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CaF2 crystal structure can
be thought of in the following way:
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The fluorine anions (green) form a face
centered cubic structure
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The calcium cations fit into the
tetrahedral spaces (blue) formed between the fluorine layers.
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Notice that there are 4 atoms (green) around
every hole (blue) but 8 holes around every atom.
thus, the ratio of
holes : atoms = 2 : 1.
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the cube itself is highlighted by 8
interconnected small silver balls (inside the green atoms).
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for the atom count, we see 4 green Ca2+
(fully) inside the cube and 8 holes (blue), each occupied by a F-.
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