Local covalent bonding

A dislocation can move no faster than its core (the region within one to two atoms of position c in Figure 5.7) so the mobility is determined by whatever barrier is presented to the core. Since the core is very localized, so must be the barrier if it is to have a substantial effect. This is why local covalent bonding leads to low mobility while the non-local bonding in metals gives high mobility. 

Since the presence of chemisorption (V and AT) states gives rise to the formation of localized covalent bonds between the adatom and substrate, we are interested in how the occurrence of localized states is governed by the values of the parameters za, zs and r/, which define the adatom-substrate interaction. Localized states exist, if one or both of (1.39) and (1.44) have real roots //,fc, which, since cosh /y,fc > 1 and e k > 1, exist for a given ri in regions of the zazs-plane depicted by the two hyperbolas... 

Both cationic adsorption and anionic adsorption belong to what is called ionic adsorption. Covalent adsorption is due to the localized covalent bonding, and metallic adsorption is due to the delocalized covalent bonding. The distinction among these three modes of chemisorption, however, is not so definite that the transition from the covalent through the metallic to the ionic adsorption may not be discontinuous, but rather continuous, in the same way as the transition of the three-dimensional solid compounds between the covalent, metallic, and ionic bonding. 

When a DE-system of p valence electrons and v atomic cores is converted by a chemical reaction into a system of localized covalent bonds and lone valence electrons, the new localized covalent bonds and lone electrons must involve the atomic cores that belonged to the DE-system, and at least all but one of the newly formed covalent bonds must be between atoms of the previous DE-system. The reverse holds for reactions that convert localized bonds and electrons into DE-systems. This corresponds to boundary conditions for the placement of the respective entries r j = r i =j= 0 (1 i,j g n) in xr-matrices. 

This leaves us nine elanents hydrogen, nitrogen, carbon, oxygen, fluorine, silicon, phosphorus, sulfur, and chlorine. All of these can bond to themselves, by localized covalent bonds that could be precisely manipulated by biochanical processes. 

In diamond (Figure 4.58), each carbon atom is tetrahedrally bonded to four other carbon atoms by single, localized covalent bonds. A very rigid three-dimensional network is formed. 

If we ignore the intermolecular forces, the electronic wave function of a molecular or ionic crystal can be approximately represented as a product of wave functions for individual molecules or ions. For example, the wave function of a sample of solid argon would be approximated by a product of atomic wave functions of the argon atoms. An approximate description of the electronic structure of network covalent crystals can include localized covalent bonding similar to that described in Chapter 21. 

---