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11.7 Metallic Bonding.docx

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CHEM 110
Ariel Fenster

October 27th 11.7 Metallic Bonding The challenge to a bonding theory for metals is to explain how bonding can occur with so few electrons. Metal Properties:  Luster or reflectivity  High electrical conductivity  High thermal conductivity  Malleability and ductility  Electronic emission Key Features of Metals:  Few Valence electrons and many empty orbitals  Metals in crystals surrounded by many neighboring atoms, regular array of the nuclei  For instance, the Electron-Sea Model; is a network of positive ions immersed in a “sea of electrons” (electrons move freely; they are mobile among array of positive charges) Coloumb’s Law: determines the Electron- Sea Model E = k Q1 Q2 / d Luster or Reflectivity:  Free electrons are not localized, therefore they are able to absorb photos of visible light, unlike electrons bound to an atom  This is why metals are opaque, they absorb visible light  Electrons at the surface of a metal are able to reradiate (at the same frequency) light that strikes the surface, which explains lustrous appearance of metals High Electrical Conductivity:  Since electrons are not localized within a bond, they are extremely mobile (they can hop and move)  Electricity comes in one side and hops out the other (electron in, electron out) October 27h 11.7 Metallic Bonding Clicker Question: Does electrical conductivity increase or decrease with increased temperature? Decreases, at higher temperatures, the increased vibration of the positive ions interferes with the motion of the electrons. By interfering with the electrons, the conductivity will decrease. High Thermal Conductivity:  Electrons can move easily, therefore electrons in hot regions migrate to cooler regions and transfer their energy to the crystal lattice  Relationship between thermal and electrical conductivity: strong relationship  those that are good thermal conductors are also good electrical conductors Malleability and Ductility:  Metals can bend; be made into a sheet (malleable)  Metals can be drawn into wire (ductile)  Off set nuclei, and move them around, the electron sea can still cope with those changes  Metal becomes deformed, but still remains a metal  a offset of electrons (positive beside positive) Electronic Emission:  Since electrons are not localized within a bond, they are extremely mobile METALLIC PROPERTIES Luster or reflectivity High electrical conductivity Since electrons are not localized within a High thermal conductivity bond, they are extremely mobile Malleability & ductility Electronic emission th October 27 11.7 Metallic Bonding Band Theory:  Atomic orbitals in metallic crystal mix to form bands of closely spaced g g emetallic orbitals e n n  EEnergy Band: collection of very closely spaced molecular orbital energy levels  Valence Band: electrons responsible for bonding (valence electrons) are found in this band  The excited state produces mobile electrons, by applying a small electric Na Na2 Na 3 Na Na 2 Na3 Na4 potential difference across the crystal  Conduction Band: an energy band that is only partly filled with electrons Example: Na y Half-filyed 2s valence band 2s r r n n ENEERGYYBANND E 2s E Na Na Na Na Na Na 2 3 4 n n Example: Mg Electrons are promoted to the p 8 orbital  Forming a conduction band. y 3p y r r VALENCE BAND n n E 3s E Mg Mg Mg Mg 2 n n CONDUCTION BAND r r e VALENCE BAND e E E CONDUCTION BAND CONDUCTION BAND th October 27 11.7 Metallic Bonding g g e VALENCE BAND e E E CONDUCTION BAND VALENCE BAND Three Possible Conductivities: Mg 1. Metals: the valence band is only partially filled; the valence band also serves as a conduction band 2. In other Metals: the valence band is full, but a conduction band overlaps it r r n n E E CONDUCTION BAND VALENCE BAND CONDUCTIONBAND VALENCE BAND CONDUCTOR CONDUCTOR CONDUCTIONBAND r r n n E E VALENCE BAND VALENCE BAND 3. Intrinsic Semiconductor: the valence band is full and the conduction band is empty; energy between the gap is small enough that some electrons INSULATOR SEMI-CONDUCTOR make the transition between them just by acquiring thermal energy 10 a. Conductivity increases with temperature b. Fixed gap c. Greater the thermal energy the more electrons can transition CONDUCTIONBAND y In semi-conductorstheconductivity r n E increaseswithtemperature.
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