Materials -- unit cell question "In terms of the atomic radius, R, determine the distance between the centers of adjacent atoms for the BCC crystal structure along the  direction." I got the answer 4R but it is wrong. I came up with this by assuming that atoms touch each other in the  direction in a BCC structure. In this direction  there is one atom that goes through the (1/2,1/2,1/2) position and half an atom at the origin and half an atom at the (1,1,1) position summing to 4 half's of an atom in length. using the answer 4R I got the lattice parameter a=4R/sqrt(3) which is the true lattice parameter of BCC. I also answered another question: "In terms of the atomic radius, R, determine the distance between the centers of adjacent atoms for the FCC crystal structure along the  direction. " and got the answer 2R*sqrt(2) which was right. This was derived by saying FCC has atoms that touch along the  direction, and then doing geometry to calculate the  direction. any help on this would be great, thanks.