Crystalline Solids Crystal Lattice ? A 3D array of points where each point has an identical environment. Unit Cell ? The repeating unit (a unit cell is to a crystal, like a ?brick? is in a house). In a given crystal all unit cells are identical. CaF2 Unit Cell Large cation & Large anion r(Cation)/r(Anion) ~0.72 Small cation & Large anion r(Cation)/r(Anion) ~0.3 Cation Anion Anions and cations want to stay in contact with each other, but when the cation becomes much smaller than the anion, anion-anion contacts prevent this. The solution is to lower the coordination number Ionic radii and coordination number Radius Ratio Rules r(cation)/r(anion) Coordination Number Coordination Geometry ? 0.73 8 Cube ? 0.41 6 Octahedron ? 0.22 4 Tetrahedron ? 0.15 3 Trigonal Planar