Structures of Metals: Close Packing.

Metallic Bonding in the Solid State:

For metals the atoms have low electronegativities; therefore the electrons are delocalized over all the atoms.  We can think of the structure of a metal as an arrangement of positive atom cores in a sea of electrons. For a more detailed picture see "Conductivity of Solids".




To maximize the bonding in a metal it makes sense to pack as many atoms around each other as possible, maximize the number of nearest neighbors (called the "coordination number") and  minimize the volume.

Close packed layers of atoms.

If we treat the atoms as spheres and consider all the atoms in the solid to be of equal size (as is the case for elemental metals), the most efficient form of packing is the close packed layer.  This is illustrated below where it is clear that close-packing of spheres is more efficient than, for example, square packing.

Below on the left is a square packed array compared to the more densely packed close packed array.
Within the square packed layer the coordination # of each atom is 4, in the close packed layer it is 6.

   

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