Van der Waals radii of the elements

Intermolecular distances in these sulfur-selenium phases are comparable to those observed for monoclinic y-sulfur and monoclinic ot-selenium The shortest distances in both isomorphic series are much shorter than the sum of the van der Waals radii of the elements. In the isomorphic series of the sulfur-rich species the shortest distanee between the molecules gets shorter with increasing selenium content of the phase but remains effectively constant in the selenium-rich series. 

VSEPR concept, valence bond description and hybridization, molecular orbital description, bond energies, covalent and van der Waals radii of the elements, intermolecular forces... 

Van der Waals Constants for Gases, 6-36 Van der Waals radii of the elements, 9-49 Vanadium see also Elements... 

The PCM algorithm is as follows. First, the cavity siuface is determined from the van der Waals radii of the atoms. That fraction of each atom s van der Waals sphere which contributes to the cavity is then divided into a nmnber of small surface elements of calculable surface area. The simplest way to to this is to define a local polar coordinate frame at tlie centre of each atom s van der Waals sphere and to use fixed increments of AO and A(p to give rectangular surface elements (Figure 11.22). The surface can also be divided using tessellation methods [Paschual-Ahuir d al. 1987]. An initial value of the point charge for each surface element is then calculated from the electric field gradient due to the solute alone ... 

Single crystal X-ray analysis of the ethynyl-l,5-azastibocine 5a showed the presence of intramolecular Sb---N interaction which should be responsible for the reactivity enhancement of the ethynyl-l,5-azastibocines in Pd-catalyzed cross-coupling reactions with organic halides (see Section 14.18.11). The distance between the antimony and nitrogen atoms (2.538(4) A) corresponds to 68% of the sum of the van der Waals radii of both elements (3.74 A). In the crystal central antimony atom exhibited a pseudo-trigonal-bipyramidal structure <2003TL8589>. 

The fundamental value of crystallographic data, and of the printed compendia themselves, is nowhere more eloquently illustrated than in the classic text. The Nature of the Chemical Bond by Pauling [2]. Here for the first time we see the systematic correlation of geometrical results from the relatively small number of structural studies available at that time. Despite this paucity of data, at least by modern day standards, the impact of this volume on chemical thinking has been enormous. The results presented there for the covalent and van der Waals radii of common elements remain in use today. It was not until the late 1950 s that a further major compilation of geometrical data was accomplished by Sutton et al. [3]. These... 

Nag S, Banerjee J, Datta D (2007) Estimation of the van der Waals radii of the d-block elements using the concept of block vtilence. New J Chem 31 832-834... 

Then we have some key pieces of information, two of which are the van der Waals characteristics of helium and neon. (Larger rare gas atoms also become important when one considers atoms further down in the periodic table.) How closely can we approximate the van der Waals characteristics of hydrogen with those of helium And how closely can we approximate the van der Waals properties of carbon with those of neon We would expect that carbon and neon would be rather similar in their properties. We know that as we go across the periodic table from left to right, the van der Waals radii of the atoms become smaller, and we know by approximately how much. The value of e for carbon would be expected to be similar to that for neon. The polarizabilities of atoms decrease somewhat as we go to the right in the periodic table. The number of electrons may increase, but they are more tightly held. But we also know that there is a big difference in polarizability between the first-row elements and second-row elements. To a first approximation, the first-row elements are similar. On the other hand, one expects a sizable difference between hydrogen and helium. 

The tendency to form hydrogen bonds exists to a much smaller extent in the elements of the second row of the Periodic System, such as chlorine and sulphur. The bond has a less ionogenic character here. It is not correct that the hydrogen atom is situated much deeper in the electron cloud, compare the Van der Waals radii of F-, O2-, Cl-and S2 of 1.35, 1.45, 1.80 and 1.90 A with the nuclear distances in the hydrogen compounds of 0.92, 0.97, 1.27 and 1.35 A, respectively. 

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