Final answer:
The smallest collection of atoms that defines the geometry of a crystal structure is known as a unit cell, which is a necessary component of the crystal lattice and repeats to form the entire crystal.
Step-by-step explanation:
The smallest collection of atoms that uniquely define the crystal structure geometry is a unit cell. A unit cell is the smallest portion of a crystal lattice that shows the three-dimensional pattern of the entire crystal. Essentially, a crystal is made up of the repetitive stacking of its unit cell in all three spatial dimensions.
In the context of crystal systems, specifically the cubic crystal system, there are three distinct types of unit cells: (1) simple cubic, (2) face-centered cubic, and (3) body-centered cubic unit cells. These variation in unit cells contribute to the different properties and arrangements found within crystalline materials.
It's important to note that although each unit cell can be conceptualized independently, they do not exist in isolation but as part of the larger repetitive lattice structure. These unit cells are the building blocks of the crystal lattice and are crucial for understanding the structure and properties of crystal materials.