Ans. Some commonly used unit cells in crystal structures include:- Simple cubic unit cell: This unit cell has atoms or ions at the corners of a cube, with no atoms in the center.- Body-centered cubic unit cell: This unit cell has atoms or ions at both the corners and the center of a cube.- Face-centered cubic unit cell: This unit cell has atoms or ions at the corners and the center of each face of a cube.- Hexagonal close-packed unit cell: This unit cell has atoms or ions arranged in a hexagonal pattern, with additional layers stacked on top.These unit cells represent the basic building blocks of crystal structures and help determine the overall arrangement of atoms or ions in a crystal lattice.