The Covalent Bonding Model

The covalent bonding model proposes that electron sharing between pairs of atoms leads to strong, localized bonds, usually within individual molecules. At first glance, however, it seems that the model is inconsistent with some of the familiar physical properties of covalent substances. After all, most are gases (such as methane and ammonia), liquids (such as benzene and water), or low-melting solids (such as sulfur and paraffin wax). Covalent bonds are strong (—200 to 500 kJ/mol), so why do covalent substances melt and boil at such low temperatures ... 

Atomic Properties and Chemical Bonds 9.3 The Covalent Bonding Model 9.5 Between the Extremes Electronegativity... 

How the Model Explains the Properties of Ionic Compounds The Covalent Bonding Model... 

The covalent bonding model proposes that electron sharing between pairs of atoms... 

These mysterious rules may be understood by the following reasoning. In the covalent bond model, an octahedral cation site in these phases implies a higher charge transfer than does a tetrahedral one. It is therefore reasonable to locate the octahedrally coordinated cations near the center of the layer. For the tetrahedral sites of the outer anion double layer, however, we have no other equivalent choice. If we used the other r set where all tetrahedra turn their bases towards the interior, then... 

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