Atomic properties metallic behavior

In the preceding chapter we looked at the elements of the third row in the periodic table to see what systematic changes occur in properties when electrons are added to the outer orbitals of the atom. We saw that there was a decided trend from metallic behavior to nonmetallic, from base-forming to acid-forming, from simple ionic compounds to simple molecular compounds. These trends are conveniently discussed... 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

One has an intuitive, simple description of transition metal behavior in terms of properties which depend on the atomic potential, such as ea, and other properties, such as the d band structure, which depend on crystal symmetry and lattice constant. This division is useful for consideration of XPS conduction band results in transition metal alloys and it may be noted that core and d levels, alike, lie higher in the metal than in the free atom. 

The BNC nanotubes can have a metallic behavior if they do not have a band gap or a semi conductor behavior if there are band gaps. The importance of this phenomenon is that the electric properties of BCN compoimds can be controlled by varying the atomic composition and atomic arrangement of the compounds. In addition, their mechanical properties could be similar to these of diamond and cubic BN, providing new super-hard materials [14]. 

This article is organized primarily according to the periodic table and secondarily with regard to the importance of substrates found to react with metal vapor atoms. Tables, lists, and figmes will display many of the known compounds prepared via the metal vapor-substrate cocondensation method and give further information about physical properties, chemical behavior, spectroscopic data, and structural details. 

The diversity in structure and bonding possible for phosphides is effectively demonstrated by the monophosphides. Monophosphides MP of the group 1 and 2 elements (El, E2) are polyphosphides with i(P ) chains and P2" dumbbells, respectively. Ell and E12 monophosphides are not known. The E3 and E13 monophosphides are the so-called normal compounds with 3x = (M) (see Section 2). With El3, they form the zinc blende structure with tetrahedral heteroatomic bonds. Ternary derivatives such as MgGeP2 and CuSi2P3 have a random distribution of the M atoms, whereas CdGeP2, crystallizes in the ordered chalcopyrite type with a TO[GeP4/2] tetrahedral net (see Section 6.4). The E3 monophosphides form the NaCl structure. CeP is remarkable because of its physical properties (metal-semiconductor transition heavy-fermion behavior). The E14 monophosphides show the break usually observed when passing the Zintl border. Binary lead phosphides are not known SiP and GeP... 

Finally, the structural modifications of elemental boron exhibit complex extended lattices of cages in the solid state, whereas those of metals possess much simpler close-packed atomic lattices. These differences are a direct reflection of atomic properties and result in the respective nonmetallic and metallic behavior. However, boron combines with most other elements including metals. There are a wide range of metal borides known with stoichiometric as well as nonstoi-chiometric atomic ratios. The amazingly varied interpenetration of the two characteristic structural motifs and the subtly balanced competition between the two modes of solid state bonding found in the metal borides constitutes further justification of our theme. This is discussed in some detail in Section II,C. 

Unlike (SN), most polymers correspond to closed-shell systems where all the electrons are paired. Such a configuration leads to insulating or semiconducting properties as noted previously. Polyacetylenes and related conjugated polymers, for example, have conductivities that classify them as semiconductors. The carbon atom in polyacetylene is sp hybridized, which leaves one p electron out of the bond-forming hybrid orbitals. In principle, such a structure might be expected to give rise to extended electronic states formed by overlap of the p (tt) electrons and thus provide a basis for metallic behavior in polymers. 

These atomic properties have a profound effect on many macroscopic properties, including metallic behavior, acid-base behavior of oxides, ionic behavior, and magnetic behavior of the elements and their compounds. 

Our main purpose for discussing atomic properties is, of course, to see how they affect element behavior. In this section, you ll see how the properties we just examined influence metallic behavior and determine the type of ion an element can form, as well as how electron configuration relates to magnetic properties. 

Describe the general properties of metals and nonmetals and understand how trends in metallic behavior relate to ion formation, oxide acidity, and magnetic behavior understand the relation between atomic and ionic size and write ion electron configurations ( 8.5) (SPs 8.6-8.8) (EPs 8.47-8.65)... 

Write electron configurations of transition metal atoms and ions compare periodic trends in atomic properties of transition elements with those of main-group elements explain why transition elements have multiple oxidation states, how their metallic behavior (type of bonding and oxide acidity) changes with oxidation state, and why many of their compounds are colored and paramagnetic ( 22.1) (SP 22.1) (EPS 22.1 -22.17)... 

Although the properties of a bare metal cluster in the gas phase, e.g. the ionization potential, depend solely on the nuclearity, it is clear that any other molecule interacting with the metal atoms influences their behavior. 

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