Sodium crystallizes in bcc unit cell with edge length 4.29 Å.  What is the length of body-diagonal of the unit cell?

7.4 Å

The correct answer is option 3. 7.4 Å.

For a body centered cubic unit cell (bcc)

Radius (r) = \(\frac{a × \sqrt{3}}{4}\) = \(\frac{4.29 × 10^{-10} × 1.732}{4}\) = 1.86 x 10^-10 m = 1.86 Å

Length of body diagonal = 4r = 4 x 1.86 = 7.44 Å