Final answer:
A unit cell is the smallest repeatable unit of a crystal lattice, necessary to form the entire lattice through three-dimensional repetition. The unit cell defines the underlying spatial arrangement of atoms in a crystal.
Step-by-step explanation:
The correct answer to the student’s question is B. Unit Cell. A unit cell is the smallest portion of a crystal lattice that when repeated in three dimensions, forms the entire lattice. This concept is fundamental in the study of crystallography and material science.
A crystal lattice is an array of points representing the periodic arrangement in which one or more atoms, ions, or molecules are repeated at each lattice point. The entire crystal is therefore seen as being made up of these basic repeating units, much like a three-dimensional pattern formed by stamping the same design over and over again.
The unit cell can be envisioned like stacking layers of identical spheres in a repeating pattern. For example, in the simple cubic structure, each corner atom is shared among eight unit cells, and thus only one-eighth of each atom belongs to any one unit cell, resulting in exactly one atom per unit cell.