Crystal Structures
Many naturally occuring solids have distinct geometrical shapes.
A solid that exists in a definite three-dimensional geometrical shape is known as a crystalline solid or crystal.
Crystalline solids have a characteristic shape regardless of the size of the piece.
Crystalline solids are composed of orderly arrangements of atoms, ions, or molecules.
There are seven basic shapes identified in naturally occuring solids.
The arrangement of atoms within the crystal is known as crystal lattice.
This arrangement can be determined by a preocess called X-ray diffraction.
The smallest portion of a crystal lattic that shows the three-dimensional pattern of the entire lattice is called a unit cell.
When the crystal systems are combined with the various possible lattice centerings, we get 14 different crystal lattices, called Bravais Lattices.
Lattice systems:
| The 7 lattice systems | Axial Distances (edge lengths) | Axial Angles | The 14 Bravais Lattices Possible Variations |
|---|---|---|---|
| 1. triclinic | a ≠ b ≠ c | α ≠ β ≠ γ ≠ 90° | Primitive |
| 2. monoclinic | a ≠ b ≠ c | α = γ = 90°, β > 90° |
Primitive, Base-centred |
| 3. orthorhombic | a ≠ b ≠ c | α = β = γ = 90° | Primitive, Body-centred, Face-centred, Base-centred |
| 4. rhombohedral | a = b = c | α = β = γ ≠ 90° | Primitive |
| 5. tetragonal | a = b ≠ c | α = β = γ = 90° | Primitive, Body-centred |
| 6. hexagonal | a = b ≠ c | α = β = 60°, γ = 120° |
Primitive |
| 7. cubic | a = b = c | α = β = γ = 90° | Primitive, Body-centred, Face-centred |