Lecture%2012 - EEE 352 LECTURE 12 Bonding-Metallic crystals...

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1 EEE 352: LECTURE 12 Bonding—Metallic crystals and Covalent bonding The Nearly Free Electron Model—Imposing Periodicity on our Particles * Combining free electrons: an approximation * Nearly free electron model Electron diffraction Energy gaps Metals are GOOD electrical conductors and typically correspond to those elements whose shells are OVERFILLED by just one or two electrons In metals these extra VALENCE electrons are SURRENDERED by each atom, thus forming a SEA of charge that may wander FREELY through the crystal and so CONDUCT electricity Metallic Crystals 1 10 6 2 6 2 2 4 3 3 3 2 2 1 : Cu s d p s p s s 1 6 2 2 3 2 2 1 : Na s p s s + + + + + + SCHEMATIC MODEL OF A METAL CRYSTAL THE IONIZED ATOMIC CORES SIT AT FIXED POSITIONS WHILE THE GREY REGIONS REPRESENT THE ELECTRON GAS THAT IS SPREAD UNIFORMLY THROUGH THE CRYSTAL [ web ] We have discussed the formation of IONIC bonds and crystals by electron transfer Molecules and crystal structures may also form by COVALENT bonding * In these cases electrons are SHARED between different atoms RATHER than being transferred as in ionic crystals * By sharing electrons the atoms involved APPEAR to fill their shells EXAMPLES of covalent MOLECULES are H 2 , Cl 2 , H 2 O, CH 4 + + IN THIS CASE A COVALENT BOND IS FORMEDBETWEEN TWO HYDROGEN ATOMS THE 1s SHELL OF BOTH ATOMS THUS APPEARS FILLED AND STABLE Covalent Molecules and Crystals The covalent bond is the basic bond in ALL organic chemistry, which includes all living creatures, their DNA, RNA, etc. [ web ] Stable CRYSTALS may also be formed by covalent bonding since the covalent bond is COMPARABLE in strength to an ionic bond A good covalent crystal is DIAMOND which forms when CARBON atoms bond * Each atom bonds covalently to FOUR other carbon atoms And so APPEARS to fill its outermost shell This gives rise to the distinctive TETRAHEDRAL bonding Covalent Molecules and Crystals 2 2 2 2 2 1 p s s EACH CARBON ATOM IN DIAMOND IS COVALENTLY BONDED TO FOUR OTHER CARBONS IN A TETRAHEDRAL BONDING ARRANGEMENT
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2 Some properties of COVALENT crystals (continued) Their bonds are highly DIRECTIONAL in nature Covalent Molecules and Crystals CONTOURS OF ELECTRON DENSITY IN COVALENTLY BONDED GERMANIUM NOTE THE HIGH CONCENTRATION MIDWAY ALONG THE COVALENT Ge-Ge BOND Ge HAS A SIMILAR CRYSTAL STRUCTURE TO DIAMOND + + Some properties of COVALENT crystals They form relatively STABLE and HARD crystals Due to their HIGH cohesive energies They are POOR electrical conductors Since there are normally NO free electrons in the crystal Covalent Molecules and Crystals CRYSTAL DIAMOND Si Ge InAs SiC ZnS COHESIVE ENERGY (eV) 7.37 4.63 3.85 5.70 12.3 6.32 ± In a crystal, the electron wave will DIFFRACT from the atoms just as an X-ray diffracts from the atoms ± For what values of the electron wavelength will diffraction occur in a crystal?
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Lecture%2012 - EEE 352 LECTURE 12 Bonding-Metallic crystals...

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