Crystal structures lab student

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					Crystal Structures Lab                v4’      Dr. Breinan          Chemistry                     p. 1
    The regular geometric shapes of crystals reflect the orderly arrangement of the atoms, ions, or
molecules that make up the crystal lattice. Many different types of compounds will form crystals when
solid. At room temperature, metals and ionic compounds are almost always in crystal form. Only a few
covalently bonded compounds (molecules) are solid at room temperature. Of the molecular compounds
that are solid, some form crystals, and others do not... the ones that do not are referred to as amorphous.
Many covalently bonded compounds that are liquid or gaseous at room temperature will crystallize at
lower temperatures (the formation of ice is a common example). In general, molecular crystals are more
complicated and will not be studied in this lab.
    In this experiment, you will gain insight into the ways in which metallic or ionic crystals are formed
by using styrofoam spheres as models of atoms or ions. You will investigate three basic crystal
structures made out of atoms: hexagonal closest packing, face-centered cubic packing, and body-
centered cubic packing. When all of the particles are the same size, as would happen in a pure metal,
these are the simplest structures which normally form. Using the styrofoam models, you will determine
the number of nearest neighbors (the coordination number) of the particles in each of these structures. In
a simple crystal structure, all atoms or ions of the same kind should have the same coordination number.
In your model the atoms will touch each other.
    When ions form crystals, the cations and anions are normally different sizes. Often times, the
resulting lattice (a term that refers to the repeating layout of particles in the crystal) resembles one of the
basic structures, however the difference in size will cause slight alterations in the packing arrangement.
The coordination number in an ionic compound refers to the number of nearest neighbors of opposite
charge. You will examine these effects in the crystal lattices of sodium chloride (NaCl).

- To gain familiarity with the geometry of crystal structures.

Pre-Lab: Complete on a separate page
1. Which of these can form crystals: metal atoms             ionic compounds      molecules
2. List the three basic crystal structures that will be studied in this lab.
3. What is meant by the coordination number of: a) an atom in a metallic crystal; b) an ion in an ionic
4. Why do you think the nearest neighbors in ionic compounds are oppositely charged?
5. What is meant by a close-packed plane?
6. Calculate the exact ratio of the radii of sodium ions to chloride ions (express as one decimal)?
7. Should separate layers be connected together?
8. Should you disassemble each model right after you complete it?

• Do not fool around or “play” with the spheres. Make only the structures you are directed to in
  the lab. Do not toss spheres. Do not make snowmen.
• Each crystal structure will be formed in several layers. Unless told to, it is better to not
  connect the three different layers in any model together.
• Remember, crystals are repeating structures: in a real crystal each layer you build would
  contain many, many more atoms. Also, many more layers like those you build would be found
  on top of and below those of your model.
• Use toothpicks to connect the spheres (pipe cleaners or coffee stirrers may serve as alternates)
• When you have completed the models, you may work on the write-up.
• Disassemble all structures and remove all toothpicks only when you are completely done with
  the entire lab (not as you complete each one).
• Use common sense when atoms don’t seem to touch only because your model is not perfect.
  Also, remember that these models contain high degrees of symmetry.
• Orient each layer exactly as shown in the diagrams.
Crystal Structures Lab            v4’       Dr. Breinan     Chemistry                     p. 2
Specific instructions for each structure:

Part A. Hexagonal Closest Packing (HCP)
1. Connect three sets of 2-inch diameter spheres as shown in Figure 1. Each of these
   arrangements of atoms demonstrate the closest together you can place spheres of the same
   size on a flat surface. This arrangement is known as a “close-packed plane.”
    Figure 1

2. Position one of the layers of 3 spheres on the laboratory table so that one triangle is pointing
    “away” from you.
3. Place the layer of 7 spheres over this layer of 3 spheres in such a way that the center sphere fits
    into the depression at the center of the first layer.
4. Now, place the second layer of 3 spheres over the center sphere of the second layer. The
    spheres in this third layer should be directly above the spheres in the first layer. The
    arrangement you have constructed, expanded to include billions of atoms, is found in crystals
    of zinc, magnesium, and many other metals. Record the number of nearest neighbors (the
    coordination number) of the central sphere in the structure you formed. Convince yourself
    that any other atom has the same coordination number if the structure extends in all
    directions. Retain this model for Part C.

Part B. Face-Centered Cubic Packing (FCC)
5. Construct the layers shown in Figure 2. Use 2-inch diameter foam spheres.

