Who discovered the molecule? Mass and size of molecules. It's interesting to know that

Molecules with a multiplicity other than unity (that is, with unpaired electrons and unsaturated valences) are radicals.

Molecules of relatively high molecular weight, consisting of repeating low-molecular-weight fragments, are called macromolecules.

From the point of view of quantum mechanics, a molecule is a system not of atoms, but of electrons and atomic nuclei interacting with each other.

The structural features of molecules determine the physical properties of a substance consisting of these molecules.

Substances that retain molecular structure in the solid state include, for example, water, carbon monoxide (IV), and many organic substances. They are characterized by low melting and boiling points. Most solid (crystalline) inorganic substances do not consist of molecules, but of other particles (ions, atoms) and exist in the form of macrobodies (sodium chloride crystal, a piece of copper, etc.).

The composition of the molecules of complex substances is expressed using chemical formulas.

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At the international congress of chemists in Karlsruhe in 1860, definitions of the concepts of molecule and atom were adopted. A molecule has been defined as the smallest particle of a chemical substance that has all of its chemical properties.

Classical theory of chemical structure

In the classical theory of chemical structure, a molecule is considered as the smallest stable particle of a substance that has all its chemical properties.

The molecule of a given substance has a constant composition, that is, the same number of atoms united by chemical bonds, while the chemical individuality of the molecule is determined precisely by the set and configuration of chemical bonds, that is, valence interactions between the atoms included in its composition, ensuring its stability and basic properties in a fairly wide range. range of external conditions. Non-valent interactions (for example, hydrogen bonds), which can often significantly influence the properties of molecules and the substance formed by them, are not taken into account as a criterion for the individuality of a molecule.

The central position of the classical theory is the provision of a chemical bond, while the presence of not only two-center bonds uniting pairs of atoms is allowed, but also the presence of multicenter (usually three-center, sometimes four-center) bonds with “bridge” atoms - such as, for example, bridge hydrogen atoms in borans, the nature of the chemical bond is not considered in the classical theory - only integral characteristics such as bond angles, dihedral angles (angles between planes formed by triplets of nuclei), bond lengths and their energies are taken into account.

Thus, a molecule in classical theory is represented by a dynamic system in which atoms are considered as material points and in which atoms and related groups of atoms can perform mechanical rotational and vibrational movements relative to some equilibrium nuclear configuration corresponding to the minimum energy of the molecule and is considered as a system of harmonic oscillators.

A molecule consists of atoms, or more precisely, of atomic nuclei, surrounded by a certain number of internal electrons and external valence electrons that form chemical bonds. The inner electrons of atoms usually do not participate in the formation of chemical bonds. The composition and structure of the molecules of a substance do not depend on the method of its preparation.

Atoms join together in a molecule in most cases through chemical bonds. Typically, such a bond is formed by one, two or three pairs of electrons shared by two atoms, forming a common electron cloud, the shape of which is described by the type of hybridization. A molecule can have positively and negatively charged atoms (ions).

The composition of a molecule is conveyed by chemical formulas. The empirical formula is established on the basis of the atomic ratio of the elements of a substance and its molecular weight.

The geometric structure of a molecule is determined by the equilibrium arrangement of atomic nuclei. The energy of interaction between atoms depends on the distance between the nuclei. At very large distances this energy is zero. If a chemical bond is formed when atoms approach each other, then the atoms are strongly attracted to each other (weak attraction is observed even without the formation of a chemical bond); with further approach, electrostatic repulsive forces of atomic nuclei begin to act. An obstacle to the close approach of atoms is also the impossibility of combining their internal electron shells.

