Chromium forms BCC lattice. Here ball represent Cr and this being BCC lattice. As you know contribution of corner ball is 1/8. And ball in centre will contribute fully to unit cell. So 8*(1/8)+ 1=2 So it contains 2 atoms per unit cell. read more
I don’t like to make assumptions but I will try to answer the question simply as it pertains to Chromium, the element. read more
There are 5 atoms in a unit cell of ZnO wurtzite structure. 3 Zn atoms and 2 oxygen atoms. read more