Question

Germanium has a cubic unit cell with a side edge of 565pm. The density of germanium is 5.36g/cm^3. What is the crystal system adopted by germaium.

Answer 1

Find the mass in grams in one cell.

mass=volume*denstiy

volume=(565pm)^3=1.8E-22cm^3

mass=1.8E-22 cm^3*5.36g/cm^3= you do it.

Ok, how many Ge atoms is that?

Now, given the number of atoms per unit cell, is it face centered, body centered, or simple cubic

Answer 2

I get 8 atom/unit cell and the answer in my book was face centered. But i am a bit confused don't FCC have 4 atoms/unit cell?

Answer 3

Yes, fcc cells have four atoms/unit cell; however, if y ou look at the link provided by Bob Pursley, you will see two things there. First, the statement that Ge is like diamond and the Ge structure is "two interpenetrating face centered unit cells". I was confused, also, when I worked the problem and obtained 8. I IMed Bob P, we discussed it, and he found the link shown above. In fact, we talked about how the student was supposed to know that Ge had the diamond structure and that it was interpenetrating face centered unit cells? We wondered if this was an upper upper class or a grad student problem. Both of us thought the problem was too difficult for lower level classes but of course we aren't teaching the course. I hope this helps explain.