Each atom in a certain valence state in a molecule can be assigned a certain atomic or covalent radius (in the case of an ionic bond, the ionic radius), which characterizes the size of the electron shell of the atom (ion) forming a chemical bond in the molecule. The size of the electron shell of a molecule is a conventional value. There is a probability (albeit very small) of finding the electrons of a molecule at a greater distance from its atomic nucleus. The practical dimensions of a molecule are determined by the equilibrium distance to which they can be brought together when the molecules are densely packed in a molecular crystal and in a liquid. At large distances, molecules attract each other; at shorter distances, they repel each other. The dimensions of a molecule can be found using X-ray diffraction analysis of molecular crystals. The order of magnitude of these dimensions can be determined from the coefficients of diffusion, thermal conductivity and viscosity of gases and from the density of the substance in the condensed state. The distance at which valence-unbonded atoms of the same or different molecules can approach each other can be characterized by the average values of the so-called van der Waals radii (Ǻ).

The van der Waals radius significantly exceeds the covalent radius. Knowing the values of van der Waals, covalent and ionic radii, it is possible to construct visual models of molecules that would reflect the shape and size of their electronic shells.

Covalent chemical bonds in a molecule are located at certain angles, which depend on the state of hybridization of atomic orbitals. Thus, molecules of saturated organic compounds are characterized by a tetrahedral (tetrahedral) arrangement of bonds formed by a carbon atom, for molecules with a double bond (C = C) - a flat arrangement of carbon atoms, for molecules of compounds with a triple bond (C º C) - a linear arrangement of bonds . Thus, a polyatomic molecule has a certain configuration in space, that is, a certain geometry of the arrangement of bonds, which cannot be changed without breaking them. A molecule is characterized by one or another symmetry of the arrangement of atoms. If a molecule does not have a plane and a center of symmetry, then it can exist in two configurations that are mirror images of each other (mirror antipodes, or stereoisomers). All the most important biological functional substances in living nature exist in the form of one specific stereoisomer.

Quantochemical theory of chemical structure

In the quantum chemical theory of chemical structure, the main parameters that determine the individuality of a molecule are its electronic and spatial (stereochemical) configurations. In this case, the configuration with the lowest energy, that is, the ground energy state, is taken as the electronic configuration that determines the properties of the molecule.

Representation of molecular structure

Molecules consist of electrons and atomic nuclei, the location of the latter in the molecule is conveyed by the structural formula (the so-called gross formula is used to convey the composition). Molecules of proteins and some artificially synthesized compounds can contain hundreds of thousands of atoms. Polymer macromolecules are considered separately.

Molecules are the object of study of the theory of the structure of molecules, quantum chemistry, the apparatus of which actively uses the achievements of quantum physics, including its relativistic sections. Also currently developing is such an area of chemistry as molecular design. To determine the structure of the molecules of a particular substance, modern science has a colossal set of tools: electron spectroscopy, vibrational spectroscopy, nuclear magnetic resonance and electron paramagnetic resonance and many others, but the only direct methods at present are diffraction methods, such as X-ray diffraction and neutron diffraction.

Interaction of atoms during the formation of a molecule

The nature of chemical bonds in a molecule remained a mystery until the creation of quantum mechanics - classical physics could not explain the saturation and direction of valence bonds. The foundations of the theory of chemical bonds were laid in 1927 by Heitler and London using the example of the simplest molecule H2. Later, the theory and calculation methods were significantly improved.

The chemical bonds in the molecules of the vast majority of organic compounds are covalent. Among inorganic compounds, there are ionic and donor-acceptor bonds, which are realized as a result of the sharing of a pair of electrons of an atom. The energy of formation of a molecule from atoms in many series of similar compounds is approximately additive. That is, we can assume that the energy of a molecule is the sum of the energies of its bonds, which have constant values in such series.

Additivity of molecular energy is not always satisfied. An example of a violation of additivity is flat molecules of organic compounds with so-called conjugated bonds, that is, with multiple bonds that alternate with single ones. Strong delocalization of the p-states of electrons leads to stabilization of the molecule. The equalization of electron density due to the collectivization of p-states of electrons across bonds is expressed in the shortening of double bonds and the lengthening of single bonds. In a regular hexagon of benzene intercarbon bonds, all bonds are identical and have a length intermediate between the lengths of a single and double bond. The conjugation of bonds is clearly manifested in molecular spectra. The modern quantum mechanical theory of chemical bonds takes into account the delocalization of not only the p-, but also the s-states of electrons, which is observed in any molecules.

