L5.pdf - Geos 306 Lecture 5 The Packing of Atoms In this...

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Geos 306 Lecture 5The Packing of AtomsIn this lecture we examine what happens when groups of atoms are brought together to form crystals.We start with the packing of atoms inmetalsbecause they are the simplest. We assume that all the atoms in the metal aresphericaland have thesame sizesince we assume that the metal is made of only one type of atom, eg Cu. Recallthe electrondensity map of Cuand how spherical the atoms appeared.There are 2 major packing models that collectively are calledclosest packing. The two models are cubic closest packing (ccp)and hexagonal closest packing (hcp). The models were formalized by John Barlow in the 1800's. Barlow was an Englishmanwho was funded by the French government to determine the most efficient way to pack cannonballs on the deck of a warship.ccp, with packing sequenceabcabcabceg. Cu, Au, Ag, Al, Ni, Pthcp, with packing sequenceababababeg. Mg, Be, Ti, ZrPut a layer of spheres (marbles) on a flat surface and push them together as tightly as you can, then you would find that eachsphere came into contact with 6 other spheres, just like we saw in the Cu electron density map. We call this layer amonolayerand label it "layera".Image from Next, we can add a 2ndlayer on top of the first. There are two places that we can add this layer. These places are equivalent,with the only difference being orientation. We call this layerb.HCP. If a 3rdlayer is put directly over the 1stlayer, then we have the sequenceaba. The 3rdlayer is called layerabecause it is

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Term
Fall
Professor
Barton
Tags
Atom, Electron, Electric charge, Ion

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