When it says Lithium crystallizes in a body centered cubic unit cell. what is the mass of one unit cell....

I did my math, but it says I have to multiply by 2. I am confused in why I have to do that?

I also have trouble understanding what is a body-centered cubic cell.

Question

When it says Lithium crystallizes in a body centered cubic unit cell. what is the mass of one unit cell....

I did my math, but it says I have to multiply by 2. I am confused in why I have to do that?

I also have trouble understanding what is a body-centered cubic cell.

aaronq · Answer

Because there are 2 atoms (represented by spheres (more like circles in this drawing)) per unit cell (cube).
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It's kinda hard to see (and impossible to draw here) how only 1/8 of each sphere at each corner is INSIDE the cube. The picture i attached is a little better. If you still cant see it, get a styrofoam sphere (from arts and crafts/dollar store) and cut it into 8 equal pieces.