Density = (Z × M) / (a³ × Nₐ)
Crystalline solids have atoms arranged in repeating three-dimensional patterns. The unit cell is the smallest repeating unit. The three cubic types are: Simple Cubic (SC) with 1 atom/cell and 52.4% packing (a = 2r); Body-Centered Cubic (BCC) with 2 atoms/cell and 68.0% packing (a = 4r/√3); Face-Centered Cubic (FCC) with 4 atoms/cell and 74.0% packing (a = 2√2·r). Crystal density is calculated from ρ = ZM/(a³Nₐ), where Z is atoms per cell, M is molar mass, a is edge length, and Nₐ is Avogadro's number. Most metals crystallize in BCC (Fe, W, Cr) or FCC (Cu, Al, Au, Ag) structures. Understanding unit cells is essential in materials science, metallurgy, and semiconductor physics.