Structure and Fundamental Properties of Solid and Liquid Bodies

(Langmuir. The Constitution and Fundamental Properties of Solids and Liquids. P. I. The Chemical News. 1917, vol. 116, 3009—3013)1.

Proceeding from the work of W. H. and W. L. Bragg on determining the structure of crystals by means of X-rays, the author comes to the conclusion that we must discard the usual conceptions of molecules and of the division of the forces acting between the particles of solid and liquid bodies into “physical” and “chemical” ones. From his point of view, both solid and liquid bodies consist of atoms held together by “chemical forces” (these forces may be of electromagnetic origin). Thus the conception of molecules loses its meaning, except in the case of gases. “We may regard a solid or a liquid body as one large molecule.” Such phenomena as melting, evaporation, adsorption, surface tension—all these are chemical phenomena.

According to the data of the Braggs, each atom of $Na$ in a rock-salt crystal is surrounded by six equidistant atoms of $Cl$, arranged around it as about a center. In exactly the same way, each atom of $Cl$ is surrounded by 6 equidistant atoms of $Na$. Thus there is no need to speak of a molecule of sodium chloride. Here the forces act directly between the atoms $Na$ and $Cl$. The monovalent character of $Na$ is lost in this case. The atom $Na$ is held by chemical forces to 6 atoms of $Cl$. If we adhere to the usual conceptions of valence, we must say that the valence of $Na$ is distributed among six atoms of $Cl$.

During evaporation, at high temperature, atoms leave the surface of the crystal in pairs, forming the molecule $NaCl$, so that molecules are obtained during evaporation, although, as such, they did not exist within the crystal. Hence it follows that evaporation must be regarded as a chemical phenomenon.

Langmuir examines a series of crystals ($ZnS$, $CaF_2$, $CaCO_3$, etc.), where he becomes convinced that each atom of one or another element is combined with a number of atoms many times greater than would follow from its normal valence.

This fact, in essence, is not in contradiction with modern theories of the chemical structure of matter. Basing himself on Werner’s theory (of primary and residual valences) and on the works of R. Abegg, Stark, J. J. Thomson, and Lewis, the author comes to the conclusion that, indeed, the structure of crystals can be explained from the standpoint of primary and residual valences. At the same time, unlike Werner, according to whom compounds of the first order are determined by primary valence ($H_2O$, $NaCl$, etc.), and only when simple compounds of the first order form compounds of higher orders does the action of residual valence appear ($BaCl_2, 2H_2O$), Langmuir comes to the conclusion that in some cases residual valence plays the predominant role, as, for example, in the case of crystals of sodium chloride or

fluorite \((CaF_2)\). Thus, for example, the atoms \(Na\) and \(Cl\) at high temperatures, forming \(NaCl\), are held by primary valence. But if the temperature is lowered, then residual valence finally replaces the primary one, for primary valence cannot be the cause of the formation of molecules close to the molecules of a solid body.

All compounds of the polar type are built of atoms connected by residual valences. If by the term “molecule” one denotes a group of atoms capable of passing from the gaseous state into the solid (or liquid) state and back again, then we must say that there are no molecules at all in the majority of inorganic compounds.

It is better to define a molecule as a group of atoms held together by atomic forces.

Introducing the concept of a “group molecule” as an aggregate of atoms found in such an interrelation that the atoms in the group can differ from the atoms outside the group in that the atoms are bound by primary valences, while the “group molecules” are bound by secondary (residual) ones, we arrive at the rule that nonpolar compounds consist of such “group molecules,” forming a large “crystalline molecule” that includes the whole solid mass.

As for the forces acting between atoms, it is necessary to assume the presence both of attractive forces and of repulsive forces. Langmuir infers the repulsive forces from the small value of the coefficient of expansion and from the values of the specific heats; from this he determines that atoms oscillate about certain equilibrium positions and finds the time of a separate oscillation of an atom to be equal to \(1.8 \cdot 10^{-13}\) sec., and the time necessary for an atom to come into thermal equilibrium with a neighboring atom to be equal to \(\frac{1}{1800}\) of the time of a separate oscillation. This indicates that the motions of an individual atom must be strongly damped.

Langmuir further comes to the conclusion that, because solids consist of atoms or “group molecules” connected by residual valences, we must expect that in the solid state any combination of atoms or “group molecules” is conceivable; and if we do not obtain any metallic compounds in the study of alloys (it must be said that Tammann’s investigations have shown that most such metallic compounds do not fit within the framework of the usual theory of valence), then this occurs exclusively because, at the moment of solidification, the atoms arrange themselves in the most stable manner; with the development of the technique of obtaining alloys, we may expect any combinations of atoms. From his point of view, minerals or complex salts of any composition are fully explicable.

From a comparison of the latent heat of vaporization, as the energy necessary for separating the atoms of a solid metal from every other atom, with the energy of compression necessary for decreasing the distance between atoms, Langmuir obtains the limits of action of atomic forces. It then turns out that the attractive force between atoms reaches a maximum when the atoms are separated by a distance 10–30% greater than the distance at which they are at the moment of equilibrium, and that “the attractive force becomes practically negligibly small when the distance between the centers of the atoms is twice as great as the distance between them at the moment of equilibrium.”

When a cleavage surface is formed in some crystal, the atoms break away when the distance increases to \(0.6 \cdot 10^{-8}\) cm. The atoms on the surface must be arranged so that the total energy in the field surrounding them is a minimum; thus, each time we split a crystal or a solid body into parts, we expend a certain amount of energy, since the field of energy of the atoms located on the surface is greater than the field of the same number of atoms inside.

In the surface layer the concentration of atoms is greater than inside the solid body, so that here there can be no continuous change of density from the solid body to space. Consequently, the transition from the solid body into the void appears as a discontinuous transition.

T. Molodoy.

1. Only the part of the article relating to the theory of solid bodies is set forth here. The remainder will be set forth upon receipt of subsequent issues. ↩