Homopolar chemical bond

Llusar R, Beltran A, Andres J, Noury S, Silvi B (1999) Topological analysis of electron density in depleted homopolar chemical bonds. J Comput Chem 20(14) 1517-1526... 

I presume that Ziegler s notation C—Na was only symbolical, since when it was proposed the homopolar and heteropolar nature of chemical bonds was not as clearly understood as it is today. 

At temperatures of 1000°C and above, A1 and Si wet covalent ceramics rather well with contact angles close to 50° for both non-reactive (A1/A1N and Si/SiC) and reactive systems (Al/SiC and Al/BN). This behaviour relates well to theoretical studies indicating the formation of metallic or covalent chemical bonds at the interfaces between A1 or Si and covalent ceramics. The ability of A1 and Si to bond strongly with ceramic surfaces appears to correlate with the degree of covalence (or, equivalently, with the degree of ionicity) of the ceramic, as shown by the data in Table 7.9 for Si on non-reactive solids. A similar tendency is observed for A1 on various solids, including solid Ai considering the metallic bond in solid Al as a homopolar Al-Al bond (Table 7.9). 

The Heitler-London method, although approximate, has the advantage that a physical picture of the nature of the chemical bond may easily be obtained. The bond is formed by two electrons, one from each atom with opposite spin and owing to the transfer of the electrons, two states arise, which on superposition yield a transitional cloud, which serves to bond the atoms together. Such a bond is termed the homopolar or covalent bond. This physical picture of the bond is clearly the basis of the electron pair bond of Lewis. 

Figure 2 gives the variation of the function along the line of two neighboring nuclei in several molecules. This figure clearly shows that for the homopolar bonds (the central bond of naphthalene, the hydrogen molecule, the lithium molecule) the result of the chemical bond is an increase of electrons between" the nuclei, while for the heteropolar bond (LiH) there is a transfer of charge from one atom towards the others. 

In qrrartz, the main chemical bond is Si-O. They are approximately eqiral amormts of homopolar and hetero polar bonds. Thus, the notion that the quartz crystal lattice is composed of Si arrd O atoms, or Si -O iorrs, reflects only the limiting... 

Fig. 2.3 Details of a dynamically evolving Gc2Se3 amorphous structure. Beside tetrahedral configurations in which a Ge atom green) is surrounded by four Se atoms yellow), ring structures can be realized, including a number of atoms ranging from four top left panel) to six bottom left panel) or more. These, eventually evolve in chains including only atoms of the same chemical species (homopolar bonds) as in the right panel. The blue balls are a short-hand visualization of the centers of the wavefunctions [43] contributing to the chemical bonds...

The third part deals with the theory of the chemical bond. It contains (of course) the seminal paper by Heitler zmd London, as well as a more general paper by London on the chemical bond and the quantum theory of homopolar valence numbers. The Heitler-London theory gave rise to the valence bond (VB) approach in quantum chemistry. There is also a paper with some exact calculations on H2 by Hylleraas, which present a computationally accurate view of the hydrogen bond. 

Another similar problem with the translation is the word Krdftespiel, which, translated literally, means a game of forces . Meant is the complex dynamic equilibrium between forces that lead to homopolar, or covalent, chemical bonds between equal atoms. The word game can be used quite well in this sense in German or Dutch, but seems silly in English. Hence my translation into equilibrium of forces or something very similar, depending on the context. 

Diatomic molecules have only one vibration along the chemical bond in homopolar AA D molecules, this vibration is Raman active but not infrared active, whereas the stretching vibration is both Raman and infrared active in heteropolar AB(D[Pg.1026]

The condensing atoms react with the surface to form atom-to-atom chemical bonds. The chemical bonding may be by metallic (homopolar) bonding where the atoms share orbital electrons, by electrostatic (coulombic, heteropolar) bonding where ions are formed due to electron loss/gain, or by electrostatic attraction (van der Waals forces) due to polarization... 

Returning to the Thomas-Fermi approximation, the approximation is indeed in agreement with the thermodynamic limit for bosonic condensation and can also be used for chemical systems, for chemical bonds are present down to the limiting case of single-chemical-bond systems (as in the paradigmatic homopolar bindings of molecular hydrogen and helium, H2 and He2). 

Therefore, the present study unfolds a possible way of reconciling fermionic and bosonic behaviors in the quantum description of chemical bonding for the paradigmatic diatomic homopolar Heitler-London case [83]. 

Table 4 Atomic and molecular data (number of electrons in atomic valtmce shell, atomic chemical hardness jj, atomic ground-state energy Eg, interatomic homopolar binding length with respect to the first Bohr radius oq. along the specific physical and chemical bonding BEC energies) for hydrogen and helium toward assessing their BEC-bonding Lennard-Jones-type potential (181), all in atomic units...

Bohr s hydrogen atom model of 1913 had provided inspiration to a few physicists, like Kossel, who were interested in chemical problems but to very few chemists concerned with the explanation of valence. First of all, the Bohr atom had a dynamic character that was not consistent with the static and stable characteristics of ordinary molecules. Second, Bohr s approach, as amended by Kossel, could not even account for the fundamental tetrahedral structure of organic molecules because it was based on a planar atomic model. Nor could it account for "homopolar" or covalent bonds, because the radii of the Bohr orbits were calculated on the basis of a Coulombic force model. Although Bohr discussed H2, HC1, H20, and CH4, physicists and physical chemists mainly took up the problem of H2, which seemed most amenable to further treatment. 11... 

---