The smallest group of particles in a crystal that retains the shape of the crystal is called the

Unit Cell

Explanation:

A unit cell is the smallest fundamental particle of a crystal structure that represents the structure of a crystal. Long-range, three-dimensional repetitiveness of this unit cell forms a crystal. A unit cell is composed of ions or atoms in a particular arrangement forming a well-defined structure. Crystal have a sharp melting and boiling point and when cut using a tool, they give a sharp edge on the pieces.