This type of crystal structure has a hexagonal prism shaped unit cell with atoms centered at all twelve corners, plus an atom centered in the middle of both of the top and bottom faces, with an additional three atoms in the plane between the top and bottom faces. Each
atom touches the six in the same plane as it, plus three in each of the planes immediately
above and below it. So therefore each atom would be touching twelve other atoms. This
pattern holds six atoms per unit cell. The ideal ratio for the distance between the top and
the bottom planes of the unit cell, over the distance between atoms on the same plane
should be 1.633, although there are some materials that deviate from this ratio. The atomic
packing factor for a Hexagonal Close Packed crystalline structure is 0.74, which is the
highest possible value for any arrangement of spheres of identical radius, hence the name
close packed. Another arrangement which is also close packed is the Face Centered Cubic.
Some of the materials that are composed of Hexagonal Close Packed unit cells at room
temperature include Beryllium, Cadmium, Cobalt, Magnesium, Zinc, and
Titanium. Note that these metals tend to have high density. This is directly a result of the
high atomic packing factor.
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