 -10 -12, and the order of magnitude of the entire diameter is 10 -8. This caused a lot of controversy and problems, since hydrogen itself rightfully belongs to the main constituent parts the entire observable Universe, and such a discrepancy forces us to make many recalculations in relation to fundamental statements.

Atom and its model

Currently, five basic models of the atom are known, differing among themselves, first of all, in the time frame of ideas about its structure. Let's take a look at the models:

• The pieces that make up matter. Democritus believed that any property of substances should be determined by its shape, mass and other practical characteristics. For example, fire can burn because its atoms are sharp. According to Democritus, even the soul is formed by atoms.
• Thomson's atomic model, created in 1904 by J. J. Thomson himself. He suggested that the atom can be taken as a positively charged body enclosed within electrons.
• Nagaoka's early planetary atomic model, created in 1904, believed that the structure of the atom was similar to that of Saturn. The nucleus is small and has a positive charge, surrounded by electrons that move around the rings.
• Atomic planetary model discovered by Bohr and Rutherford. In 1911, E. Rutherford, after conducting a number of experiments, began to believe that the atom is similar to a planetary system, where electrons have orbits along which they move around the nucleus. However, this assumption went against the data of classical electrodynamics. To prove the validity of this theory, Niels Bohr introduced the concept of postulates, asserting and showing that the electron does not need to expend energy, since it is in a certain, special energy state. The study of the atom later led to the appearance quantum mechanics, which was able to explain many of the contradictions that could be observed.
• The quantum mechanical atomic model states that the central basis of the considered particle consists of a nucleus formed from protons, as well as neutrons and electrons moving around it.

Structural features

The size of an atom previously predetermined that it was an indivisible particle. However, many experiences and experiments have shown us that it is built from subatomic particles. Any atom consists of electrons, protons and neutrons, with the exception of hydrogen - 1, which does not include the latter.

The Standard Model shows that protons and neutrons are formed through interactions between quarks. They belong to fermions, along with leptons. There are currently 6 types of quarks. Protons owe their formation to two u-quarks and one d-quark, and the neutron to one u-quark and two d-quarks. The nuclear force of the strong type, which binds quarks, is transmitted with the help of gluons.

The movement of electrons in atomic space is predetermined by their "desire" to be closer to the nucleus, in other words, to be attracted, as well as the Coulomb forces of interaction between them. These same types of forces keep each electron in a potential barrier that surrounds the nucleus. The orbit of the electrons determines the size of the diameter of the atom, which is equal to a straight line passing from one point in the circle to another, and also through the center.

The atom has its spin, which is represented by its own momentum and is beyond understanding common nature matter. Described using quantum mechanics.

Dimensions and weight

Each nucleus of an atom with the same number of protons belongs to the total chemical element. Isotopes include representatives of atoms of the same element, but having a difference in neutron number.

Since in physics the structure of the atom indicates that their bulk is made up of protons and neutrons, then the total amount of these particles has a mass number. The expression of atomic mass at rest occurs through the use of atomic mass units (amu), which are otherwise called daltons (Da).

The size of an atom has no clearly defined boundaries. Therefore, it is determined by measuring the distance between the nuclei of the same type of atoms chemically bonded to each other. Another method of measurement is possible when calculating the duration of the path from the nucleus to the furthest of the available electronic orbits of a stable type. The periodic system of elements of D. I. Mendeleev arranges atoms in size, from smaller to larger, in the direction of the column from top to bottom, movement in the direction from left to right is also based on a decrease in their size.

Decay time

All chem. elements have isotopes, one and up. They contain an unstable core, subject to radioactive decay, resulting in the emission of particles or electromagnetic radiation. A radioactive isotope is one in which the value of the radius of the strong interaction goes beyond the far points of the diameter. If we consider the aurum as an example, then the isotope will be the Au atom, beyond the diameter of which radiating particles "fly out" in all directions. Initially, the diameter of the gold atom corresponds to the value of two radii, each of which is equal to 144 pc, and particles that go beyond this distance from the nucleus will be considered isotopes. There are three types of decay: alpha, beta and gamma radiation.

