Q2(c). Iron: atomic radius from the b.c.c. cell; volume change on going to f.c.c.
Iron crystallizes in a b.c.c. unit cell at room temperature (ρ = 7.86 g/cm³). Calculate the radius of an iron atom in this crystal. At temperatures more than 910 °C, iron prefers to be in f.c.c. If we neglect the temperature dependence of radius of iron on the grounds that it is negligible, use this information to determine whether iron expands or contracts when it undergoes transformation from b.c.c. to f.c.c. structure. The atomic mass of iron is 55.845 u.
Answer in brief
r = √3a/4 = 124.2 pm (1.242 Å), from the cell edge a = 286.8 pm; iron contracts, the volume per atom falling from 11.80 to 10.84 ų, a decrease of 8.1 %.
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Coversbcc to fcc transitionatomic radiusunit cell densitypacking fractioniron allotropy
All parts of Question 2
- 2(a) Hydrogen atom: the transition n = 101 → 10010 marks
- 2(b) Shapes of the sp³d, sp³d², dsp² and sd³ hybrids and the d orbitals used20 marks
- 2(c) Iron: atomic radius from the b.c.c. cell; volume change on going to f.c.c.20 marks