Molecular structure simple ionic compounds

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. 

A variety of solid materials, including metals, semiconductors, molecular solids, and ionic compounds, show a remarkable deviation from the simple Hooke s law in the pressure dependence of volume at 1-lOGPa. The deviation is quite often a prelude to a pressure-induced transition to a denser structure. - " For example, Fe metal changes its strac-ture from cubic (a-phase) to hexagonal (e-phase) at 11 GPa... 

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. 

In last century the knowledge of defects in a solid, especially an oxide, has been explored comprehensively. The contribution of Schottky and Wagner successfully put the problem on a quantitative basis and promote the discovery of semiconductor transistor. The idea of non-stoichiometry was developed by Berthollet more than a hundred years ago and the controversy between berthollides, which do not obey the Dalton s law, and daltonides, which follow Dalton s law of constant and multiple proportions based originally upon the study of simple ionic and molecular species, encouraged the scientific debates on how existence of point defect in a compound is is it random statistic distribution or the structure related The experimental data are the best way to explore the truth. Indirect and direct observations of atom... 

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. 

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