Which term describes categories of crystals based on their symmetry and internal structure?

• A. Crystalline
• B. Crystal Axes
• C. Crystal Systems
• D. Defect

Answer: (C)

Crystals are grouped by how their atoms are arranged in three-dimensional space, focusing on symmetry and the arrangement of the unit cell. This categorization is known as crystal systems. Each system reflects a specific combination of unit-cell lengths, angles, and symmetry elements, defining how the overall lattice repeats. That direct link to symmetry and internal arrangement is why crystal systems is the best term for describing these categories. The word crystalline describes the material having a crystal structure rather than naming the categories themselves; crystal axes are the reference directions used to describe geometry, not a classification; and a defect refers to irregularities in the lattice, not to how crystals are grouped by symmetry.