Question

Question #82125

Answer 1

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 `"BAs"`.

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

The unit cell of `"BAs"` is

(From Wikimedia Commons)

There are 4 `"As"` atoms (purple) inside the unit cell.

We count the `"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 `"B"` atoms and 4 `"As"` atoms or 4 formula units of `"BAs"`.