L08MetSecBds145s10

L08MetSecBds145s10 - Lecture 8: Metallic Bonding; Secondary...

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ENGR 145: Chemistry of Materials Case Western Reserve University Lecture 8: Metallic Bonding; Secondary Bonding Reading assignment: C&R §2.6-2.7; §12.5-12.6; OGC §10.2-10.3 Learning objectives: Understand metallic bonds as one extreme on the metallic-covalent continuum of bond character Use the bond type triangle to identify the predominant bond type in simple compounds Explain qualitatively how discrete atomic states of free atoms become bands in solids Understand the difference in band structures between metallic solids and semiconductors Describe absorption and emission of photons of the band-gap energy in semiconductors, and applications of these phenomena Recognize the role of secondary bonds in solids hydrogen bonds van der Waals bonds 1 Lecture 8, spring 2010
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ENGR 145: Chemistry of Materials Case Western Reserve University Covalent vs. Metallic Character Covalent bonding: sharing of electrons The electrons are localized, mostly between the atoms Metallic bonding: sharing of electrons The electrons are distributed throughout the solid or M + M + electron “gas” or “sea” 2 Lecture 8, spring 2010
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ENGR 145: Chemistry of Materials Case Western Reserve University + + e M M loosely bound valence electrons forming a “gas” or a “sea” ion cores (Flled shells) Metallic Bonding 3 Lecture 8, spring 2010
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metallic-covalent bonding spectrum 100% covalent localized electrons A B 100% metallic delocalized electrons B B Average of electronegativity χ avg = | A + B | /2 : increasing avg covalent-ionic bonding spectrum 100% covalent 100% ionic A + B - B B Δ and polarity transfer of electrons sharing of electrons dipole moment Difference in electronegativity, Δ = | A B | When at least one of the atoms in a bond has high electronegativity: When both atoms in a bond have similar electronegativity: 4
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ENGR 145: Chemistry of Materials Case Western Reserve University Bond Type Triangle increasing χ avg increasing Δ C – covalent avg large Δ small C B – ionic avg medium Δ large A – metallic avg small Δ small D – semimetallic B A D 5 Lecture 8, spring 2010 avg Δχ
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ENGR 145: Chemistry of Materials Case Western Reserve University Transitions between bond types are actually gradual , not sharp
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This note was uploaded on 01/30/2011 for the course EMSE 103 taught by Professor Ggh during the Spring '10 term at Case Western.

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L08MetSecBds145s10 - Lecture 8: Metallic Bonding; Secondary...

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