Silicium atom

From a chemical point of view, silica gel is silicium dioxide in the crystal structure each silicium atom is surrounded by four oxygen atoms. There are some specific... 

Vanadium (element No. 23) is comparatively abundant in the universe. At 0.0001%, its cosmic abundance is comparable to that of copper and zinc. The cosmic abundance is, e.g., reflected in chondritic meteorites, which contain 220 atoms of the isotope in relation to 10 silicium atoms. Cosmic formation of vanadium is based on the a, y cascade up to Cr, followed by the reaction sequence ... 

Als Zwischenprodukt bei der Synthese von 1-Glykosyl-lH-indazol (s. S. 840) werder 1-Tri-methylsilyl-lH- bzw. 2-Trimethylsilyl-2H-indazol durch Reaktion von Indazol mit Hexamethylsilazan hergestellt530 60f -f,n7. Der Angriffsort des Silicium-Atoms (N-l oder N-2) wurde nicht bestimmt606,607 bzw. als lH-Isomeres angegeben530. 

Heterogeneous catalysis The best known catalyst is titanium silicate - (TS-1) a synthetic zeolite of ZSM family which does not contain any aluminium and in which titanium atoms replace some silicium atoms within the crystal lattice (Ti/Si 5 %) (ref. 3). A process based on this catalysis is used in Italy (ref. 4). Although its performance levels are good, and the HQ/PC ratio can be modified (ref. 5) the high cost of the catalyst and its uncertain lifetime are the major handicaps preventing its extension. 

Tectosilicates and feldspars If in the tridimensional framework corresponding to the stoichiometric formula SiO (e.g., tridymite or cristobalite) a certain proportion of silicium atoms is substituted by aluminum atoms, in order for the lattice to preserve its structure, it is necessary that the crystal acquire one supplementary electron per aluminum atom. In order to restore the electroneutrality, the crystals accept ions, e.g., Na, which penetrate the holes of the lattice and become linked to the oxygen atoms. 

Cristobalite framework in which one-third of the silicium atoms are substituted by aluminum atoms KCAilSi Og) (leucite). 

Tridymite framework, where half of silicium atoms are... 

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