Question

See the image below..?

Please explain your answer.

Answer 1

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.

(From geologycafe.com)