Molecule

MOLECULE (from Latin molecula - small particle), the smallest particle of a substance that retains all its chemical properties. The molecule is the limit of subdivision of the substance as such: when attempts are made to further reduce it, the molecule must divide into its constituent parts - atoms (see Atom), which have different chemical properties. Therefore, the chemical formula of a molecule is at the same time the chemical formula of any amount of this substance that consists of a collection of a huge number of identical molecules. During the last decades, as a result of the significant expansion of knowledge about the finest structure of matter, the concept of the molecule has somewhat changed and been clarified: only in gases do molecules exist individually and are at enormous (compared to their size) distances from each other; in liquids they combine ('associate') with each other into larger or smaller and stable aggregates; in solid (crystalline) bodies they are so close that the boundaries between individual molecules disappear and in many cases a regular alternation of atoms (or ions) results, which cannot be attributed to specific molecules. However, for gases and for (diluted) solutions the concept of the molecule has fully retained its meaning. In the following, when speaking of molecules, it must be kept in mind, mainly, these two forms of matter. For them 689 the law of Avogadro (Avogadro) is valid, according to which in equal volumes of different substances in gaseous or vapor state (at the same pressure and temperature) there is contained an equal number of molecules. At atmospheric pressure (760 mm) and 0° in a volume equal to 22.4 liters, there is contained such an amount of any substance which is equal to its molecular weight: 2.0156 g H₂, 32.000 g O₂ and so on. This volume is called the gram-molecular. The number of molecules in this volume [or in other (liquid and solid) states of the substance, in a weight amount equal to (in grams) its molecular weight (this amount is abbreviatedly denoted by the word 'mole')] according to Avogadro's law is the same for all substances, i.e. it represents a universal constant. It is usually called Avogadro's number and denoted by the letter N. Sometimes it is called Loschmidt's number (Loschmidt) and denoted by the letter N_L. At present there exists a whole series of independent methods for determining N, giving very close results. The most accurate of these methods give the value N = 6.06·10²³. Dividing this enormous number into the volume of a mole of substance in the condensed state (liquid or solid), one can find the upper limit of the dimensions of the space occupied by one molecule. Other methods also allow determining, although with less accuracy than N, the dimensions of molecules or rather the dimensions of their spheres of action. They are different for different substances, increasing with the complexity of the chemical composition, but on average their order of magnitude is 10^-7 cm. It is clear that with such dimensions molecules are invisible even in the best microscope or ultramicroscope, and all our knowledge about molecules can be obtained only by indirect methods. The mass of an individual molecule is very small. It can be found with great accuracy for each substance by dividing the mole by Avogadro's number. Thus, for hydrogen it is 0.324·10^-23 g, for helium 0.639·10^-23 g and so on. As was indicated above, molecules consist of even smaller structural units - atoms. Therefore, the question arises about the structure of molecules, about how atoms are arranged in molecules, what forces bind them together, what energy must be expended to separate them, and so on. This question has special significance for chemistry, as it reduces to the question of the structure and formation of chemical compounds. The problem of the structure of complex molecules constitutes the most important part of modern organic chemistry and has been developed with great completeness during the 19th century. As for simpler molecules, mainly inorganic compounds, their structure and formation have only been clarified recently in connection with the theories of atomic structure, which have constituted a new chapter in physical chemistry, called in recent years chemical physics. It turned out that the bond between atoms in a molecule can be different. In this sense, all molecules can be divided into two large groups: heteropolar and homopolar molecules. Between the extreme representatives of both groups there exists a large number of gradual transitions, and it is not always possible to determine to which type a molecule of a given compound belongs. However, typical heteropolar molecules, such as NaCl, sharply differ from typical homopolar molecules, such as H₂. The former consist of atoms or groups of opposite chemical nature, moreover charged with electricity of opposite sign, i.e. of ions, for example Na⁺ and Cl^-, Ca⁺⁺ and CO₃^-- and so on. The latter, on the contrary, consist of identical or similar in nature atoms or groups, devoid of electric charge: H₂, N₂, O₂, Cl₂, C₂H₆ and so on. In molecules of the first kind, the bond between the components is apparently carried out by attractive forces of an electrostatic nature, i.e. by the attraction of their opposite charges; the nature of the bonding forces in homopolar molecules is less clear, although it can probably also be reduced to electrical forces. -The picture of the formation of heteropolar molecules from the point of view of the electronic theory of atomic structure is given by Kossel (Kossel): in the formation of a molecule, for example NaCl, the valence electron of Na is pulled toward Cl, giving it a negative charge and leaving Na positively charged. The resulting ions Na⁺ and Cl^- mutually attract each other electrostatically, and in each of them in the outer shell (see Atom) there are eight electrons, as in the atoms of the nearest noble gases. Following this path, Kossel explained the formation not only of simple heteropolar compounds of the NaCl type, but also of much more complex complex compounds (see), and with the help of energy considerations it was possible to calculate the coordination number (see Coordination theory).-In typical homopolar molecules, a complete pulling of electrons from some components of the molecule to others probably does not occur. It is believed that here the electrons that carry out the chemical bond do not leave the atom from which they originated, but become common for the components being connected, surrounding their nuclei with their orbits and spending part of their time in the sphere of action of each of the components of the molecule. Finally, in molecules of an intermediate type, called semipolar (Lux), the valence electrons are common to the components of the molecule, but to a greater or lesser degree shifted toward one of them. In the question of the structure of molecules, enormous successes have been achieved in recent years by studying the optical and electrical properties of molecules. Molecular spectra are significantly more complex than the linear spectra of atoms: they consist of bands which, with strong instruments, are resolved into a series of the finest, very close to each other lines. The origin of these bands is explained as follows. Whereas in atoms spectral lines appear only as a result of jumps of electrons from outer orbits to inner ones, in molecules, in addition to these abrupt changes in state and energy reserve, vibrations of the components of the molecule relative to each other and rotation of the entire molecule as a whole are possible. All these three types of motion 592: according to the principles of quantum theory (see) can be accompanied not by continuous, but only by completely definite discrete changes in the energy reserve of molecules. Each of them, under certain conditions, can be accompanied by the emission of a separate spectral line. Changes in rotational motion are associated with very small changes in energy, small quanta, and the corresponding lines lie in the far infrared (see Infrared rays) region. Changes in vibrational motion lead to significantly (100-200 times) larger changes in energy, and the corresponding lines are in the short infrared part of the spectrum. Jumps of electrons in molecules cause the appearance of lines in the visible and ultraviolet part of the spectrum. These three series of motions are interconnected, and for example, the vibrational motion of the components of a diatomic molecule along the line connecting their centers always simultaneously causes a change in the rotational motion. This is why the appearance of a line of the vibrational spectrum always causes the appearance of a series of lines of the rotational spectrum, located near the first at very small distances and in weak optical instruments merging with it into one spectral band of the rotation-vibration spectrum. A jump of an electron excites, in addition to the main line, a whole series of lines of the vibrational and rotational spectrum, merging with the first into a wide band in the visible or ultraviolet part of the emission or absorption spectrum (absorption, see).-The detailed study of molecular spectra has led to the accurate determination of the length (distance between atoms) of certain diatomic molecules, their moment of inertia and important energy quantities, such as for example the heat of dissociation of many molecules into atoms. Important results have also been obtained from the study of the electrical properties of molecules. Polar molecules, consisting of electrically heterogeneous components, are completely neutral only at large distances compared to their length.

