Energy diatomic metals

Figure 4. Comparison of theoretical and experimental bond dissociation energies for first row diatomic metal hydride ions. Data from reference 27.

The Knudsen effusion method In conjunction with mass spectrometrlc analysis has been used to determine the bond energies and appearance potentials of diatomic metals and small metallic clusters. The experimental bond energies are reported and Interpreted In terms of various empirical models of bonding, such as the Pauling model of a polar single bond, the empirical valence bond model for certain multiply-bonded dlatomlcs, the atomic cell model, and bond additivity concepts. The stability of positive Ions of metal molecules Is also discussed. 

The most commonly measured property for these types of molecules is their dissociation energy or atomization energy. According to a recent review ( ) these have been experimentally determined for approximately 50 homonuclear diatomic metal molecules, 15 polyatomic metal molecules (including germanium but excluding silicon and antimony), 110 diatomic intermetallic compounds and more than 20 polyatomic intermetallic molecules. 

The ejcperimental bond energies of diatomic metals and small clusters have very recently been reviewed ( ). Therefore, only the atomization energies that have since become available have been listed in Table IV. Also Included are previously known molecules for which revised values have been reported. The error limits for AuCs and AuRb have been Increased by the difference in the value for Dq(Au2) in references (20) and (16). For Biln and Bi2ln the error limits have been estimated as twice the third law standard deviation plus half of the difference between the reported second and third law values (21). 

As already mentioned in Section 2.2.8, measurement of the heats of formation of a variety of atoms and radicals allows the thermochemical derivation of a large number of heats of dissociation involving tfiem. It is not intended to discuss these methods and results comprehensively, but mention should be made of one particular thermochemical cycle which has been widely applied. The dissociation energy of a diatomic metallic halide or oxide, Z)(MX) is related to other thermochemical quantities by... 

Table 3-2 Bond Energies (AHdbe) and Charge Transfer [8 = A7SZMy/2] for Diatomic Metal-Chlorine and Metal-Oxygen Molecules...

This qualitative description of the interactions in the metal is compatible with quantum mechanical treatments which have been given the problem,6 and it leads to an understanding of such properties as the ratio of about 1.5 of crystal energy of alkali metals to bond energy of their diatomic molecules (the increase being the contribution of the resonance energy), and the increase in interatomic distance by about 15 percent from the diatomic molecule to the crystal. 

It is this resonance energy that would be in the main responsible for the difference in energy of the crystal and the gas of diatomic molecules Li2. But the heat of formation of Li2 molecules from atoms is only 6-6 kcal./g.-atom, whereas that of the metal is 39kcal./g.-atom. It seems unlikely, by comparison for example with the analogous case of Kekule-like resonance in aromatic molecules, that the great difference, 32-4 kcal./g.-atom, could result from the synchronized resonance, of type f Li—Li Li Li)... 

The energy values and the derived quantities for the diatomic molecules of the alkali metals are given in table 1. It is seen that the amount of p character is calculated to he between 5 and 14 %. 

To describe the band structure of metals, we use the approach employed above to describe the bonding in molecules. First, we consider a chain of two atoms. The result is the same as that obtained for a homonuclear diatomic molecule we find two energy levels, the lower one bonding and the upper one antibonding. Upon adding additional atoms, we obtain an additional energy level per added electron, until a continuous band arises (Fig. 6.9). To describe the electron band of a metal in a... 

In diatomic molecules such as N2, O2, and CO the valence electrons are located on the 5cr, Ijt and 2jt orbitals, as shown by Fig. 6.6. [Note that the 5cr level is below the Ijt level due to interaction with the 4cr level, which was not included in the figure.] In general, the Ijt level is filled and sufficiently low in energy that the interaction with a metal surface is primarily though the 5cr and 2jt orbitals. Note that the former is bonding and the latter antibonding for the molecule. We discuss the adsorption of CO on d metals. CO is the favorite test molecule of surface scientists, as it is stable and shows a rich chemistry upon adsorption that is conveniently tracked by vibrational spectroscopy. 

The experiments with a beam of silver particles were conducted at room temperature. The energy of dissociation of diatomic molecules of silver is 1.78 eV, the heat of evaporation of silver molecules is 95 kcal/mol [46], and the heat of evaporation of an uniatomic silver is 64 kcal/mol. Mass-spectrometric studies [46] of silver vapour above a metallic silver showed that the ratio of number densities of ions Ag /Ag2 is equal to two. In other studies [47], a considerably larger value of this ratio was found. At 1037 - 1147 C molecular mass of silver particles in vapours was found to be 278 90 [46], i.e., an average number of atoms in a molecule of silver is 2.56. 

Figure A.15 Energy diagram for the adsorption of a simple diatomic molecule on a d-metal. Chemisorption orbitals are constructed from both the bonding and the antibonding levels of the molecule. As the latter becomes partially occupied, the intramolecular bond of the adsorbate has been activated.

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