# Question 82125

Mar 25, 2017

A unit cell can contain more than one formula unit of a compound.

#### Explanation:

A formula unit in chemistry is the smallest whole-number ratio of atoms in a compound.

For example, a formula unit of boron arsenide is $\text{BAs}$.

A unit cell may contain more than one formula unit of a compound.

The unit cell of $\text{BAs}$ is

(From Wikimedia Commons)

There are 4 $\text{As}$ atoms (purple) inside the unit cell.

We count the $\text{B}$ atoms (pink) as follows:

8 color(red)(cancel(color(black)("corners"))) × "⅛ B atom"/(1 color(red)(cancel(color(black)("corner")))) + 6 color(red)(cancel(color(black)("faces"))) × "½ B atom"/(1 color(red)(cancel(color(black)("face")))) = "4 B atoms"#

Hence, a unit cell contains 4 $\text{B}$ atoms and 4 $\text{As}$ atoms or 4 formula units of $\text{BAs}$.