Consider the unit cells shown here for three different structures that are commonly observed for metallic elements. (b) Which structure(s) corresponds to the least dense packing of atoms?

A unit cell is the smallest repeating unit of a crystal lattice that reflects the overall symmetry and structure of the entire crystal. It defines the arrangement of atoms in a solid and can vary in shape and size. Understanding unit cells is crucial for analyzing the packing efficiency and density of different crystalline structures.

Atomic packing efficiency (APE) refers to the fraction of volume in a crystal structure that is occupied by atoms. It is calculated by comparing the volume occupied by the atoms to the total volume of the unit cell. Structures with higher APE have denser packing, while those with lower APE indicate less dense arrangements, which is essential for determining which structure corresponds to the least dense packing.

Different types of crystal structures, such as face-centered cubic (FCC), body-centered cubic (BCC), and simple cubic (SC), exhibit varying atomic arrangements and densities. Each structure has unique characteristics that influence the physical properties of the material. Identifying these structures helps in understanding their packing efficiency and density, which is key to answering the question about the least dense packing.