Final answer:
The BCC structure has only two atoms in one cell because there is one-eighth of an atom at each of the eight corners which adds up to one atom, and one atom in the center, totaling two atoms. This structure has a coordination number of eight and occupies about 68% of the total volume.
Step-by-step explanation:
The body-centered cubic (BCC) solid has only two atoms in one cell. This crystal structure has atoms at all of the corners of a cube and a single atom in the center. The corners of the BCC unit cell each contribute one-eighth of an atom to the unit cell (because each corner atom is shared by eight unit cells), which sums up to one whole atom (8 × ⅛ = 1). When you add the one atom from the center, you get a total of two atoms per unit cell. This efficient packing results in the BCC structure occupying about 68% of the total volume, and metals with this structure include elements like K, Ba, Cr, Mo, W, and Fe at room temperature.
Each atom in a BCC arrangement touches four atoms in the layer above it and four atoms in the layer below it, thus having a coordination number of eight. The BCC structure is an example of atomic packing in solids and is often compared with other structures like face-centered cubic (fcc) and hexagonal close-packed (hcp), each of which has a different coordination number and space occupancy.