In the vast majority of cases, the total spin of the valence electrons in a molecule is zero. Molecules containing unpaired electrons - free radicals (for example, atomic hydrogen H, methyl CH 3) are usually unstable, since when they interact with each other, a significant decrease in energy occurs due to the formation of covalent bonds.

Intermolecular interaction

Spectra and structure of molecules

Electrical, optical, magnetic and other properties of molecules are related to the wave functions and energies of various states of the molecules. Molecular spectra provide information about the states of molecules and the probability of transition between them.

The vibration frequencies in the spectra are determined by the masses of atoms, their location and the dynamics of interatomic interactions. The frequencies in the spectra depend on the moments of inertia of the molecules, the determination of which from spectroscopic data allows one to obtain accurate values of interatomic distances in the molecule. The total number of lines and bands in the vibrational spectrum of a molecule depends on its symmetry.

Electronic transitions in molecules characterize the structure of their electronic shells and the state of chemical bonds. The spectra of molecules that have a greater number of bonds are characterized by long-wave absorption bands falling in the visible region. Substances that are built from such molecules are characterized by color; These substances include all organic dyes.

Molecules in chemistry, physics and biology

The concept of a molecule is fundamental to chemistry, and science owes most of the information about the structure and functionality of molecules to chemical research. Chemistry determines the structure of molecules based on chemical reactions and, conversely, based on the structure of the molecule, determines what the course of reactions will be.

The structure and properties of a molecule determine the physical phenomena that are studied by molecular physics. In physics, the concept of a molecule is used to explain the properties of gases, liquids and solids. The mobility of molecules determines the ability of a substance to diffuse, its viscosity, thermal conductivity, etc. The first direct experimental evidence of the existence of molecules was obtained by the French physicist Jean Perrin in 1906 while studying Brownian motion.

Since all living organisms exist on the basis of finely balanced chemical and non-chemical interactions between molecules, the study of the structure and properties of molecules is of fundamental importance for biology and natural science in general.

The development of biology, chemistry and molecular physics led to the emergence of molecular biology, which studies the basic phenomena of life based on the structure and properties of biologically functional molecules.

May contain positively and negatively charged, i.e.; in this case are implemented. In addition to those indicated, there are also weaker interactions between. Repulsive forces act between valence-unbonded bonds.

The development of the doctrine of structure is inextricably linked with success, first of all. The theory of structure, created in the 60s. 19th century the works of A. M. Butlerov, F. A. Kekule, A. S. Cooper and others, made it possible to represent or by structural formulas expressing the sequence of valence in. With the same empirical formula, there can be different structures with different properties (phenomenon). These are, for example, C 5 H 5 OH and (CH 3) 2 O. These compounds differ:

In some cases, isomeric ones quickly transform into one another and a dynamic relationship is established between them (see). Subsequently, J. H. Van't Hoff and independently the French chemist A. J. Le Bel came to an understanding of the spatial arrangement in and to an explanation of the phenomenon. A. Werner (1893) extended the general ideas of the theory of structure to inorganic ones. By the beginning of the 20th century. had a detailed theory based on the study of only their chemical properties. It is remarkable that direct physical research methods, developed later, in the vast majority of cases completely confirmed those established by studying macroscopic quantities, and not individual ones.

Equilibrium internuclear distances r 0 and energies D (at 25° C) of some diatomic

	r 0, Ǻ		r 0 , Ǻ






				C-Br…………….
Cº C……………...		C-I………………
C-H……………..		C-S……………..
C-O……………..		O-H…………….
C=O……………...		N-H……………..
C-N……………..		S-H……………..







