enma2 - Pauli Exclusion principle For an individual atom at...

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Pauli Exclusion principle: For an individual atom, at most two electrons, which necessarily have opposite spins, can occupy the same state % ionic character= (1-exp(-(0.25)(X A -X B ) 2 ))x100 Covalently bonded substances have low MP due to weak intermolecular bonds (Van der waals) FCC Coordination number 12 = a 2R2 BCC coordination number 8 =( )/ a 4R 3 Simple cubic coordination number 6 = a 2R = APF volume atomsvolume unit cell HCP (6 atoms per cell) = = V c332a2 wHere a 2r Density = ρ nAwVcNa N a =6.02*10 23 atoms/mol Coordination number Cation anion radius ratio 2 <.155 3 .155-.225 4 .225-.414 6 .414-.732 8 .732-1.0 Structur e Name Structu re type Anion packi ng Catio n # Anion # Exampl es Rock salt AX FCC 6 6 NaCl, MgO, FeO CsCl AX Simpl e cublic 8 8 CsCl Zinc Blende AX FCC 4 4 ZnS, SiC Fluorite AX 2 Simpl e cubic 8 4 CaF 2 (Ca is in every other cube Perovski te ABX 3 FCC 12(A) 6(B) 6 BaTiO 3 Spinel AB 2 X 4 FCC 4(A) 6(B) 4 MgAl 2 O 4 Rock salt = + a 2rcat 2ran Seven crystal systems <111> family of directions {111} family of planes Chapter 4 = DP Mnm where m is average mol. Weight of repeat unit % = ( - ) ( - ) crys ρc ρs ρa ρs ρc ρa C is crystalline, a is totally amorphous, and s is density tbd • Crosslinking occurs at double bonds in the main chain • Crosslinking increases E, uts, and oxidation resistance σ • Useful rubbers are 1-5 w/o sulfur, higher crosslinked rubbers are too stiff, hard, and brittle. Linear Polymers – Mer units are joined together end-to-end in single chains. – The long chains are flexible and may be thought of as spaghetti. – Extensive vanderWaals and H-bonding – Examples: HDPE, PVC, PTFE, PMMA, polystyrene Branched Polymers – Long side branch chains are connected to the main ones – Reduced chain packing efficiency (lowers strength and density) – Examples: LDPE Crosslinked Polymers – Adjacent linear chains are joined to one another at various positions along their length by covalent bonds. – Crosslinking is accomplished by additive atoms or molecules – Examples: Vulcanized Rubbers, Elastomers Network Polymers – Trifunctional mer units having three active covalent bonds form threedimensional networks. – Three dimensional networks also possible in highly
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This note was uploaded on 08/02/2011 for the course ENMA 300 taught by Professor Alsheikhly during the Spring '11 term at University of Maryland Baltimore.

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enma2 - Pauli Exclusion principle For an individual atom at...

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