L08MetSecBds145s09r2

L08MetSecBds145s09r2 - Lecture 8: Metallic Bonding;...

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Lecture 8, spring 2009 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
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Lecture 8, spring 2009 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
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Lecture 8, spring 2009 ENGR 145: Chemistry of Materials Case Western Reserve University - + + e M M loosely bound valence electrons forming a “gas” or a “sea” ion cores (filled shells) Metallic Bonding 3
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Lecture 8, spring 2009 ENGR 145: Chemistry of Materials Case Western Reserve University metallic-covalent bonding spectrum 100% covalent localized electrons A B 100% metallic delocalized electrons A B A B Average of electronegativity χ avg = | A + B | /2 : increasing avg covalent-ionic bonding spectrum 100% covalent 100% ionic A + B - A B A B increasing 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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Lecture 8, spring 2009 ENGR 145: Chemistry of Materials Case Western Reserve University
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L08MetSecBds145s09r2 - Lecture 8: Metallic Bonding;...

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