Question
Mon March 24, 2014 By: Pb Nazar

# Why formula unit of hexagonal close packing is 6 while that cubic close packing is 4 ?

Tue March 25, 2014

For cubic closed packed structure or FCC:

Contribution by atoms on the corners = 1/8 x 8 = 1 atom

Contribution by atoms on the faces = ½ x 6 = 3

Therefore, number of atoms present in the unit cell = 1+3 = 4 atoms

For hexagonal closest packed structure:

Contribution by atoms of the hex shape at top = 1/6 x 6 = 1 atom

Contribution by atoms of the hex shape at bottom = 1/6 x 6 = 1 atom

Contribution by the one atom at the centre of top and bottom hex each = 1

Contribution by the three atoms in the center = 3 atoms

Therefore, number of atoms per unit cell = 1+1+1+3 = 6 atoms

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