At small distances from their ends (poles), the asymmetric arrangement of electrical charges within them becomes apparent: on one pole positive charges predominate, on the other negative charges. The distance between the centers of gravity (poles) of these charges, multiplied by the magnitude of these charges, gives the dipole moment of such an M., called a dipole. The asymmetric arrangement of opposite charges, which gives the M. a dipole character, is inherent in some M. constantly (permanent dipole moment), while in others it can be caused by external influence, e.g., by placing the M. in an electric field (induced or induced dipole moment). The study of permanent dipole moments of M., carried out mainly by Debye (P. Debye) and his school, led to the determination of the length of dipole M. and the arrangement of constituent atoms in space. In addition, it was possible to explain the phenomenon of association in liquids and in vapors (see Liquids), deviations in the behavior of binary mixtures from simple laws of mixing, the nature of the so-called Van der Waals forces of attraction between M., etc. At the same time, it was possible to determine which parts of M. (groups or radicals of complex compounds) determine its polar properties, to assign a specific dipole moment to each such polar group, and by additive summation of these (taking into account the arrangement of individual groups in space) to calculate the dipole moments of complex M. in close agreement with experimental data. An induced or induced dipole moment can appear in an M. that does not have a permanent dipole moment under the influence of light falling on it, which is a rapidly changing electromagnetic field. The appearance of this moment is reflected in the refraction of light when it passes through such polarizable M. The study of molecular refraction ^^j - (where n is the refractive index, M is the molecular weight, q is the density) of a number of compounds led to the finding of additively combining atomic and ionic refractions of individual components of M. and to elucidating the structure of some M. In its application to polar M. of salt-like compounds, the study of refractions led to the finding of deformation of electron shells of ions in chemical compounds, which allowed Fajans (K. Fajans) to explain a number of features of various salts (vapor pressure, boiling and melting points, solubility, color, and many others). In recent years, a new physical method for studying M. has been found, which has already given very interesting results. This method, almost simultaneously found by Raman in India and Mandel'shtam and Landsberg in Moscow, consists in obtaining spectra of scattering of the substances under study, in which, along with the usual lines of incident light, a series of satellites, i.e., lines of longer and shorter wavelength, appears. The method of X-ray analysis also proved very useful for studying the structure of M., allowing to determine the arrangement in space and absolute distances between atoms that make up M. of complex compounds, such as naphthalene, hexamethylenetetramine, and others. Dipole moments of individual groups in complex M. Bond or radical: C-H H-O C=O C-O...C-Cl C-OH C-C C-NO3 \OH Dipole MOM. D 10* 0,4 1,6 2,3 0,7 1,5 1,6 1,0 3,7 Dipole moments of complex M: Para

TX„1ПЯ_ O-di- Composition:

Toluol chlor

z.zrJi't^ chlor- Phenol Acetone phenol

kрезол benzol Dip. mom. calc. D 10": 0,4 2,3 1,5 2,6 1,6 2,7 Dip. MOM. EXP. D 10": 0,45 2,4 1,5 2,2 1,63 2,70