			In the vast majority of cases, the total valence in is equal to zero, i.e., they are pairwise saturated. , containing unpaired - (for example, atomic H · · , methyl CH · · 3) are usually unstable, because when they combine with each other, a significant decrease in energy occurs due to the formation of valence bonds. The most effective method for studying the structure is (). Electrical and optical properties. Behavior in an electric field is determined by the basic electrical characteristics - constant and . means a discrepancy between the centers of gravity of positive and negative charges, i.e. electrical asymmetry. Accordingly, those with a center, for example H 2, are deprived of a constant; on the contrary, in HCl they are shifted towards Cl and are equal to 1.03 D (1.03 × 10 -18 CGS units). characterized by the ability of any electron shell to shift under the influence of an electric field, as a result of which an induced one is created. The values of and are found experimentally using dielectric constant measurements. In the case of additivity of properties, it can be represented by the sum of connections (taking into account their direction), the same applies to. Elements with or odd numbers have nuclear spin paramagnetism. Such nuclei are characterized

A substance can be in three states of aggregation: solid, liquid and gaseous. Molecular physics is a branch of physics that studies the physical properties of bodies in various states of aggregation based on their molecular structure.

Thermal movement- random (chaotic) movement of atoms or molecules of a substance.

FUNDAMENTALS OF MOLECULAR KINETIC THEORY

Molecular kinetic theory is a theory that explains thermal phenomena in macroscopic bodies and the properties of these bodies based on their molecular structure.

Basic principles of molecular kinetic theory:

matter consists of particles - molecules and atoms, separated by spaces,
these particles move chaotically,
particles interact with each other.

MASS AND SIZES OF MOLECULES

The masses of molecules and atoms are very small. For example, the mass of one hydrogen molecule is approximately 3.34 * 10 -27 kg, oxygen - 5.32 * 10 -26 kg. Mass of one carbon atom m 0C =1.995*10 -26 kg

Relative molecular (or atomic) mass of a substance Mr is the ratio of the mass of a molecule (or atom) of a given substance to 1/12 of the mass of a carbon atom: (atomic mass unit).

The amount of a substance is the ratio of the number of molecules N in a given body to the number of atoms in 0.012 kg of carbon N A:

Mole- the amount of a substance containing as many molecules as there are atoms in 0.012 kg of carbon.

The number of molecules or atoms in 1 mole of a substance is called Avogadro's constant:

Molar mass- mass of 1 mole of substance:

The molar and relative molecular mass of a substance are related by the relationship: M = M r * 10 -3 kg/mol.

SPEED OF MOLECULES

Despite the random nature of the movement of molecules, their distribution of velocities has the character of a certain pattern, which called Maxwell's distribution.

The graph characterizing this distribution is called the Maxwell distribution curve. It shows that in a system of molecules at a given temperature there are very fast and very slow, but most of the molecules move at a certain speed, which is called the most probable. As the temperature increases, this most likely rate increases.

IDEAL GAS IN MOLECULAR KINETIC THEORY

Ideal gas is a simplified gas model in which:

gas molecules are considered material points,
molecules do not interact with each other
molecules colliding with obstacles experience elastic interactions.

In other words, the movement of individual molecules of an ideal gas obeys the laws of mechanics. Real gases behave like ideal gases at sufficiently high rarefaction, when the distances between molecules are many times larger than their sizes.

The basic equation of molecular kinetic theory can be written as

Speed called the mean square speed.

TEMPERATURE

Any macroscopic body or group of macroscopic bodies is called thermodynamic system.

Thermal or thermodynamic equilibrium- a state of a thermodynamic system in which all its macroscopic parameters remain unchanged: volume, pressure do not change, heat exchange does not occur, there are no transitions from one state of aggregation to another, etc. Under constant external conditions, any thermodynamic system spontaneously goes into a state of thermal equilibrium.

Temperature- a physical quantity characterizing the state of thermal equilibrium of a system of bodies: all bodies of the system that are in thermal equilibrium with each other have the same temperature.

Absolute zero temperature- the limiting temperature at which the pressure of an ideal gas at constant volume must be equal to zero or the volume of an ideal gas at constant pressure must be equal to zero.

Thermometer- a device for measuring temperature. Typically, thermometers are calibrated on the Celsius scale: the crystallization temperature of water (ice melting) corresponds to 0°C, its boiling point - 100°C.

