[Il Divo]

Hey, all, just a quick question regarding molecular crystals. I've been asked to determine the # of atoms occupying a face-centered orthorhombic structure.

Now, if this were a one component material (essentially any pure metal), this would be completely straight forward. One atom at each corner + an atom along each face would lead to 4 atoms total in the unit cell.

My issue is that I'm working within the context of diatomic halogens. Looking up some of their images on-line, they don't seem to fall within that neat little arrangement that would describe the above metal orthorhombic structure (one atom at each corner, 1 along each face).

I was just curious if anyone had any tips on how to determine # atoms for molecular solids in a unit cell. Do I just say "There are 4 halogen atoms per unit cell"? Or would it potentially be "4 halogen *molecules* per unit cell? Or something else completely that I'm missing?

Thanks in advance.

[mjc123]

Well, what replaces metal atoms in a fco structure? Halogen atoms or halogen molecules?
You could just do it the same way as for the metal, just slightly more complicated, go through the unit cell counting how many atoms are in the unit cell, how many cells is each atom split between?
Or you could look up the unit cell parameters for (say) iodine, and calculate the density on the assumption that there are n I₂ molecules per unit cell. What value of n gives the best match to the reported density?