What term refers to a three-dimensional arrangement of atoms in a crystal?

The correct term for a three-dimensional arrangement of atoms in a crystal is "crystal lattice." A crystal lattice describes the organized, repeating pattern of atoms, ions, or molecules in a crystalline material. This concept is foundational in the study of crystal structures, as it encompasses the spatial arrangement and periodicity that characterizes the crystal.

While the term "unit cell" refers to the smallest repeating unit in a crystal lattice, it does not denote the entire three-dimensional arrangement itself. "Molecular structure" pertains more to the arrangement of atoms within a molecule, and "atomic framework" is not a standard term used in crystallography to describe the specific arrangement of crystals. Thus, the distinction lies in the comprehensive nature of the crystal lattice compared to these other terms, making it the most accurate choice for this question.