Metals crystalline arrangement

The atomic radius of an element is considered to be half the interatomic distance between identical (singly bonded) atoms. This may apply to iron, say, in its metallic state, in which case the quantity may be regarded as the metallic radius of the iron atom, or to a molecule such as Cl2. The difference between the two examples is sufficient to demonstrate that some degree of caution is necessary when comparing the atomic radii of different elements. It is best to limit such comparisons to elements with similar types of bonding, metals for example. Even that restriction is subject to the drawback that the metallic elements have at least three different crystalline arrangements with possibly different coordination numbers (the number of nearest neighbours for any one atom). 

Lorentz1 advanced a theory of metals that accounts in a qualitative way for some of their characteristic properties and that has been extensively developed in recent years by the application of quantum mechanics. He thought of a metal as a crystalline arrangement of hard spheres (the metal cations), with free electrons moving in the interstices.. This free-electron theory provides a simple explanation of metallic luster and other optical properties, of high thermal and electric conductivity, of high values of heat capacity and entropy, and of certain other properties. 

Supported metal particles were compared to the idealized crystal shapes available in various crystalline arrangements. Some of the shapes derived from fee crystals (the crystalline orientation of most catalytically active metals) are depicted in Fig. 3.3. The various types of surface atoms were differentiated by the number of... 

In this section, we start by explaining the nature of the chemical bond of metals. We will see that metals usually arrange themselves in a regular, crystalline order. Therefore, we will afterwards discuss the structure of crystals and, finally, explain how a metallic material is composed of such crystals. 

Structures analyzed above as metal clusters could be in principle also considered as alkali metals strongly contaminated by oxygen, leading thus to a quasi-infinite crystalline arrangement. Their classification as a cluster species therefore needs further evidence. 

Amphiboles. The crystalline stmcture common to amphibole minerals consists of two ribbons of siUcate tetrahedra placed back to back as shown in Figure 5. The plane of anionic valency sites created by this double ribbon arrangement is neutralized by the metal cations. The unit cell has seven cationic sites of three different types these sites can host a large variety of metal cations without substantial dismption of the lattice. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

In the solid state, aluminum chloride exists in a crystalline lattice. Each aluminum atom is surrounded by six chlorine atoms arranged around the metal atoms at the comers of an octahedron. Aluminum bromide and aluminum iodide form AI2 Xj molecules in all three phases. 

---