We may consider a crystal as a collection of identical unit cells.
For a substance containing more than 1 kind of atom, the ratio of atoms
in the unit cell must be exactly the same as in the entire crystal.
For example, the unit cell H3 contains 1 pink and (6 x 1/3 = 2) purple
circles, or a ratio of 1:2.
H3 unit cell
The full lattice must also have a 1:2 ratio. Because the drawing
does not end at the edges of the unit cells, the ratio you see (10:30)
is not representative.
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Structure of Crystals
From Unit Cell to Lattice
From Lattice to Unit Cell
Packing & Geometry
Simple Cubic Metals
Close Packed Structures
Body Centered Cubic
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