Calculating the Mass of an Aluminum Unit Cell

[tica86]

Aluminum has a density of 2.699 g/cm3 , and the atoms are packed in a face-centered cubic crystal lattice. Find the mass of a unit cell.

-----I already know the answer is 1.792*10^-22g/unit cell

I have a question about how you get the answer:
I know you multiply 4.480*10^-23g * 4 atoms/1 unit cell= the answer

where does the 4 atoms come from?

 

[Borek]

Geometry of the cell. In fcc all atoms lie on the cell walls, so they are only partially belong to the cell. Sum these fractions.

 

[tica86]

Geometry of the cell. In fcc all atoms lie on the cell walls, so they are only partially belong to the cell. Sum these fractions.

got it, thanks!