Molecule (Franz. molecule, from lat. moles - mass) is the least capable of independent existence particle of a substance that has its chemical properties.
The teaching about the structure and properties of molecules acquired exceptional interest for knowing the submicroscopic structure of cells and tissues, and also the mechanism of biological processes at the molecular level. Major successes in the study of the structure of M and, in particular, M. such biopolymers, proteins, and nucleic acids, showed that the most important function of these substances in organisms are carried out at the level of individual molecules and therefore should test molecular phenomena. Established, for example, that the function of proteins, as enzymatic, structural, Oxytocics, immune, transport (reversible binding to and transfer of vital substances) are played at the molecular level and are directly determined by the structure and properties of M. these substances. Heredity and variability of organisms associated with a particular structure and properties Meters of nucleic acids, in which all the genetic information necessary for the synthesis of proteins in the body. Small deviations in the structure or composition of molecules of a number of biologically important substances or changes in the molecular mechanism of some metabolic processes are the cause of several diseases (such as sickle cell anemia, a hereditary galactosemia, diabetes mellitus and others), called molecular disease.
The molecule of each substance consists of a certain number of atoms (see) one chemical element (a simple matter) or the various elements (a complex matter), United by chemical (valence) links. The composition of the PM expressed by a chemical formula, which marks elements indicate the type of atoms forming Meters, and the numbers at the bottom right, show the number of atoms of each element is part of the M. Thus, a chemical formula of glucose Sunee follows that M glucose consists of 6 carbon atoms, 12 atoms of hydrogen and 6 atoms of oxygen. Molecules of inert gases and vapors of some metals odnotonnye. This is a very simple M. the Most difficult are M proteins (see), nucleic acids (see) and other biopolymers, which consists of many thousands of atoms.
To find the chemical formula M it is necessary to determine the approximate molecular weight (see) of the substance and a simple empirical formula of his Feet, the Last output of the percentage composition of the substance and atomic weights (see) of chemical elements that are included with this substance. So, for example, chemical analysis found that benzene consists of 92,26% of carbon and 7,74% of hydrogen. Hence, the ratio of the number of carbon atoms to the number of hydrogen atoms in the benzene molecule is:
where 12,011 and 1,008 - the atomic weight of carbon and hydrogen, respectively. Therefore, the simplest formula of benzene should be CH. Comparing the simplest formula of benzene with its approximate molecular weight (78,1)found empirically determine its actual or true formula With6N6.
Sizes M expressed in A. for example, the diameter of Meters of water, assuming that it has a spherical shape is 3.8 A. M. high-molecular substances much more, such as the linear sizes large and small axes M rod-shaped fibrinogen bull 700 and 40 a, And tobacco mosaic virus - 2800 and 152 respectively. Measure the relative weight of the molecule is a molecular weight (see), the value of which varies from several units up to million.
The sequence in which the atoms are connected in M (chemical structure M. A. M. Butlerov), represent the so-called structural formulas. For example, the chemical structure of acetic acid With2N4O2 are the following structural formula:
where each line represents a single valence (see), the number of lines that are appropriate to the atom, it is valency in this connection.
Chemical structure Meters, found on the basis of determining molecular weight, chemical composition and study of chemical properties of the substance and finally through its synthesis of substances, the chemical structure of which is known, is an important factor in determining the properties of substances, in particular its pharmacological effects, toxicity and biological functions. The difference in the properties of the isomers (see Isomerism) is an example of the dependence of properties of substances from chemical structure of their molecules. Atomic composition M. isomers of the same, for example, dimethyl ether and ethyl alcohol, being isomers, have the same chemical formula2N6O, however, the structural formulas are different:
and this explains their different properties.
The ability of atoms to form a certain number of chemical bonds with other atoms in the monastery is called the valence of the given atom. In the formation of the chemical (valence) connection is regrouping external (valence) electrons interacting atoms, which resulted in the outer electron shells of the atoms in the molecule acquires a stable structure, typical of the atoms of inert gases (see) and usually consisting of eight electrons (e-octet). Depending on the way of rearrangement of valence electrons there are several basic types of chemical bonds.
Ion (electrovalent) communication occurs between the atoms of the elements of widely differing chemical properties, for example between alkali metal atoms and atoms of halogen. This metal atom gives up an electron to the halogen atom (Fig. 1).

Fig. 1. The molecule of sodium chloride.

