Xenon, atomic volume

The atomic volumes of the alkali metals increase with atomic number, as do those of the inert gases. Notice, however, that the volume occupied by an alkali atom is somewhat larger than that of the adjacent inert gas (with the exception of the lithium and helium—helium is the cause of this anomaly). The sodium atom in sodium metal occupies 30% more volume than does neon. Cesium occupies close to twice the volume of xenon. 

Calculate the ratio of the number of electrons in a neutral xenon atom to the number in a neutral neon atom. Compare this number to the ratio of the atomic volumes of these two elements. On the basis of these two ratios, discuss the effects of electron-electron repulsions and electron-nuclear attractions on atomic size. 

The NMR chemical shift of I29xe adsorbed on molecular sieves reflects all the interactions between the electron cloud of the xenon atoms and their environment in the intracrystalline void volume [1]. This nucleus therefore proved to be an ideal probe for investigating various zeolitic properties such as pore dimensions [2, 3], location of the countercations [4, 5], distribution of adsorbed or occluded phases [6-8] and framework polarisability [8, 9]. 

Generally, a, reflects the effects of two-body collisions between xenon atoms. The change in slope can be interpreted as a change in free volume upon coking. One may estimate effective relative internal volume accessible to xenon gas per gram of material from the slopes ... 

Fraissard et al (8) have shown that the chemical shift 5 of adsorbed xenon can be expressed as the sum of several terms corresponding to the various perturbations suffered by the xenon atom in a confined volume ... 

In the case of an isotropic Xe distribution (large cavities), the slope d8/dn of the straight section of the d-curve is inversely proportional to the free volume of the cavities accessible to the xenon atoms. 

At low Tt, Co + ions remain in the supercage forming octahedral complexes, Co(H20)62+, as observed by Kazanskii et al. (6) by optical measurements these complexes can generate a secondary sphere of coordination which retains water molecules inside the supercage. The void volume accessible to xenon atoms then... 

It occurred to Bartlett that the ionization energies of the oxygen molecule (1180 kj/mol) and the xenon atom (1167 kj/mol) were remarkably similar. He decided to try the same reaction as above with xenon replacing the diatomic oxygen. He prepared known volumes of xenon (in slight excess) and platinum hexafluoride and carefully noted the pressure of each. When he allowed the two gases to mix. 

The effect of these weak attractions between particles is a decrease in the number of collisions with the surfaces of the container and a corresponding decrease in the pressure compared to that of an ideal gas. We can see the effect of intermolecular forces when we compare the pressure of 1.0 mol of xenon gas to the pressure of 1.0 mol of an ideal gas as a function of temperature and at a fixed volume of 1.0 L, as shown in Figure 5.25 t. At high temperature, the pressure of the xenon gas is nearly identical to that of an ideal gas. But at lower temperatures, the pressure of xenon is less than that of an ideal gas. At the lower temperatures, the xenon atoms spend more time interacting with each other and less time colliding with the walls, making the actual pressure less than that predicted by the ideal gas law. 

On tlio basis of present knowledge, the degassers should be so designed that a large interfacial area is provided and that the liquid metal surface is as turbulent as possible. Theoretically, a degasser should work with good efficiency. A theoretical analysis by McMillan (BNL-353) showed that xenon has a tremendous tendency to concentrate on liquid bismuth surfaces. For a spherical volume, the number of xenon atoms on the surface vas estimated to be about 10 times the number dissolved in bismuth at 300°C. At 500°C this ratio came close to 10°. 

Occasionally, the presence of holes inside the molecule was described. One hole in myoglobin can accomodate a xenon atom (Schoenborn et ai, 1965). Lee and Richards (1971) have determined and listed cavities in three proteins, myoglobin, lysozyme, and ribonuclease S. In myoglobin, the larger cavities have respective volumes of 2.85 and 2.29 A. The heme bound water molecule forms parts of the boundary of a cavity of 1.781 A. Another cavity of 0.853 A can accomodate a xenon atom. One or two cavities in ribonuclease S appear to be unoccupied. In many cases holes inside proteins are filled with solvent. In lysozyme one large and two smaller cavities were reported from the electron density map, and three buried water molecules were found (Lee and Richards, 1971 Richards, 1974). In chymotrypsin, water molecules are inserted into cavities. 

In the past 40 years, compounds have been isolated in which xenon is bonded to several nonmetals (N, C, and Cl) in addition to fluorine and oxygen. In the year 2000, it was reported [Science, Volume 290. page 117) that a compound had been isolated in which a metal atom was bonded to xenon. This compound is a dark red solid stable at temperatures below -40°C it is believed to contain the [AuXe4F+ cation. 

Xenon is found in trace amounts in the atmosphere. It makes up just 0.086 ppm by volume of air. Xenon is the rarest of the noble gases. For every thousand-million atoms of air, there are only 87 atoms of xenon. Even so, it is recovered in commercial amounts by boiling off the xenon from fractional distillation of liquid air. Small amounts of xenon have been found in some minerals and meteorites, but not in amounts great enough to exploit. 

Evidence that clustering of rare gas atoms occurs around ions comes from (a) ion mobility measurements, and (b) volume changes occurring on electron attachment to solutes. The mobility of positive ions in xenon decreases with increasing pressure and at pressures near 100 bar is 1.3 X 10 cm /Vs [see Fig. 3(a)] near room temperature. An estimate of the size of the cluster moving with the ion may be obtained from such data using the Stokes equation. 

Crystals of the compound of empirical formula FiiPtXe are orthorhombic with unit-cell dimensions a = 8-16, h = 16-81. c = 5-73 K, V = 785-4 A . The unit cell volume is consistent with Z = 4, since with 44 fluorine atoms in the unit cell the volume per fluorine atom has its usual value of 18 A. Successful refinement of the structure is proceeding in space group Pmnb (No. 62). Three-dimensional intensity data were collected with Mo-radiation on a G.E. spectrogoniometer equipped with a scintillation counter. For the subsequent structure analysis 565 observed reflexions were used. The platinum and xenon positions were determined from a three-dimensional Patterson map, and the fluorine atom positions from subsequent electron-density maps. Block diagonal least-squares refinement has led to an f -value of 0-15. Further refinements which take account of imaginary terms in the anomalous dispersion corrections are in progress. 

In view of the chemistry of this inert element, the main application of Xe NMR is as a surface probe for studying meso and microporous solids and the free volume in polymers. The relaxation time for Xe adsorbed in solids is typically 10 ms to a few seconds. The use of Xe NMR as a probe for studying microporous solids has been extensively reviewed by Barrie and Klinowski (1992). A more recent example of the use of Xe NMR to study surface interactions is provided by a study of borosilicalites with the ZSM-5 structure (Ngokoli-Kekele et al. 1998). The Xe shift of adsorbed xenon (referred to the shift of the pure gas extrapolated to zero pressure) was found to change regularly with boron content, with a discontinuity at a boron content of about one atom per unit cell ascribed to a change in the distribution of boron atoms in the lattice. A similar correlation between the Xe NMR shift and the aluminium content has been reported for the zeolite ZSM-5, in which the discontinuity occurred at about 2 Al atoms per unit cell (Chen et al. 1992). 

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