# Crystals.pdf - Solids u2013 the Crystals Just as the human...

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Solids – the Crystals Just as the human body is made up of invisible cells, crystals are also made up of repeating geometrically shaped units called Unit Cells . When the crystal grows, these invisible units cells are stacked on top of each other growing in various directions in a characteristic manner. This characteristic type of growth is called the crystal’s habit . The 7 crystal systems are based on the type of unit cell. For example, a crystal built by cubic unit cells will have right angles between its faces. Sometimes it is difficult classifying a crystal into one of the 7 categories below. For example, NaCl crystals look like a cube. It is in the cubic crystal system since the unit cells are cubic. Yet KMnO 4 does not look like a cube. Yet all the lines of symmetry (equal lines, all at 90 o ) places it in the cubic system. Another way of looking at it is that the geometric container that a KMnO 4 crystal fits best in is a cube. trigonal Crystals are characterized by the length of their sides & the angles between them. ex: Cubic (isometric) means sides are equal & all at 90 o
The 7 types of unit cells along with lattice structures Lattice Structure = how atoms or molecules are packed in a unit cell. How we count particles in a unit cell
Lattice Structures Coordination # = 6 % efficiency = 52% % empty space = 48% Total particles owned per cell = 1 Coordination # = 8 % efficiency = 68% % empty space = 32% Total particles owned per cell = 2 Coordination # = 12 % efficiency = 74% % empty space = 26% Total particles owned per cell = 4 Hexagonal Close-Packed Lattice Coordination # = 12 % efficiency = 74% % empty space = 26% Total particles owned per cell = not listed
How the crystal structure and the lattice structure affects the physical properties. Simple Body centered Face centered Hexagonal close-packed Substance Unit cell Lattice Structure Density Melting Point (K) Sodium Osmium Bromine Silicon Fluorine Tin Tungsten X-Ray diffraction helps us in identifying crystal structure
Crystallography problem : Determining the lattice structure of a crystal using X-Ray diffraction: Sodium crystallizes in a __________unit cell (look at periodic table) that through X-Ray diffraction has an edge of 430pm. The density of Sodium is 0.963g/cm 3 and the atomic mass is 23. How many atoms of Na are in that unit cell?_________ Therefore what type of lattice structure must we have? ________________(check your answer with the periodic table.)
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