See the image below..?

enter image source here

Please explain your answer.

1 Answer
Ernest Z.
Sep 24, 2016

The compound could be #"CaF"_2#.

Explanation:

The big atoms (#bb"B"#) are counted as:

  • 8 completely inside the cell = 8 × 1 atom = 8 atoms

The small atoms (#bb"A"#) are counted as:

  • 1 at each corner = 8 × ⅛ atom = 1 atom
  • 1 at each face = 6 × ½ atom = 3 atoms

Total = 4 #"A"# atoms.

The formula is #"B"_8"A"_4# or #"AB"_2#.

The formula must be either #"CaF"_2# or #"SO"_2#.

#"SO"_2# is a covalent compound. It would exist as discrete molecules in the unit cell.

Hence, the compound must be #"CaF"_2# (fluorite).

Here's a better picture of the fluorite unit cell, that makes it easier to count the atoms.

Fluorite unit cell
(From geologycafe.com)

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