Molecular Physics

molecular physics [mə′lek·yə·lər ′fiz·iks]

(physics)

The study of the behavior and structure of molecules, including the quantum-mechanical explanation of several kinds of chemical binding between atoms in a molecule, directed valence, the polarizability of molecules, the quantization of vibrational, rotational, and electronic motions of molecules, and the phenomena arising from intermolecular forces.

---

Molecular physics

The study of the physical properties of molecules. Molecules possess a far richer variety of physical and chemical properties than do isolated atoms. This is attributable primarily to the greater complexity of molecular structure, as compared to that of the constituent atoms. Molecules also possess additional energy modes because they can vibrate; that is, the constituent nuclei oscillate about their equilibrium positions and rotate when unhindered. These modes give rise to additional spectroscopic properties, as compared to those of an atom; molecular spectroscopy in the optical, infrared, and microwave regions is one of the physical chemist's most powerful means of identifying and understanding molecular structure. Molecular spectroscopy has also given rise to the rapidly growing field of molecular astronomy.

Molecular physics is primarily concerned with the study of properties of isolated molecules, as contrasted to the more general study of molecular reactions, which is the domain of physical chemistry. Such properties, in addition to the broad field of spectroscopy, include electron affinities (for the formation of molecular negative ions); polarizabilities (the “distortability” of the molecule along its various symmetry axes by external electric fields); magnetic and electric multipole moments, attributable to the distributions of electric charge; currents and spins of the molecule; and the (nonreactive) interactions of molecules with other molecules, atoms, and ions. See Infrared spectroscopy, Intermolecular forces, Microwave spectroscopy, Molecular beams, Molecular structure and spectra, Spectroscopy

---

Molecular Physics 

the branch of physics that deals with the physical properties of bodies in various states of aggregation on the basis of an examination of their microscopic (molecular) structure. Problems of molecular physics are solved using methods of physical statistics, thermodynamics, and physical kinetics; they are related to the study of the motion and interaction of the particles (atoms, molecules, and ions) that constitute physical bodies. The atomistic concepts of the structure of matter that were advanced by ancient philosophers were used successfully in chemistry in the early 19th century (J. Dalton, 1801), thus greatly fostering the development of molecular physics. The kinetic theory of gases was the first branch of molecular physics to be formulated. Classical statistical physics came into being as a result of the works of J. Maxwell (1858-60), L. Boltzmann (1868), and J. Gibbs (1871-1902), who developed the kinetic molecular theory of gases.

Quantitative concepts of the interaction of molecules (molecular forces) developed in the theory of capillary phenomena. The classic works of A. Clairaut (1743), P. Laplace (1806), T. Young (1805), S. Poisson, K. Gauss (1830-31), Gibbs (1874-78), and I. S. Gromeka (1879 and 1886) gave rise to the theory of surface phenomena. Intermolecular interactions were taken into account by J. van der Waals (1873) in his explanation of the physical properties of real gases and liquids.

In the early 1920’s molecular physics entered a new period of development, characterized by proofs of the construction of bodies from molecules in the works of J. Perrin and T. Svedberg (1906) and M. Smoluchowski and A. Einstein (1904-06) on the Brownian motion of microparticles and by research on the molecular structure of substances. The use of X-ray diffraction for these purposes in the works of M. von Laue (1912), W. H. Bragg and W. L. Bragg (1913), G. V. Vul’f (1913), A. F.Ioffe (1924), G. W. Steward (1927-31), J. Bernal (1933), and V. I. Danilov (1936), as well as the subsequent use of electron and neutron diffraction, made it possible to obtain precise data on the structure of crystalline solids and liquids. The study of molecular interactions on the basis of the concepts of quantum mechanics was developed in the works of M. Born (1937-39), P. Debye (1930’s), F. London (1927), and W. Heitler (1927). The theory of transition from one state of aggregation to another, which was outlined in the 19th century by van der Waals and W. Thomson (Lord Kelvin) and developed in the works of Gibbs, L. Landau (1937), M. Volmer (1930’s), and their followers, was transformed into an important independent branch of molecular physics—the modern theory of the formation of a new phase. The combination of statistical methods with modern concepts of the structure of substances in the works of Ia. I. Frenkel’ (1926 and subsequently), H. Eyring (1935-36), and J. Bernal led to the molecular physics of liquids and solids.

The range of problems encompassed by molecular physics is broad. It considers the structure of gases, liquids, and solids and the changes in them under the influence of external conditions (pressure, temperature, and electric and magnetic fields); transfer phenomena (diffusion, thermal conductivity, and internal friction); phase equilibrium and the processes of phase transitions (such as crystallization and melting or evaporation and condensation); the critical state of a substance; and surface phenomena at the interface of unlike phases.

The intensive development of molecular physics led to the separation of a number of major independent branches, such as statistical physics, physical kinetics, solid-state physics, physical chemistry, and molecular biology.

Modern science and technology are using an increasingly large number of new substances and materials. The particular structural features of these bodies that have been revealed have led to the development of various scientific approaches to their study. For example, such special fields of science as metal physics, polymer physics, plasma physics, crystal physics, the physical chemistry of disperse systems and surface phenomena, and the theory of heat and mass transfer have been developed on the basis of the general theoretical concepts of molecular physics. A new branch of science, physicochemical mechanics, which constitutes the theoretical foundation of modern materials science by pointing out ways of developing technically important materials with required physical properties, may also be included here. In spite of all the differences in the objects and methods of investigation, the fundamental idea behind molecular physics— the description of the macroscopic properties of matter on the basis of the peculiarities of the microscopic (molecular) picture of its structure—is retained.

REFERENCES

Kikoin, I. K., and A. K. Kikoin. Molekuliarnaia fizika. Moscow, 1963.
Hirschfelder, J., C. Curtis, and R. Bird. Molekuliarnaia teoriia gazov i zhidkostei. Moscow, 1961. (Translated from English.)
Frenkel’, la. I.Sobr. izbr. trudov. Vol. 3: Kineticheskaia teoriia zhidkostei. Moscow-Leningrad, 1959.
Frank-Kamenetskii, D. A. Diffuziia i teploperedacha v khimicheskoi kinetike, 2nd ed. Moscow, 1967.
Kittel, C. Vvedenie v fiziku tverdogo tela. Moscow, 1957: (Translated from English.)
Likhtman, V. I, E. D. Shchukin, and P. A. Rebinder. Fiziko-khimicheskaia mekhanika metallov. Moscow, 1962.

P. A. REBINDER, B. V. DERIAGIN, AND N. V. CHURAEV