Kelvin introduced the absolute temperature scale, according to which zero temperature corresponds to absolute zero, the unit of temperature on the Kelvin scale is equal to the degree Celsius: [T] = 1 K(Kelvin).

Relationship between temperature in energy units and temperature in Kelvin:

Where k= 1.38*10 -23 J/K - Boltzmann's constant.

Relationship between the absolute scale and the Celsius scale:

T = t + 273

Where t- temperature in degrees Celsius.

The average kinetic energy of the chaotic movement of gas molecules is proportional to the absolute temperature:

Mean square speed of molecules

Taking into account equality (1), the basic equation of molecular kinetic theory can be written as follows:

EQUATION OF STATE OF AN IDEAL GAS

Let a gas of mass m occupy a volume V at a temperature T and pressure R, A M- molar mass of the gas. By definition, the concentration of gas molecules is: n = N/V, Where N-number of molecules.

Let's substitute this expression into the basic equation of molecular kinetic theory:

Size R is called the universal gas constant, and the equation written in the form

called the ideal gas equation of state or the Mendeleev-Clapeyron equation. Normal conditions - gas pressure is equal to atmospheric ( R= 101.325 kPa) at ice melting temperature ( T = 273,15TO).

1. Isothermal process

The process of changing the state of a thermodynamic system at a constant temperature is called isothermal.

If T =const, then

Boyle-Mariotte Law

For a given mass of gas, the product of the gas pressure and its volume is constant if the gas temperature does not change: p 1 V 1 =p 2 V 2 at T = const

A graph of a process occurring at a constant temperature is called an isotherm.

2. Isobaric process

The process of changing the state of a thermodynamic system at constant pressure is called isobaric.

Gay-Lussac's Law

The volume of a given mass of gas at constant pressure is directly proportional to the absolute temperature:

If a gas, having a volume V 0, is under normal conditions: and then, at constant pressure, goes into a state with temperature T and volume V, then we can write

Having designated

we get V=V 0 T

The coefficient is called the temperature coefficient of volumetric expansion of gases. The graph of a process occurring at constant pressure is called isobar.

3.Isochoric process

The process of changing the state of a thermodynamic system at a constant volume is called isochoric. If V = const, That

Charles's Law

The pressure of a given mass of gas at constant volume is directly proportional to the absolute temperature:

If a gas, having a volume V 0, is under normal conditions:

and then, maintaining volume, goes into a state with temperature T and pressure R, then we can write

The graph of a process occurring at constant volume is called isochore.

Example. What is the pressure of compressed air in a 20 liter cylinder at 12°C if the mass of this air is 2 kg?

From the equation of state of an ideal gas

Let's determine the pressure value.

A molecule (French molecule, from Latin moles - mass) is the smallest particle of a substance capable of independent existence, possessing its chemical properties.

The study of the structure and properties of molecules has acquired exceptional interest for understanding the submicroscopic structure of cells and tissues, as well as the mechanism of biological processes at the molecular level. Great advances in the study of the structure of molecules and, in particular, the molecules of biopolymers such as proteins and nucleic acids have shown that the most important functions of these substances in organisms are carried out at the level of individual molecules and therefore should be studied as molecular phenomena. It has been established, for example, that such functions of proteins as enzymatic, structural, contractile, immune, transport (reversible binding and transfer of vital substances) occur at the molecular level and are directly determined by the structure and properties of the molecules of these substances. Heredity and variability of organisms are associated with the special structure and properties of nucleic acid molecules, which contain all the genetic information necessary for the synthesis of proteins in the body. Small deviations in the structure or composition of the molecules of a number of biologically important substances or changes in the molecular mechanism of certain metabolic processes are the cause of a number of diseases (for example, sickle cell anemia, hereditary galactosemia, diabetes mellitus, etc.), called molecular diseases.

