The Structure of Covalent Compounds

The electronic structure of molecules of covalent compounds involving the principal groups of the periodic table can usually be written by counting the number of valence electrons in the molecule and then distributing the valence electrons as unshared electron pairs and shared electron pairs in such a way that each atom achieves an argononic structure. 

The structures of the isomeric molecules ethyl alcohol, CH3CH2OH, and dimethyl ether, (CHg)20. 

In this molecule, the hydrogen fluoride molecule, there is a single covalent bond that holds the hydrogen atom and the fluorine atom firmly together. 

It is often convenient to represent this electronic structure by using a dash as a symbol for the covalent bond instead of the dots representing the shared electron pair. Sometimes, especially when the electronic structure of the molecule is under discussion, the unshared pairs in the outer shell of each atom are represented, but often they are omitted  

These substances are strong acids when they are dissolved in water the proton leaves the molecule, and attaches itself to a water molecule to form a hydronium ion, HgO, the halogen being left as a halide ion. 

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Lewis structures can be used to draw the structures of covalent compounds. You should learn the rules for drawing these structures. 

The direction of a chemical bond directly determines the structure of covalent compounds. For example, in diamond (its electron configuration being ls 2s 2p ), four hybrid sp orbitals are formed due to the destruction of a spin bond at s levels and the excitation of three electrons at p levels, they are directed from the centre to the vertices of regular tetrahedron. The angle between the axes of orbitals is equal to 109°28. ... 

While sharing of electrons, i.e., covalent bonding, is the major component of the cohesive force in intermetallics, rationalization of their structure formation based on such chemical bonding is not trivial, because of the failure of the common electron counting rules that chemists have developed over the years from the studies of covalent compounds. The origin of the problem is the well-delo-... 

Chemists use two parameters, bond lengths and bond angles, to describe the 3D structures of covalent compounds. A bond length is the average distance between the nuclei of the atoms that are covalently bonded together. A bond angle is the angle formed by the interaction of two covalent bonds at the atom common to both. 

The structures of technetium compounds are insufficiently known to give satisfactory values for ionic radii. Its covalent radius is very little less than that of rhenium both metals have the same crystal structure (h.c.p.) and the atomic volume of Re is very little greater (p. 480). 

The structure of diamond, a covalent crystal, is shown in Fig. 7.1. How is it related to some of the structures of ionic compounds discussed in this chapter ... 

Structural formulae such as those which we have just quoted arise from an attempt to interpret ionic structures in terms of chemical principles which are strictly applicable only to covalent and molecular compounds. If a single molecule of calcium carbonate could exist, say as a gas, it might well have the structure represented above, but it is of little significance to discuss the structure of a compound in a state in which it does not occur. If any formula is employed to represent the structure in the solid state it must be one which simply expresses the co-ordination of each of the several atoms in turn, e.g. 

In chapters 8 and io we have discussed the structures of some compounds of composition AmBnOz, and we have seen that these maybe broadly divided into two classes. On the one hand we have complex oxides, in which B is an electropositive element and in which the binding forces are predominantly ionic throughout the structure. On the other hand we have the salts of the inorganic acids, which are characterized by the existence of discrete complex anions bound to the cations A by ionic bonds but bound within themselves by covalent bonds. These complex anions are generally relatively simple in form, and although occasionally polynuclear (as in the thionates) they are always finite in extent. 

Three factors are critical in determining the structure of ceramic compounds crystal stoichiometry, the radius ratio, and the propensity for covalency and tetrahedral coordination. 

We have seen that carbon forms four covalent bonds and hydrogen forms only one covalent bond (Section 1.4). This means that there is only one possible structure for an alkane with molecular formula CH4 (methane) and only one structure for an alkane with molecular formula C2H6 (ethane). We examined the structures of these compounds in Section 1.7. There is also only one possible stracture for an alkane with molecular formula CsHg (propane). 

In Chapter 7, you learned about the structure of ionic compounds— substances formed from ionic bonds. The covalent molecules you have read about in this chapter have structures that are different from those of ionic compounds. In studying the molecular structures of covalent compounds, models are used as representations of the molecule. 

In Chapter 8 we used Lewis structures to account for the formulas of covalent compounds. (Section 8.5) Lewis structures, however, do not indicate the shapes of molecules they simply show the number and types of bonds. For example, the Lewis structure of CCI4 tells us only that four Cl atoms are bonded to a central C atom ... 

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