CH 2200 Lecture 17: Chinese common words 59

si4 lewis structure

Sillimanite is one of the aluminosilicate polymorphs (Al2SiO5). To balance this formula, you find that the aluminum must be in both 6+ and 4+ oxidation states. Thus the more accurate formula looks like Al6+Al4+Si4+O5. The Al6+ all reside in octahedral coordination, while the Al4+ and Si4+ mix on tetrahedral sites. There are two symmetrically distinct tetrahedral sites per unit cell called T1 and T2. There are four formula units per unit cell and thus there are 8 total T-sites in the unit cell, four T1 and four T2 filled with 4 Al4+ and 4 Si4+. In fully disordered sillimanite the Al4+ and Si4+ are evenly distributed over these tetrahedral sites (two of each element in each of the two T-sites). Calculate the entropy of configuration (Smixing) for fully disordered sillimanite at 0K. Recall, S = klnΩtotal and Ωtotal = Ωsite1Ω site2

(a) Explain briefly why a given cation might have a maximum total coordination number. Tell what this number is for each period of the periodic table (starting with the second period). HALURES

(b) Which total coordination number (maximum or penultimate) is likely to apply in determining the formulas of fluoro anions? Apply the appropriate number to the case of Si⁴⁺ and predict the formula of the fluoro anion of Si⁴⁺. (c) Explain the difference between the coordination number and the total coordination number of an element.