Bonds directional character

This type of argument leads us to picture a metal as an array of positive ions located at the crystal lattice sites, immersed in a sea of mobile electrons. The idea of a more or less uniform electron sea emphasizes an important difference between metallic bonding and ordinary covalent bonding. In molecular covalent bonds the electrons are localized in a way that fixes the positions of the atoms quite rigidly. We say that the bonds have directional character— the electrons tend to remain concentrated in certain regions of space. In contrast, the valence electrons in a metal are spread almost uniformly throughout the crystal, so the metallic bond does not exert the directional influence of the ordinary covalent bond. 

The nonlocalized or mobile electrons account for the many unique features of metals. Since metallic bonds do not have strong directional character, it is not surprising that many metals can be easily deformed without shattering their crystal structure. Under the influence of a stress, one... 

We have already learned that metals may be deformed easily and we have explained this in terms of the absence of directional character in metallic bonding. In view of this principle, it is not surprising that two-element or three-element metallic crystals exist. In some of these, regular arrangements of two or more types of atoms are found. The composition then is expressed in simple integer ratios, so these are called metallic compounds. In other cases, a fraction of the atoms of the major constituent have been replaced by atoms of one or more other elements. Such a substance is called a solid solution. These metals containing two or more types of atoms are called alloys. 

Ions stack together in the regular crystalline structure corresponding to lowest energy. The structure adopted depends on the radius ratio of cation and anion. Covalent character in an ionic bond itnposes a directional character on the bonding. 

Figure 4. Association of 812(8,2)12 polyhedra in 8 5 showing (a) the directed character of intericosahedral bonds, as well as the existence of large voids in the boron framework, (b) the filling of voids by additional 8 and Y atoms (partial projection onto the ac plane according to ref. 9).

The properties of a substance depend in part upon the type of bonds, between the atoms of the substance and in part upon the atomic arrangement and the distribution of the bonds. The atomic arrangement is itself determined to a great extent by the nature of the bonds the directed character of covalent bonds (as in the tetrahedral carbon atom) plays an especially important part in determining the configura-... 

The description of the bond orbitals of the carbon atom as sp8 tetrahedral hybrid orbitals is satisfactory in many respects, but it can be improved. One improvement is that of concentrating the orbitals more closely about the bond direction by the introduction of some d and/character.21... 

The size and shape of molecules are as much a part of molecular structure as is the order in which the component atoms are bonded. Contrary to the impression you may get from structural formulas, complex molecules are not flat and formless, but have well-defined spatial arrangements that are determined by the lengths and directional character of their chemical bonds. It is not easy to visualize the possible arrangements of the bonds in space and it is very helpful to have some kind of mechanical model that reflects the molecular geometry, including at least an approximation to the relative lengths of the bonds. Ball-and-stick models such as the ones used by Patemo (Section 1-ID) fill this purpose admirably. 

The greater the amount of orbital overlap, the stronger the bond. This leads to a directional character to the bond when other than s orbitals are involved. 

If the lateral distances of the atoms are changed, this is called surface reconstruction. This is, for instance, observed with the (100) faces of Au, Ir, Pt, and W. Figure 8.4 shows two types of surface reconstruction that lead to a doubling of the lattice spacing in one direction. Semiconductor surfaces tend to exhibit surface reconstruction due to the directional character of the dangling bonds at the surface. 

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