Structural approach to glass formation

Modification can be understood as follows. Si02 can be considered as forming three dimensional network of tetrahedral [Si04/2] units. All the four oxygens in Si04 tetrahedra are shared and therefore the unit as such is 

The dotted line across O is to indicate that it is shared between tetrahedra. The dotted line is often not indicated but understood. The glass therefore contains Si-O-Si linkages. When a highly ionic oxide like Na20 (2Na + 0 ) is added to Si02, the following reaction occurs. 

It may be appropriate to note here that model building is an integral part of research in glasses. Models can be built by hand or using a 

3-dimensions, for example one can construct the cm for Si02 from the cm of Si by introducing oxygen on every Si-Si bond. 

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