What is a Unit Cell?

• A. A single atom.
• B. The smallest repeating pattern of crystalline solids.
• C. A large cluster of atoms in the crystal.
• D. The entire crystal.

Answer: (B)

A unit cell is the smallest repeating block of a crystal lattice that, when translated through space, builds the entire crystal. It encodes the arrangement of atoms within that repeating region (the basis) and is defined by the lattice parameters and angles. Because the unit cell represents the fundamental repeating unit, it isn’t limited to a single atom—the cell may contain several atoms depending on the structure. Repeating copies of this unit cell tile space to form the whole crystal. A single atom or a large, non-repeating cluster does not capture the periodic nature of the lattice, and the entire crystal is simply many unit cells arranged together.