The molecule of each substance consists of a certain number of atoms (see) of one chemical element (simple substance) or different elements (complex substance), united through chemical (valence) bonds. The composition of a molecule is expressed by a chemical formula in which the signs of the elements indicate the type of atoms that form the molecule, and the numbers at the bottom right indicate how many atoms of each element are included in the molecule. Thus, from the chemical formula of glucose C 6 H 12 O 6 it follows that a glucose molecule consists of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. The molecules of inert gases and vapors of some metals are monatomic. These are the simplest molecules. The most complex molecules are proteins (see), nucleic acids (see) and other biopolymers, consisting of many thousands of atoms.

To find the chemical formula of a molecule, it is necessary to determine the approximate molecular weight (cm) of the substance under study and the simplest (empirical) formula of its molecule. The latter is derived from the percentage composition of a given substance and the atomic weights (see) of the chemical elements that make up this substance. For example, chemical analysis has established that benzene consists of 92.26% carbon and 7.74% hydrogen. It follows that the ratio of the number of carbon atoms to the number of hydrogen atoms in a benzene molecule is equal to:

where 12.011 and 1.008 are the atomic weights of carbon and hydrogen, respectively. Therefore, the simplest formula for benzene should be CH. By comparing the simplest formula of benzene with its approximate molecular weight (78.1), found experimentally, its actual, or true, formula C 6 H 6 is determined.

The sizes of molecules are expressed in A. For example, the diameter of a water molecule, assuming that it has a spherical shape, is 3.8 A. Molecules of high-molecular substances are much larger, for example, the linear dimensions of the large and small axes of the rod-shaped bovine fibrinogen molecules are 700 and 40 A, and tobacco mosaic virus - 2800 and 152 A, respectively. A measure of the relative mass of a molecule is molecular weight (cm), the value of which ranges from several units to millions.

The sequence in which atoms are connected in a molecule (the chemical structure of molecules according to A.M. Butlerov) is depicted by so-called structural formulas. For example, the chemical structure of acetic acid C 2 H 4 O 2 is represented by the following structural formula:

where each line denotes a unit of valency (cm), the number of lines approaching an atom is equal to its valence in a given compound.

The chemical structure of a molecule, found on the basis of the determination of molecular weight, chemical composition and the study of the chemical properties of the substance under study and finally confirmed by its synthesis from substances whose chemical structure is known, is an important factor determining the properties of the substance, in particular its pharmacological action, toxicity and biological functions . The difference in the properties of isomers (see Isomerism) is an example of the dependence of the properties of substances on the chemical structure of their molecules. The atomic composition of the molecules of isomers is the same, for example, dimethyl ether and ethyl alcohol, being isomers, have the same chemical formulas C 2 H 6 O, but their structural formulas are different:

which explains their different properties.

The ability of an atom to form a certain number of chemical bonds with other atoms in molecules is called the valence of a given atom. When a chemical (valence) bond is formed, a rearrangement of the outer (valence) electrons of interacting atoms occurs, as a result of which the outer electron shells of the atoms in the molecule acquire a stable structure characteristic of atoms of inert gases (see) and usually consisting of eight electrons (electron octet). Depending on the method of rearrangement of valence electrons, several main types of chemical bonds are distinguished.

Ionic (electrovalent) bonds occur between atoms of elements that differ greatly in chemical properties, for example, between alkali metal atoms and halogen atoms. In this case, the metal atom gives up an electron to the halogen atom (Fig. 1).

Rice. 1. Formation of a sodium chloride molecule.

An atom that donates an electron becomes a positively charged ion. An atom that accepts an electron becomes a negatively charged ion. Oppositely charged ions arising in this way attract each other, forming a molecule. Molecules and compounds with ionic bonds (for example, salts and oxides of metals of the first and second groups of the periodic table of elements) are called heteropolar. An ionic bond is characterized by high strength (bond energy), i.e., the work required to break the molecule into individual ions.

A covalent (atomic) bond occurs when atoms with identical or similar properties interact. In this case, each of the connecting atoms gives up one or several valence electrons to form a pair (or several pairs of electrons), which becomes common to both atoms. A generalized pair of electrons, enveloping the nuclei of connecting atoms in their movement, holds them one near the other. Molecules with covalent bonds include molecules of simple gases, oxides and hydrogen compounds, non-metals and many organic compounds:

The dots indicate electrons located on the outer electron shells of atoms, and the chemical symbols indicate the nuclei of atoms with all electron shells except the outer ones. The pair of electrons bonding atoms corresponds to the valence feature in common structural formulas.

