As electronegativity. Act as good glass formers.

Asdiscussed earlier glasses are amorphous solids of disordered arrangement ofatoms, so no particular theory is on the formation of glass network. But stillthere are number of theories which describe the formation and structure of theglasses. These theories depend upon number of factors like their bonddissociation energies, their abilities to from covalent or ionic bonds, theircationic and ionic radii ratios etc. The brief description of these theories isdiscussed below:1.                 GoldschmidtTheory:   Theoryon the glasses given by the Goldschmidt was an empirical statement on thestructure of glasses.

According to Goldschmidt formation of glasses (RnOm)is depends upon the radius of the cations and anions. If the ratio of ionicradii of cation to ionic radii of anion lies in the range from 0.2 to 0.4 onlythen compounds form glass in which cations will form tetrahedral coordinationwith oxygen; i.e.                                           (1.1)Inthis ratio range cations formed tetrahedral coordination with oxygen which madeassumption that only materials having tetrahedrally coordinated cation can beform glasses during cooling.

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   2.                 SmekalTheory:  Thistheory was based upon the nature of the bonds in the material. According tothis theory only whose materials are preferably form glasses, have intermediatecharacter to form bonds.

That means which forms partially ionic and partiallycovalent bonds. The reason behind partially ionic and partially covalent isthat bonds purely ionic in nature do not form network structure and purelycovalent in nature provides well define bond angels in the network. Thiswell-defined arrangement prevents the formation of disorder-ness in thematerials.

    3.                 StanworthTheory: This theory was based upon the partial ioniccharacter of the material. He categorized the oxides according to theirelectronegativity. He classified the electronegativity of cations in three wayswhich fall under the categories of glass former, intermediate and modifiers.This classification can be understand from the following flow chart:     Ionic Character & electronegativity If cation-oxygen bond character is 50% ionic in nature. Cations have high electronegativity.   Act as good glass formers. If cation-oxygen bond character is > 50% ionic in nature.

Cations have slightly low electronegativity.   Act as glass intermediates. If cation-oxygen bond character is highly ionic in nature. Cations have very low electronegativity.   Act as glass modifiers.

Group I Group II Group III              4.     SunTheory: Like Stanworth, Sun classified the glass former,intermediate and modifiers according to the bond dissociation energies ofcations with anions. 


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