Molecular structure simple covalent compounds

Since the immediate electronic environment of the deuterium nucleus in most compounds is relatively simple, deuterium quadrupole coupling constants show very systematic variations with molecular structure. In covalent compounds, except those with hydrogen bonds or multicentered bonds (as in many boron deuterides), deuterium is bonded to only one atom. The bonding electrons have essentially cylindrical symmetry, and the only asymmetry in the electronic environment of deuterium comes from indirectly bonded atoms. Therefore, in many cases, 17 is very small 0.1). 

As the valency of the metal increases, the bonding in these simple binary compounds becomes more covalent and the highly symmetrical structures characteristic of the simple ionic compounds occur far less frequently, with molecular and layer structures being common. Many thousands of inorganic crystal structures exist, ffere we describe just a few of those that are commonly encountered and those that occur in later chapters. 

We have used the concepts of the resonance methods many times in previous chapters to explain the chemical behavior of compounds and to describe the structures of compounds that cannot be represented satisfactorily by a single valence-bond structure (e.g., benzene, Section 6-5). We shall assume, therefore, that you are familiar with the qualitative ideas of resonance theory, and that you are aware that the so-called resonance and valence-bond methods are in fact synonymous. The further treatment given here emphasizes more directly the quantum-mechanical nature of valence-bond theory. The basis of molecular-orbital theory also is described and compared with valence-bond theory. First, however, we shall discuss general characteristics of simple covalent bonds that we would expect either theory to explain. 

In this chapter we shall discuss the structures of a number of simple compounds of composition AmXz, and also of some complex oxides and sulphides of composition AmBnXz. We shall find among these compounds representatives of ionic, covalent and molecular structures. 

When a compound has strong covalent character then we expect it to be molecular with all the typical properties associated with simple molecular structures, such as relatively low melting and boiling points and non-electrolyte behaviour in water. This means that the solution will contain molecules and be non-conducting. Simple molecular substances are also non-conducting in the solid and liquid states. However, some molecular substances react with water to release ions. This is known as hydrolysis. An example is anhydrous aluminium chloride, which reacts with water in a hydrolysis reaction to form aluminium hydroxide and hydrochloric acid. 

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

Compounds containing covalent bonds have molecules whose structures can be classified as either simple molecular or giant molecular. 

Such research into the assemblies of chiral steroidal compounds gave us a chance to think about the hierarchical structure of materials in the universe [42], We have the simple idea that the atoms combine together through stronger covalent bonds to yield molecules which serve as information carriers, and that the molecules combine together through noncovalent bonds to yield molecular assemblies which serve to express the information. Chiral carbon chains... 

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