Crystal Structure
Definition and meaning of Crystal Structure in chemistry.
Crystal structure refers to the specific, orderly three-dimensional arrangement of atoms, ions, or molecules within a crystalline solid, described by a repeating unit cell, while the crystal system classifies that unit cell by its geometric symmetry.
In more detail
The unit cell is the smallest repeating box that, when translated in three dimensions, generates the entire crystal lattice. Crystal systems are classified by the unit cell's edge lengths (a, b, c) and angles (alpha, beta, gamma) into seven categories: cubic, tetragonal, orthorhombic, rhombohedral (trigonal), hexagonal, monoclinic, and triclinic. Combining these with centering types (primitive, body-centered, face-centered, base-centered) yields the 14 Bravais lattices, which underlie all known crystalline materials. Crystal structure governs a solid's density, cleavage, optical properties, and diffraction pattern.
Key facts
| Field | Physical Chemistry |
|---|---|
| Number of crystal systems | 7 (cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic, triclinic) |
| Number of Bravais lattices | 14 |
| Determined experimentally by | X-ray diffraction (Bragg's law) |
Sodium chloride (NaCl) crystallizes in the cubic system with a face-centered cubic (rock salt) structure, in which Na+ and Cl- ions alternate at the corners and face centers of the unit cell, each ion surrounded octahedrally by six ions of opposite charge.
Frequently asked questions
What is the difference between a crystal system and a unit cell?
The unit cell is the actual repeating three-dimensional block of atoms/ions with specific dimensions; the crystal system is the symmetry category (based on relative axis lengths and angles) that the unit cell belongs to.
How is crystal structure determined experimentally?
X-ray diffraction is the standard method: a beam of X-rays scattered by the periodic lattice produces a diffraction pattern that, via Bragg's law, reveals atomic spacing and arrangement.