Molecules in which the electrical centers of gravity of negative (electrons) and positive (atomic nuclei) charges coincide are called homeopolar. These include, for example, molecules of simple gases and hydrocarbons. If the electrical centers of gravity of negative and positive charges in molecules do not coincide, the molecules are called polar (for example, molecules of water, ammonia, hydrogen halides, alcohols, ketones, aldehydes, ethers). A polar molecule behaves like a dipole, that is, a system of two electric charges e+ and e-, equal in magnitude but opposite in sign, located at a distance h from one another (Fig. 2).

Rice. 2. Dipole diagram.

The product e·h=μ is called the dipole moment of the molecule. The latter is a measure of the polarity of the molecule. Substances consisting of polar molecules have higher boiling points, specific heats, heat of vaporization, and surface tension than substances consisting of homeopolar molecules. The interaction between polar molecules is one of the reasons for the association of molecules in liquids, and the interaction of polar solvent molecules with polar molecules or solute ions is the solvation of the latter. The rate of diffusion of polar molecules through the cell membrane is less than that for homeopolar molecules.

A coordination (semipolar, donor-acceptor) bond is a type of covalent bond that occurs between atoms that are part of different molecules, one of which has a lone pair of electrons, and the other lacks two electrons to form a stable outer electron shell. This type of connection is typical for complex compounds. For example, the combination of an ammonia molecule NH 3 with a molecule of boron fluoride BF3 into a complex molecule of boron fluoride ammonia is carried out by a lone pair of nitrogen electrons

The nitrogen atom serves as a donor, the boron atom as an electron pair acceptor.

A hydrogen bond occurs between a hydrogen atom covalently bonded to an F, O, or N atom and F, O, or N atoms located in other molecules. The strength of the hydrogen bond is low (5-10 kcal/mol), but is sufficient for the formation of molecular associations in liquids and solutions. In water, for example, such associations have the following structure (hydrogen bonds are indicated by dotted lines):

Hydrogen bonds occur not only between molecules, but also between atoms within the same molecule; These are so-called intramolecular hydrogen bonds (hydrogen bridges). An example of such a bond is the hydrogen bond between the hydrogen atom and the oxygen atom in the o-methyl salicylate molecule:

Due to the presence of this bond, the properties of o-methyl salicylate differ sharply from the properties of the m- and n-isomers. The presence of hydrogen bridges in the molecules of nucleic acids, proteins and other polymers largely determines the lability of these molecules. Hydrogen bonds play a significant role in the submicroscopic structure of protoplasm.

With the help of X-ray, electron, and neutron diffraction, molecular spectroscopy and nuclear magnetic resonance, it was possible to establish the spatial arrangement of individual atoms in a molecule, that is, the geometric configuration of the molecules of a number of substances, including molecules of biologically important substances.

The definition of the spatial configuration of molecules consists of the definition of the so-called skeleton of a molecule, i.e., the spatial arrangement of the nuclei of the atoms that form it, and the distribution of electrons within a given molecule.

The core of the molecule is found based on data on the bond length and bond angles determined using the above methods. Bond length is the distance between the centers of two atoms in a molecule connected to each other by a covalent bond. The smaller angle formed by straight lines connecting the centers of two atoms A 1 and A 2 with the center of the third atom A 3 in a given molecule is called the bond angle. The core of the molecule is not absolutely rigid. For example, in molecules of organic compounds, carbon atoms can rotate around single (simple) bonds, while the relative position of the nuclei changes, but the sequence of connections of atoms in the molecule, the length of the bonds and bond angles remain constant. These different forms of molecules resulting from the rotation of a carbon atom around a single bond are called conformations. Different conformations of the same molecule easily and reversibly transform into each other, which explains the absence of rotational isomers and the transition of molecules into the form most suitable for the occurrence of a particular reaction.

