Final answer:
Iron has a body-centered cubic (BCC) crystal structure below 910 °C, with atoms at all corners and the center of the cube, resulting in two atoms per unit cell and a coordination number of eight.
Step-by-step explanation:
The crystal structure of iron, specifically at temperatures below 910 °C, is body-centered cubic (BCC). In this arrangement, the cubic unit cell has atoms at all of its corners, with one atom located in the center of the cube. There is effectively one-eighth of an atom at each of the eight corners, which together make up one whole atom, plus the one atom at the center, totaling two atoms per unit cell.
The BCC crystal structure is more densely packed than a simple cubic structure, with atoms occupying about 68% of the total volume. Among the metals that crystallize in a BCC arrangement are potassium (K), barium (Ba), chromium (Cr), molybdenum (Mo), tungsten (W), and iron (Fe) at room temperature. These elements are isomorphous, meaning they crystallize with the same structure.
In a BCC structure, each atom touches four atoms in the layer above it and four in the layer below, resulting in a coordination number of eight. This refers to the number of nearest-neighbor atoms an atom has in its crystal structure.