The concept of valence and the presence of energy levels

We have already got acquainted with the answers to such questions: what is the diameter of an atom, its size, got acquainted with the concept of the decay of an atom, etc. However, in addition to this, there are also such characteristics of atoms as the magnitude of energy levels and valency.

Electrons moving around the atomic nucleus have potential energy and are in a bound state, located at an excited level. According to the quantum model, an electron only occupies a discrete number of energy levels.

Valence is the general ability of atoms that have free space on the electron shell to establish bonds chemical type with other atomic units. Through the establishment of chemical bonds, atoms try to fill their layer of the outer valence shell.

Ionization

As a result of the impact high value tension on an atom, it can undergo irreversible deformation, which is accompanied by electron detachment.

This results in the ionization of the atoms, during which they donate an electron(s) and undergo a transformation from a stable state into ions with a positive charge, otherwise referred to as cations. This process requires a certain energy, which is called the ionization potential.

Summing up

The study of questions about the structure, features of interaction, qualitative parameters, about what is the diameter of an atom and what size it has, all this allowed the human mind to do incredible work that helps to better understand and understand the structure of all matter around us. The same questions made it possible for a person to discover the concepts of the electronegativity of an atom, its dispersed attraction, valence possibilities, determine the duration radioactive decay and much more.

Atom size determined by the radius of its outer electron shell. The dimensions of all atoms are ~ 10 -10 m. And the size of the nucleus is 5 orders of magnitude smaller, in total - 10 -15 m. Visually, this can be represented as follows: if the atom is increased to the size of a 20-story building, then the nucleus of the atom will look like a millimeter speck of dust in the central room in this house. However, it is difficult to imagine a house, the mass of which is almost completely concentrated in this speck of dust. And the atom is just that.

Atoms are very small and very light. An atom is as many times lighter than an apple as an apple is the globe. If the world "gets heavier" so that an atom begins to weigh like a drop of water, then people in such a world will become heavy, like planets: children - like Mercury and Mars, and adults - like Venus and Earth.

You can't see an atom even with a microscope. The best optical microscopes make it possible to distinguish the details of an object if the distance between them is ~0.2 µm. In an electron microscope, this distance was reduced to ~2-3 Å. For the first time, it was possible to distinguish and photograph individual atoms using an ion projector. But no one saw how the atom is arranged inside. All data on the structure of atoms are obtained from experiments on particle scattering.

Mass of the atomic nucleus several thousand times the mass of its electron shell. This is due to the fact that the nuclei of atoms consist of very heavy, in comparison with the electron, particles - protons. p and neutrons n. Their masses are almost the same and about 2000 times the mass of an electron. Wherein proton- positively charged particles, and neutron- neutral. The charge of a proton is equal in magnitude to the charge of an electron. The number of protons in the nucleus is equal to the number of electrons in the shell, and this ensures the electrical neutrality of the atom. The number of neutrons can be different, in the nucleus of a light hydrogen atom there are no neutrons at all, and in the nucleus of a carbon atom there can be 6, and 7, and 8.

Mass of an electronme ≈ 0.91. 10 -30 kg, proton massm p≈ 1.673. 10 -27 kg = 1836m e , neutron massm n \u003d 1.675. 10 -27 kg≈ 1840 me.

mass of an atom less than the sum of the masses of the nucleus and electrons per size ∆m, called mass defect, which arises due to the Coulomb interaction of the nucleus and electrons. The mass defect of atoms (unlike nuclei) is very small, and although it increases with increasing Z, not a single atom exceeds the mass of an electron. material from the site

Of course, an atom cannot be put on a scale and weighed, it is too small. The masses of atoms were first determined by chemists. Moreover, they measured them in relative units, taking the mass of a hydrogen atom as a unit and using Dalton's law, according to which chemicals are formed when atoms of chemical elements are combined in a strictly defined proportion. And now the masses of atoms are most often measured in relative units, but 1/12 of the mass of the carbon atom C 12.1 a.m. is used as the atomic mass unit (a.m.u.). e.m. = 1.66057 . 10 -27 kg.