Systematics atomic volume

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

The properties of elements change in a systematic way through a period this is indicated in Figure 5-1, which shows the atomic volume (the atomic weight divided by the density that is, the volume in cm of 1 g-atom of the element) of the elements at 0° C and 1 4tm, as a function of the atomic number. 

The designers of the lecture room were, of course, proved correct. Only a few years later a systematic order was, indeed, recognized. An extraordinary double discovery was made in 1869. The German chemist Julius Lothar Meyer (1830-1895) noticed a remarkable periodicity during his rigorous scientific analysis of the atomic weights and volumes. He remained content with only a mild curiosity in this realization, as his interests lay primarily in physicochemical problems. He was objective and driven only by facts he was wary of hypotheses... 

Considering other families of similar compounds, the contributions given by Guillermet and Frisk (1992), Guillermet and Grimvall (1991) (cohesive and thermodynamic properties, atomic average volumes, etc. of nitrides, borides, etc. of transition metals) are other examples of systematic descriptions of selected groups of phases and of the use of special interpolation and extrapolation procedures to predict specific properties. 

Matter is composed of spherical-like atoms. No two atomic cores—the nuclei plus inner shell electrons—can occupy the same volume of space, and it is impossible for spheres to fill all space completely. Consequently, spherical atoms coalesce into a solid with void spaces called interstices. A mathematical construct known as a space lattice may be envisioned, which is comprised of equidistant lattice points representing the geometric centers of structural motifs. The lattice points are equidistant since a lattice possesses translational invariance. A motif may be a single atom, a collection of atoms, an entire molecule, some fraction of a molecule, or an assembly of molecules. The motif is also referred to as the basis or, sometimes, the asymmetric unit, since it has no symmetry of its own. For example, in rock salt a sodium and chloride ion pair constitutes the asymmetric unit. This ion pair is repeated systematically, using point symmetry and translational symmetry operations, to form the space lattice of the crystal. 

Thin layer chromatography and gas-liquid chromatography have been widely applied for the separation and for the identification of thiazoles in reaction mixtures. From systematic studies it appears that thiazole, alkyl- and aryl-thiazoles and benzothiazoles are best separated on stationary phase of low polarity in GLC and with eluents of low polarity in TLC. It has been possible to correlate, for these series of compounds, the RF of TLC or the specific volume of retention in GLC with the number of carbon atoms in the aliphatic side chain, and also with the rate constants of quaternization of the cyclic nitrogen atom. This last observation indicates a significant participation of the nitrogen atom in the chromatographic processes (67BSF846). 

Probably the most systematic predictive and scaling studies with emphasis on an as complete as possible implementation of atomic, molecular and surface effects for a divertor configuration have been carried out for the ASDEX Upgrade tokamak. Making connection here in particular to the chapter by U. Fantz in this volume (and references therein) we discuss here the initially unexpected role played by the molecular chemistry in dense cold divertor plasmas. 

---