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Properties of metals

Properties of metals. Metals (75% of elements) Lustrous (reflect light) (almost) all solids malleable & ductile good conductors of heat and electricity oxides are basic, ionic solids aqueous cations (n+). Bonding in metals. Free Electron Model

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Properties of metals

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  1. Properties of metals • Metals (75% of elements) • Lustrous (reflect light) • (almost) all solids • malleable & ductile • good conductors of heat and electricity • oxides are basic, ionic solids • aqueous cations (n+)

  2. Bonding in metals • Free Electron Model • Metals are positive ions in “sea” of nearly free electrons • Electrons bond metal ions together but are free to roam the crystal lattice. • Explains high electrical and thermal conductivity

  3. Bonding in Metals • Band Theory • Atomic orbitals (AO) mix to form molecular orbitals (MO). • Start with 2 AO, end with 2 MO • Start with n AOs, end up with n MOs • In metals energy difference between orbitals in valence band is small. • Orbital form a continuous “band” of allowed energy states.

  4. Conduction and Insulation Metal Valence electrons do not fill available orbitals (not enough electrons) Insulator or semiconductor Valence band is full (or completely empty). Energy gap separates valence band from empty orbitals.

  5. Band Gaps • Insulators: The energy gap is > 3.0 eV (= 290 kJ/mol) in insulators • Semiconductors: The energy gap is between 0.05 and 3.0 eV in semiconductors (kJ/mol)

  6. SEMICONDUCTORS Add impurities(dopants)to semi-conductor If impurities donate extra electrons, then the semiconductor is n-type e.g. P impurities in Si. If impurities accept electrons, then the semiconductor is p-type e.g. B impurities in Si. n-type: negative charge carriers (electrons). p-type: apparent positive charge carriers (holes).

  7. Silicon • Properties: • shiny, silvery gray • brittle • poor thermal conductor • semiconductor • Uses: • alloy (with Al, Mg) • silicone polymers • electronics, solar cells: • very pure silicon (<1ppb) is required.

  8. Zone refining to get pure Si

  9. Diodes • A diode is a semiconductor with a p-type material bonded to an n-type material. • Solar cells (photovoltaics) and light emitting diodes (LEDs) are both diode devices. When no current flows

  10. Diodes • A diode allows current to flow in only one direction • Electrons can flow from n-type to p-type under forward bias • In a solar cell, light excitation makes current flow in the opposite direction Current flows when the diode is forward biased

  11. Light Emitting Diode When electrons combine with holes, light is emitted. The energy of light (E = h) is the same as the band gap energy Eg The band gap energy depends on the material used to make the diode.

  12. LED Materials

  13. LEDs: LightEmittingDiodes More energy efficient than incandescent lighting LEDs producing visible light are typically made from doped Aluminum-Gallium-Arsenide (AlGaAs)

  14. Where are LEDs used?

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