Question

the atomic packaging factor of iron decreases by what percentage you when it is cooled from 1000 to 900°c

Answer 1

It is known that the atomic packing factor (APF) of the body center cubic (BCC) unit cell (APF=[(The volume of atom x number of atom in a unit cell)/volume of the unit cell)] is less (APF=0.68) than that of the one in the Face center cubic (FCC)unit cell (APF=0.74). In other words, the gamma iron phase (FCC) is less susceptible to host foreign atoms than the alpha iron phase(BCC).So to answer your question, the volume change (decrease) in iron is accompanied with phase transformation of alpha iron to gamma iron, 0.74-0.68=0.06