Atom radiating electron, it becomes positively charged ion. Atom-makers electron becomes negatively charged ion. Appearing by oppositely charged ions pull against each other, forming a molecule. M. and connection with ionic bonds (for example, salts and oxides of metals of groups I and II of the periodic system of elements) are called heteropolar. Ionic bond is characterized by high durability (binding energy), i.e. the work needed to break the molecules on a separate ions.
Covalent (nuclear) communication occurs in the interaction of the same or similar properties of atoms. Each of connecting atoms gives one or several valence electrons in the formation of pairs (or more pairs of electrons), which is becoming common for both atoms. Generalized pair of electrons, covering in its motion kernel connecting atoms, keeps them one near another. To M. with the covalent bond Meters are simple gases, oxides and hydrogen compounds't metals and many organic compounds:

Dots indicate the electrons located on the outer electron shells of atoms, chemical signs - the nuclei of atoms with all electronic shells, in addition to the exterior. A pair of electrons, connecting the atoms that corresponds stretching the boundaries in the ordinary structural formulas.
Molecules, in which the electric centers of gravity is negative (electrons) and positive (the nuclei of atoms) charges are the same, called Homo-polar. These include, for example, M. simple gases and hydrocarbons. If the electrical centers of gravity of negative and positive charges in the M not the same, M. called the Arctic (e.g., M of water, ammonia, hydrogen halides, alcohols, ketones, aldehydes, esters). Polar molecule behaves like a dipole, i.e. the system of two electric charges e+ and e - equal in magnitude, but opposite in sign, located on pekatoros distance h from one another (Fig. 2).

Fig. 2. The scheme of the dipole.

Work e·h=n are called the dipole moment of the molecule. The latter is a measure of polarity M. substances consisting of polar Meters, have a higher boiling point, heat capacity, heat of vaporization and surface tension than substance consisting of Homo-polar molecules. Interaction between polar M is one of the reasons of the Association of molecules in liquids and interaction of polar Meters with polar solvent M or ions of the solute - solvation last. The diffusion rate polar Meters through the membrane of cells less than that of Homo-polar M.
Coordination (semiplena, donor-acceptor) communication-a form covalent bonds occur between the atoms that are part of different molecules, one of which has a lone pair of electrons, and the other lacks two electrons for education for sustainable outer electron shell. This kind of communication is characteristic of complex compounds. So, for example, the connection M ammonia NH3 with M fluoride boron BF3 in complex M ameacada fluoride boron is unshared pair of electrons nitrogen

The nitrogen atom serves as a donor, boron acceptor electron pair.
Hydrogen bond between the hydrogen atom, covalently bound to the atom F, O or N and F atoms, or N in other molecules. The strength of the hydrogen bond is a small (5-10 kcal/mol), but enough to form associations M in liquids and solutions. In water, for example, such associations shall have the following structure (hydrogen bonds are broken):

Hydrogen bonds arise not only between the Feet, but also between the atoms within the same Meters; this is the so-called intramolecular hydrogen bonds (hydrogen bridges). An example of such communication can serve as a hydrogen bond between the hydrogen atom and the oxygen atom in M. o-methyl salicylate:

Due to the presence of this connection properties o-methyl salicylate sharply differ from the properties of m - and n-isomers. The presence of hydrogen bridges in molecules of nucleic acids, proteins and other polymers largely determines the lability of these Meters of Hydrogen bonds play a significant role in submicroscopic structure of protoplasm.
With the help of roentgen, electron-, neutron diffraction, molecular spectroscopy and nuclear magnetic resonance managed to establish the spatial location of the individual atoms in M, i.e., geometrical configuration M. of a number of substances, including M biologically important substances.
The definition of a spatial configuration of the molecules composed of a definition of the so-called skeleton M, i.e., the spatial location of its forming nuclei of atoms, and distribution of electrons within this M
The skeleton Meters are based on the bond length and magnitude of the valence angles determined using the above methods. The bond length is the distance between the centers of two atoms in the convent, linked together by covalent bond. Less largest angle formed by the lines connecting the centers of the two atoms A1 and A2 with the center of the third atom A3 in the molecule, called the valence angle. The skeleton M is not absolutely rigid. For example, in the convent of organic compounds carbon atoms can rotate about the ordinary (plain) links, this changes the relative positions of the nuclei, but remain constant sequence of atoms in Meters, length relationships and valence angles. These various forms M. arising from the rotation of the carbon atom around ordinary communication, called conformations. Different conformations of the same Meters and easily reversible pass each other, what explains the absence of isomers of rotation and transition M. in the form most appropriate for the occurrence of one or another reaction.
The distribution of electrons in molecules are mainly using theoretical calculations, which are based on two main principles of quantum chemistry. The first of them States that the electrons in atoms and M can be only at discrete and absolutely certain energy levels. According to the second principle of electrons in atoms and M not be considered as a point particle, position, and speed in M (or atom) can accurately determine for each point in time. In fact, according to quantum mechanics, can be determined only the probability of finding an electron in some areas of space at a given time. So you can imagine that the electron charge, as if spread out within a specific region of space in the form of the electron cloud, the distribution of which is in space is determined by a mathematical function (called the wave function of an electron or its molecular orbital (or atomic orbital if its distribution is determined in the atom).
Howled shown that not all electrons in M. equally essential for its chemical properties. For example, in a molecule with a large number of double bonds, which include the vast majority of compounds, which play a dominant role in the vital functions, the electrons can be divided into two types. The first is a s-electrons involved in the formation of the ordinary relations, to the second - p-electrons involved in the formation of double bonds. The first form a rigid skeleton, M. and localized pairs between neighboring atoms. The second form is much more vague cloud covering the entire periphery Meters In such M. all their basic properties are associated p-electrons, which is more flexible compared with a s-electrons and thus more easily able to participate in various processes.