The distribution of electrons in molecules is found mainly using theoretical calculations, which are based on two basic principles of quantum chemistry. The first of them states that electrons in atoms and molecules can only be located at discrete and completely defined energy levels. According to the second principle, electrons in atoms and molecules cannot be considered as point particles, the position and speed of which in a molecule (or atom) can be accurately determined for each moment of time. In reality, as quantum mechanics teaches, you can only determine the probability of an electron being in some regions of space at a given moment in time. Therefore, one can imagine that the charge of an electron is, as it were, “spread out” in a certain region of space in the form of an electron cloud, the distribution of which in space is determined by the corresponding mathematical function (called the wave function of the electron or its molecular orbital (or atomic orbital, if its distribution is determined in an atom) .

It was shown that not all electrons in a molecule are equally important for its chemical properties. For example, in a molecule with a large number of double bonds, which includes the vast majority of compounds that play a dominant role in vital processes, electrons can be divided into two types. The first type includes σ-electrons involved in the formation of single bonds, the second type includes p-electrons involved in the formation of double bonds. The former form a rigid skeleton of the molecule and are localized in pairs between neighboring atoms. The latter form a much more diffuse cloud, covering the entire periphery of the molecule. In such molecules, all their basic properties are due to p-electrons, which are more labile compared to σ-electrons and therefore can more easily participate in various types of processes.

Every day we use some objects: we take them in our hands, perform any manipulations on them - turn them over, examine them, and ultimately break them. Have you ever thought about what these objects are made of? "What can we think about here? Made of metal/wood/plastic/fabric!" - many of us will answer in bewilderment. This is partly the correct answer. What are these materials made of - metal, wood, plastic, fabric and many other substances? Today we will discuss this issue.

Molecule and atom: definition

For a knowledgeable person, the answer is simple and banal: from atoms and molecules. But some people get puzzled and start asking questions: “What are an atom and a molecule? What do they look like?” etc. Let's answer these questions in order. Well, first of all, what are an atom and a molecule? Let us tell you right away that these definitions are not the same thing. And even more than that, these are completely different terms. So, an atom is the smallest part of a chemical element, which is the bearer of its properties, a particle of substance of scanty mass and size. A molecule is an electrically neutral particle that is formed by several connected atoms.

What is an atom: structure

An atom consists of an electron shell and (photo). In turn, the core consists of protons and neutrons, and the shell consists of electrons. In an atom, protons are positively charged, electrons are negatively charged, and neutrons are not charged at all. If the number of protons corresponds, then the atom is electrically neutral, i.e. If we touch a substance formed from molecules with such atoms, we will not feel the slightest electrical impulse. And even super-powerful computers will not catch it due to the absence of the latter. But it happens that there are more protons than electrons, and vice versa. Then it would be more correct to call such atoms ions. If there are more protons in it, then it is electrically positive, but if electrons predominate, it is electrically negative. Each specific atom has a strict number of protons, neutrons and electrons. And it can be calculated. A template for solving problems of finding the number of these particles looks like this:

Chem. element - R (insert element name)
Protons (p) - ?
Electrons (e) - ?
Neutrons (n) - ?
Solution:
p = serial number of chemical. element R in the periodic table named after D.I. Mendeleev
e = p
n = A r (R) - No. R

What is a molecule: structure

A molecule is the smallest particle of a chemical substance, that is, it is already directly included in its composition. A molecule of a certain substance consists of several identical or different atoms. The structural features of molecules depend on the physical properties of the substance in which they are present. Molecules are made up of electrons and atoms. The location of the latter can be found using the structural formula. allows you to determine the progress of a chemical reaction. They are usually neutral (have no electrical charge) and have no unpaired electrons (all valences are saturated). However, they can also be charged, in which case their correct name is ions. Molecules can also have unpaired electrons and unsaturated valencies - in this case they are called radicals.

Conclusion

Now you know what an atom is and all substances, without exception, are composed of molecules, and the latter, in turn, are built of atoms. The physical properties of a substance determine the arrangement and connection of atoms and molecules in it.