Covalent characteristics

Self-Test 14.7A Explain why beryllium compounds have covalent characteristics. 

In the right part of the Table hydrides having covalent characteristics are observed. For the elements of the last groups several covalent hydrogen compounds are known numerous compounds are given by B, C, Si, Ge, N, P, O, S, Se. In the usual ambient conditions, as it is well-known, the carbon chemistry is especially rich of hydrides (that is of several series of hydrocarbons). In this class of compounds more or less complex molecules and structures are found the catenation (chainforming) tendency is a characteristic shown by several elements in this portion of the Periodic Table. 

A) Basic and ionic characteristics of the oxides of the main group elements in their highest oxidation states increases as one moves down a column. Increasing acidic and covalent characteristics increase as one moves to the right. 

How can you determine whether the bonds that hold a compound together are ionic or covalent Examining the physical properties of the compound is one method. This method is not always satisfactory, however. Often a compound has some ionic characteristics and some covalent characteristics. You saw this in the previous Thought Lab. 

All the M2+ ions are smaller and considerably less polarizable than the isoelec-tronic M+ ions. Thus deviations from complete ionicity in their salts due to polarization of the cations are even less important. However, for Mg + and, to an exceptional degree for Be2+, polarization of anions by the cations does produce a degree of covalence for compounds of Mg and makes covalence characteristic for Be. Accordingly, only an estimated ionic radius can be given for Be2+ the charge/radius ratio... 

In contrast to the above, the somewhat (albeit lower) covalent characteristics... 

Co-ordination compounds of the alkali- and alkaline-earth metals with covalent characteristics have been reviewed. 

Before discussing each type, we should recognize that the distinction between ionic and covalent bonding is not always clear-cut. Some compounds are clearly ionic, and some are clearly covalent, but many others possess both ionic and covalent characteristics. 

Beryllium shows some covalent characteristics in some of its compounds, unlike the other alkaline earth halides. Give a possible explanation for this phenomenon. 

ZnO normally has the hexagonal (wurtzite) crystal structure with lattice parameters a = 3.25 A and c = 5.12A (space group P63mc). The Zn atoms are tetrahedraDy coordinated to four O atoms, where the Zn d-electrons hybridize with the oxygen p-electrons. Layers occupied by zinc atoms alternate with layers occupied by oxygen atoms [94]. Whilst a bond between the Zn and O atoms exhibits covalent characteristic in the c-direction, it is mostly ionic in the o-direchon [95] consequently, ZnO single crystals have highly anisotropic properties. 

Aluminum has a larger atom and is clearly more metallic than boron yet, in its compounds, it has borderline ionic and covalent characteristics ]... 

PLDL did not have covalent characteristics. It may suggest that in these nanocomposite samples the carbon nanotubes are located between the polymer chains and probably have an influence on the supramolecular structure of polylactide during solidification as a result of the interaction of secondary bonding. 

Lewis Theory An Overview—Lewis symbol represents the valence electrons of an atom by using dots placed around the chemical symbol. A Lewis structure is a combination of Lewis symbols used to represent chemical bonding. Normally, all the electrons in a Lewis structure are paired, and each atom in the structure acquires an octet—that is, there are eight electrons in the valence shell. In Lewis theory, chemical bonds are classified as ionic bonds, which are formed by electron transfer between atoms, or covalent bonds, which are formed by electrons shared between atoms. Most bonds, however, have partial ionic and partial covalent characteristics. 

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