Question #82125

1 Answer
Ernest Z.
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 "BAs".

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

The unit cell of "BAs" is

BAs
(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".

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