Packing Efficiency

The formula for density is:

where:
= number of atoms per unit cell,
= molar mass,
= Avogadro’s number,
= edge length.
For fcc, , .
For bcc, , .
Let be the molar mass of the metal. Then the density for fcc is:

And for bcc:

The ratio of densities is:

Simplify:


Nearest Integer:
The nearest integer is: