The corundum (Al2O3) has a rhombohedral Bravais lattice, but it closely approximates a hexagonal lattice and...

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Chemistry

The corundum (Al2O3) has a rhombohedral Bravais lattice, but itclosely approximates a hexagonal lattice and is usually regarded ashexagonal. There are 30 ions per lattice site (and per unit cell).The Al2O3 formula requires that these 30 ions be divided as 12Al3+and 18 O2-. The Al2O3 structure closely approximates close-packedO2- sheets with two-thirds of the small interstices between sheetsfilled with Al3+. In other words, close-packed O2- ion containinglayers have repeated stacking and the Al3+ ions fill two-thirds ofthe octahedral interstices between adjacent layers. (22 points) Thedensity of Al2O3 is ï²Al2O3 = 3.8 gram/cm3, Atomic masses are : MAl= 26.98 gram/mol, MO = 16.00 gram/mol Ionic radii are : rAl3+ =0.05 nm, rO2- = 0.14 nm a) Draw 3 layers/sheets of O2- ions andAl3+ ions in between them. b) Draw the corundum unit cell and showthe O2- and Al3+ ions in the unit cell. c) How many Al2O3 moleculesare there in 1 cm3 ? How many atoms are there in 1 cm3 of Al2O3? d)How many Al2O3 molecules are there in 1 gram? How many atoms arethere in 1 gram of Al2O3? e)Calculate the atomic packing factor ofAl2O3

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3.7 Ratings (344 Votes)

In this hcp packing there is one octahedral void corresponding to each atom constituting the close packing In corundum only 23rd of the octahedral voids are occupied It means corresponding to each oxide are 23 See Answer

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