My Huyen Pham

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Image Contributed by NASA

Crystals, or crystalline, are one of the two broad categories of arrangement of the components of a solid. Crystals make a small percentage of our lives compared to amorphous solids, but they are simpler to understand. Crystalline solids' structure has a repetitive arrangement of their components. For example, say a line of military trainees, in an amorphous arrangement, trainees would be randomly around the area; but with a crystalline arrangement, the trainees would be lined up neatly in rows and columns, analogy-wise of course.

The study of crystals is called crystallography, which emerged in the mid-1600s. Many people today think of crystals as those shining minerals in the caves or buried deep inside the earth. It is true that they can be found there but you can find crystals in your own kitchen! How are they made? What are they made of?

Examples of Crystals shown:




  • Insulin crystals

  • Sodium Chloride (or Halite)

  • Calcite

  • Quartz






Crystalline Solids


As stated earlier before, crystal structures are objects that contains repeating arrangements of their components of each unit cell. Unit cells are the arrangement of the components that are repeated. There are three types of crystalline solids: ionic solids, molecular solids, and atomic solids. Each type is composed of different components, hence, the names of each type. Ionic solids are made of ions, molecular solids are made of molecules, and atomic solids are made of atoms.

Atomic solids occur when noble gases are cooled in extremely low cooling points. The bonds are linked together by weak London dispersion forces.

Ionic solids contain charged particles, positive cations and negative anions. They are linked by electrostatic attractions.

Molecular Solids has the lowest melting points out of the three solids due to the tight bonding of intermolecular forces or dipole-dipole attraction.

pyrite.png
Model of Pyrite's Unit Cell


Components of the unit cell of a crystal will be the same in every unit cell of that crystal. Each kind of crystalline has a different unit cell composition, and has a unique structure to add to it. There are seven structures of unit cells. Here is an example of different cube models of crystals. Each sphere represents a component of the crystal.


015_cubic_crystal_structures.jpg
Each unit cell is not exactly a cube, but the components shape it as one.



Source Citation
"Crystals and crystallography." World of Scientific Discovery. Ed. Kimberley A. McGrath and Bridget Travers. Online ed. Detroit: Thomson Gale, 2007. Student Resource Center - Gold. Web. 16 Nov. 2009. <http://find.galegroup.com/gps/start.do?prodId=IPS&userGroupName=jeff53810>.