Packing of atoms and molecules

To modei the atom as a "hard" sphere, one measures the distance two atoms approach in a coiiision or the spacing they assume in a crystai, the van der Waais radius. Naturally, the van der Waals radius changes for different approaches of the atoms, but it permits an easy modeiing of the packing of atoms and molecules in crystals and in the liquid. 

Initial reconstruction caused by flame armealing is stopped when the surface is cooled in the atmosphere, though not in water. The rate of transition from unreconstructed to reconstructed surface is determined by the height of the activation barrier [348], especially at the room temperature. Reconstruction may be removed by adsorption of atoms and molecules [349], since unreconstructed, and thus, more open surface, interacts with the adsorbates stronger than does the densely packed surface. Therefore, the removal of reconstructed surface proceeds from the less to the more energetically favored state [348]. Reconstruction coupled with the formation of more dense surface structure may lead to quite a strong increase in the number of surface atoms. For instance, the Au(100)-(1 X 1) Au(100)-(hex) reconstruction is accompanied by the increase in the number of surface atoms by 24%. 

It had long been known that ultrahigh pressures could pack the atoms and molecules of some substances into more substantial configurations. Gases, as we have already noted, could be liquefied by putting them under pressure, and some metals that did not superconduct on their own eventually did so when pressure was applied. Among the pioneers in the field was Percy Williams Bridgman, an American physicist at Harvard University whose contributions resulted in new techniques that increased laboratory pressures nearly a hundredfold, and won him a Nobel Prize in 1946. Once, when he reached a pressure twenty thousand times that of normal atmospheric pressure— physicists use the term 20,000 atmospheres—he burst the metal containers used in the experiment. Eventually, he developed more resilient materials, and succeeded in creat-... 

What is steric repulsion The generic term refers to the space-filling property of atoms and molecules, as manifested in crystal packing densities, molecular collision cross-sections, and other lines of experimental evidence. Indeed, space-filling molecular models are among the most useful tools of the chemistry studenL and atomic radii are among the first properties called to the student s attention to illustrate atomic periodicity trends. 

The major difference, then, between the 3 phases we have discussed is that the solid consists of an assemblage of close-packed molecules which we have shown to have arisen when we removed enough energy from the molecules so as to cause them to condense and to form the solid state. Let us now examine the properties of atoms or molecules when they are crowded together to form a "close-packed" solid. 

In forming a solid from atoms or molecules, part of the problem lies in the fact that we must handle a very large number of atoms or molecules. For example, 6.023 x 1023 (60.23 septiUion) atoms comprise one mole and we must stack each of these in a symmetrical manner to form the close-packed... 

Chapter 6. The outer contour in this map is for a density of 0.001 au, which has been found to represent fairly well the outer surface of a free molecule in the gas phase, giving a value of 190 pm for the radius in the direction opposite the bond and 215 pm in the perpendicular direction. In the solid state molecules are squashed together by intermolecular forces giving smaller van der Waals radii. Figure 5.2b shows a diagram of the packing of the Cl2 molecules in one layer of the solid state structure of chlorine. From the intermolecular distances in the direction opposite the bond direction and perpendicular to this direction we can derive values of 157 pm and 171 pm for the two radii of a chlorine atom in the CI2 molecule in the solid state. These values are much smaller than the values for the free molecule in the gas phase. Clearly the Cl2 molecule is substantially compressed in the solid state. This example show clearly that the van der Waals of an atom radius is not a well defined concept because, as we have stated, atoms in molecules are not spherical and are also compressible. 

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

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