Magnesium Metal

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Download the magnesium structure in pdb format (courtesy of WebElements.com)

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Download the magnesium structure in pdb format (courtesy of WebElements.com)... or
Download the Magnesium unit cell ( 5kB Jun8 07) in .cmdf format, which is viewable in CrystalMaker.

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Questions

1. All of the atoms have the same atomic radius. What is the coordination number of any Mg atom in this structure?
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It is extremely difficult to visualize the CN of Mg in this structure without the complete "ball-and-stick" rendering showing the bonds between the Mg atoms. For CrystalMaker users, perhaps the best way to answer this question is by using the "Add/Delete Bonds" tool to draw the bonds from a central Mg (within the body of the structure) to each of its nearest neighbors. Rotate the structure after adding each bond to make sure that it the the same length as the other bonds you draw.

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The CN of Mg metal is 12.

2. Orient the stucture so that you are looking down the c-axis. You should see a repetition of the Mg atoms along the a-axis. What is the stacking arrangement of the layers (ABCABC, or ABAB) along the a-axis? Is this a cubic close packed (CCP) or hexagonal close packed (HCP) structure?
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The atoms repeat every other layer (ABAB...), so the close packing is hexagonal.
3. Looking down on the planes of atoms you see hexagonal rings connected to form a network.
   
   What happens if we rotate these planes? Remember to consider the atoms shown in this view as part of an infinite plane of atoms. Rotate the the structure 120°. Do similar atoms occupy exactly the same positions, or are they in a different arrangement? Try the same with a 90° and 180° rotation.
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   The atoms repeat for 120 and 180° rotations.