A unit cell is the smallest repeating structural unit of a crystalline solid. It acts like a small tile that, when repeated in three dimensions, builds the entire crystal structure or crystal lattice. There are several different types of unit cells based on their geometry and arrangement of atoms:
Categories of Unit Cells (Bravais lattices):
- Cubic (a = b = c; angles = 90°)
- Tetragonal (a = b ≠ c; angles = 90°)
- Monoclinic (a ≠ b ≠ c; two angles = 90°, one ≠ 90°)
- Orthorhombic (a ≠ b ≠ c; angles = 90°)
- Rhombohedral (a = b = c; angles ≠ 90°)
- Hexagonal (a = b ≠ c; two angles = 90°, one = 120°)
- Triclinic (a ≠ b ≠ c; angles ≠ 90°)
The three primary types of cubic unit cells:
- Simple Cubic (Primitive Cubic)
- Atoms only at the 8 corners
- Contains effectively 1 atom per unit cell (1/8th of each corner atom × 8 corners)
- Least densely packed
- Body-Centered Cubic (BCC)
- Atoms at 8 corners and 1 atom at the center of the cube
- Contains 2 atoms per unit cell (1 from corners + 1 center atom)
- More densely packed than simple cubic
- Coordination number: 8
- Face-Centered Cubic (FCC)
- Atoms at 8 corners and centers of all 6 faces
- Contains 4 atoms per unit cell (1 from corners + 3 from faces)
- Most densely packed of the cubic types
- Coordination number: 12
The unit cell's edges and internal angles characterize its type, and atoms may be located at corners, faces, or inside the cell depending on the crystal structure. In summary:
- A unit cell is the fundamental building block of a crystal.
- Its types are determined by the cell edge lengths and angles as well as atom placement.
- The three main cubic types are simple cubic, body-centered cubic, and face-centered cubic, each with differing atomic arrangements and densities.
This classification is key to understanding properties of crystalline materials like metals and salts. If you want, I can provide more on other unit cell types or examples of materials crystallizing in each type.
