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

Here's why:

* Crystal Structure: Crystals are characterized by their regular, repeating arrangement of atoms, ions, or molecules.

* Unit Cell: The unit cell is the basic building block of this arrangement. It's the smallest repeating unit that, when stacked together in three dimensions, forms the entire crystal lattice.

* Shape Retention: The unit cell contains all the information about the crystal's shape, symmetry, and dimensions.

* Repeating Pattern: The crystal's overall structure is created by the repeating pattern of unit cells.

Let me know if you'd like more information on crystal structures or unit cells!