The density of body-centred cubic is a little challenging:
Get the mass of TWO atoms of the element that is being packed. You can base this off of n=N/NA where N = 2 atoms, and then use mass = moles times molar mass
Get the volume of the unit cell. This is tougher but doable. We derive the formula edge length a = 4/sqrt(3) times r, which gives us the side length of the unit cell. We cube this to get the volume.
Density equals mass over volume with those two numbers. Easy! ha

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