Atomic Structure: Crystals of Pure Compounds
Upon successful completion of the lab experiment, the student will:
- recall and define the following fundamental chemistry terms:
- Unit cell of crystals
- Crystal structure
- perform the following calculations:
- Atoms per unit cell
- Empty space in a unit cell
- determine the structure designation of a crystal
- determine the unit cell designation for the crystal of a chemical compound
- calculate the number of atoms in and empty space in a crystal unit cell
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Crystal or Lattice Structures
When pure compounds solidify, there are specific structures that form. We call these structures crystals or lattices. Various factors effect this arrangement and if there are other substances in the solution that is solidifying, the crystals will have non-standard structures. When light is shined through the crystals, refraction occurs and the refraction patterns can be used as a first step identifying the compound or substance. Perhaps one of the most important use of lattice refraction patterns to discover the actual chemical structure of a complex chemical compound was when James Watson and Francis Crick used x-ray crystallography to identify the double helix structure of DNA. During X-ray crystallography, light in the x-ray energy range is shined through a pure crystal of a compound and the refraction of the x-rays is used to calculate the locations of specific atoms in the crystal.
When we look at the macroscopic structures of crystals, we can categorize them into seven types. These types are:
- Cubic or Isometric - not always cube shaped! You'll also find octahedrons (eight faces) and dodecahedrons (10 faces).
- Tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms.
- Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together).
- Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon.
- Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division.
- Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes.
- Monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids.
Ionic Crystal Unit Cells
If we focus on the simplest arrangements of the atoms in the ionic lattice (crystal) structure we will find that there are four designations that occur.
- If you do not have black party plates, cut the black construction paper to fit tightly in the bottom of each plate and place inside.
- Make a supersaturated solution with the Epsom salt by bringing 120 mL (about 0.5 cup) of distilled water to the almost-boiling point, then transfer the hot water to a beaker or transparent plastic cup. Add 5 tablespoons Epsom salt, stir, and continue adding Epsom salt until no more salt can be absorbed by the water. You will know that the solution is supersaturated with the salt remains in the bottom of the cup or beaker no matter how hard you stir.
- Pour the solution into a party plate and carefully label the plate with tape that has the name of the compound.
- Repeat steps 1 -3 with each of the remaining compounds. Make sure to rinse the beaker or plactic cup with distilled water each time, before making the new solution. For the alum, begin with 3 tablespoons; for the salt begin with 1 tablespoon, and for the sugar begin with 4 tablespoons. The saturated sugar solution will be thick.
- Set the saucers in an undisturbed place and observe them over the next week until all the liquid evaporates. When all the liquid is gone you should see crystals coating the sides and bottoms of the plates.
- Examine the crystals with the magnifying glass. Draw or sketch what you see.
Atomic Structure: Crystals of Pure Compounds Names ________________
NOTE: Because the Sketch of the crystal will be hand-drawn, you will need to scan your completed report and email the pdf file for the report (individual images will not be accepted).
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Questions to be Answered
- For each of the compounds, draw or sketch what you see with the magnifying glass on the above table. Look up the proper crystal strtucture type for each compound and indicate it on the table above. Does what you saw for each of your compound crystals match the standard structure type? If not, what would cause the differences between what you saw from the experiment and the standard structure type?
- Identify the compounds that are ionic. Indicate the unit cell designation for all compounds on the table above.
- For each of the unit cell types, calculate the total number of atoms in each cell. Remember that only a portion of each atom is "in" the cell for all of the unit cells with the exception of the body-centered cubic cell, that has one compete atom in it, in addition to the partical atoms at the corners.
- How much empty space is there in a simple cubic structure (this should be a percentage or decimal equivalent)? A body-centered cubic structure?
- About how small would an atom have to be to fit in between the atoms in a body-centered cubic crystal structure (in general terms, as a percentage of the atoms already in the unit cell)?