Close Packing in 3D

Nature is efficient. When atoms pack together, they try to minimize "void space." The most efficient way to pack spheres of equal size leads to an Atomic Packing Factor of 0.74.

The Two Paths of the Third Layer

Every close-packed structure starts with a hexagonal base (Layer A). The next layer (Layer B) sits in the "valleys" of Layer A.

When we add the third layer, we have two choices:

1. HCP (ABAB...): The third layer atoms are placed in the voids that are directly above the atoms of Layer A.
2. FCC (ABCABC...): The third layer atoms are placed in the "alternative" voids that were left empty by Layer B.

Interactive Stacking Lab

Use the controls to build the lattice layer by layer. Pay close attention to the C-Layer—notice how it doesn't align with either A or B.

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Sanjib's Tip for JEE: Remember that in both HCP and FCC, the Coordination Number is 12. Each atom touches 6 in its own layer, 3 above, and 3 below. The only difference is the orientation of those triangles!