At 912 °C, pure iron transforms from BCC α-iron (lattice parameter 0.2904 nm) to FCC γ-iron (lattice parameter 0.3647 nm). Which correctly describes the change in volume per atom? Both lattice parameters are values at 912 °C.

Answer: A. It contracts by about 1%

The volume per atom is the unit-cell volume divided by the number of atoms in the cell. For BCC it is 0.2904³/2 ≈ 0.01225 nm³, and for FCC 0.3647³/4 ≈ 0.01213 nm³, so the change is about −0.97%: a contraction of about 1%. The volume decreases because the structure becomes the more densely packed FCC; calling it an expansion is a sign error. Doubling comes from comparing unit-cell volumes directly (0.3647³/0.2904³ ≈ 1.98), overlooking that the BCC cell holds 2 atoms and the FCC cell 4. This volume change can be observed as a transformation point in dilatometry.