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08 - Chemistry 2000 Lecture 8 Metallic bonding Marc R...

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Chemistry 2000 Lecture 8: Metallic bonding Marc R. Roussel
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The free electron model of metals I Among all the properties of metals, the ones that stand out the most are probably their very high electrical and thermal conductivities. I One early and surprisingly successful model of metals is the free electron model which assumes that the valence electrons are free to travel throughout the metal. I Another way to think about it is that the lattice sites are occupied by cations. The valence electrons roam the rest of the space. I Because the cationic cores are small, we typically treat the metal as an empty box containing electrons, i.e. ignore the cations completely.
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Example: A sodium ion has a radius of 99 pm, but the metallic radius is almost twice as large, at 186 pm. I Sodium crystallizes in a bcc lattice. I The volume per atom (including voids around the atoms) is 3 . 98 × 10 - 29 m 3 . I The volume of the ion is 4 . 06 × 10 - 30 m 3 . I Only about 10% of the space is occupied by the ions. The rest is available to the valence electrons.
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Properties explained by the free electron model High electrical conductivity: Current is carried by the mobile electrons. High thermal conductivity: Heat can also be carried by the mobile electrons. Optical properties: Electrons can have a wide range of energies and so can absorb and re-emit at a variety of wavelengths, which makes metals opaque and reflective. Electromagnetic theory is required to give a more detailed account of the optical properties of metals.
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