# See the image below..?

Sep 24, 2016

The compound could be ${\text{CaF}}_{2}$.

#### Explanation:

The big atoms ($\boldsymbol{\text{B}}$) are counted as:

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

The small atoms ($\boldsymbol{\text{A}}$) are counted as:

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

Total = 4 $\text{A}$ atoms.

The formula is ${\text{B"_8"A}}_{4}$ or ${\text{AB}}_{2}$.

The formula must be either ${\text{CaF}}_{2}$ or ${\text{SO}}_{2}$.

${\text{SO}}_{2}$ is a covalent compound. It would exist as discrete molecules in the unit cell.

Hence, the compound must be ${\text{CaF}}_{2}$ (fluorite).

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

(From geologycafe.com)