Primary Valence Chain Lattices

A specially good example showing how a large group of important compounds can develop all the above described types—primary valence lattices, primary valence networks and primary valence chains—is afforded by the silicates whose structure elucidation is due in the first place to the experiments of W. L. Bragg and his co-workers over many years. For these very compounds the old method borrowed from organic chemistry of description by the aid of constitutional formulas was shown to be inappropriate. 

Proceeding from graphite, let us finally imagine a carbon lattice, which coheres in one direction through primary valences and in the two other directions by secondary valences. Such a structure is impossible from carbon atoms alone where tetrahedral valence holds, but we can conceive of its being realized, e.g. from a carbon chain of double bonds alone. Its density must lie, according to the distance law, between 0.8 and 1.1, according to the particular manner of separation between the chains. 

If experience gained from layer lattices is. applied to the chain lattices, which are also frequently named fiber lattices, it is to be expected that the cohesion of the primary valence chains will stamp the whole structure and that permutoid reaction will be present to a still higher degree than in layer lattices, because here the strong cohesion due to primary valence acts only in a single direction. We may further assume that the diffraction effects of such primary valence chains will exhibit certain similarities to diffraction phenomena in linear lattices. 

Natural high polymeric compounds, like high molecular weight mixtures, show only reflections corresponding to the intra-molecular periodicity, so that here also primary valence chain lattices are involved. 

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