    Figure 2

6. Place the first layer, which contains 5 spheres, on the laboratory table. Place the second layer
    over the first in such a way that the 4 spheres of the second layer rest in the spaces between
    the corner spheres of the first layer.
7. Now, place the third layer over the other two in such a way the spheres of the third layer are
    directly over the spheres in the first layer. Record the coordination number of an atom in this
    arrangement. Hint: remember that this structure repeats in all directions. You may need to
    imagining more layers. Note how there is a sphere in the center of each side (also called a
    “face”) of a cube, hence the name “face-centered cubic.” It is this type of packing that is
    characteristically found in copper, silver, aluminum, and many other metals.

Part C. Comparison of Hexagonal Closest Packing and Face-Centered Cubic Packing
This is a difficult exercise... if you do not see it immediately, move on to parts D and E. Your
   instructor will come around and show you how to do this.
8. Return to the model of hexagonal closest packing constructed in Part A. Rearrange the top
    layer (of 3 spheres) by rotating it 60 degrees (the triangle will now point ”toward” you)
Crystal Structures Lab                    v4’    Dr. Breinan    Chemistry                   p. 3
9. Now, as you rotate this rearranged model, look for 4 spheres forming a square facing you.
    Once you have found the four-sphere square, obtain the model made in Part B. Remove the
    top layer of the Part B model and place it over the four-sphere square of rearranged model A.
    Note that this new model contains a face-centered cube, just as model B did.

Part D. Body-Centered Cubic Packing (BCC)
10. Use 2-inch foam spheres to construct the layers depicted in Figure 3. Leave a space of
   approximately 0.5 cm between the spheres in the square pattern.

      Figure 3

11. Place the single sphere in the center of the first layer and then position the third layer in such
    a way that its spheres are directly over the spheres of the first layer. NOTE: the top and
    bottom layers should NOT touch- if they do, reduce the distance between the spheres in the
    squares. Record the coordination number of an atom in this structure. Note how there is a
    sphere in the center of a cube (in the center of the “body”) hence the name “body-centered
    cubic.” This type of packing is typical of the alkali metals, which include sodium and

Part E. The Sodium Chloride Lattice
Alternate assignment: if small spheres are not available, instead of making the model in class,
    use the diagram below or picture in your text to answer the questions on the sodium chloride
12. Ionic crystals are formed by packing positive and negative ions alternately into a lattice. A
    single sodium ion has a diameter of 0.20 nm while a chloride ion has a diameter of 0.36 nm.
    Because the diameters are in the ratio of roughly 1:2, the relative sizes of Na+ and Cl- can be
    approximated by using 1-inch and 2-inch spheres.
13. Use model B, with its 2-inch spheres, to represent the face-centered cubic arrangement of the
    chloride ions in a sodium chloride crystal. Insert the thirteen 1-inch spheres, representing
    sodium ions, into the holes between the chloride ions in each layer. Note that NaCl lattice is
    an “interpenetrating set of face-centered cubes” (each overlaps the other), one set of cubes
    made up of Na+ ions and the other made up of Cl- ions. Record the coordination number of
    sodium ions. Record the coordination number of chloride ions.

                              Na a
         Na                                 Cl
                        Cl                                     Cl                       Na Cl
                                                                       Na     Cl
 Cl                                  Na
              Na Na
         Cl                  Cl Cl
                        Na                                     Na     Cl               Cl   Na
Na                                   Cl                                        Na
              Cl Cl
         Na                                 Cl
                        Cl                                      Cl    Na               Na Cl
Cl                                   Na                                        Cl
Space between ions added for clarity                           Space “filling” model
Crystal Structures Lab                v4’     Dr. Breinan          Chemistry                        p. 4
Questions Sheet:
          structure                                particle                      number
   Hexagonal closest packing                      any atom
     Face-centered cubic                          any atom
     Body-centered cubic                          any atom
       Sodium chloride                           sodium ion
       Sodium chloride                           chloride ion

1. Think about the three basic structures (hexagonal closest packing, face-centered cubic packing, and
  body-centered cubic packing) and their coordination numbers. Note that all three structures were made
  from identical spheres (same size and mass) just as the atoms in a metal element are identical.
  Suppose one element could be put together in any one of these structures. Do you think the density of
  the element would change in these three different crystal structures? Comment on all three structures
  and explain your reasoning.

2. Some materials can change structures as they are heated or cooled. Iron exists as three different forms
  as shown in the table below. Even though each of these forms are solid, they are still referred to as
  different phases! The changes in structure occur at the temperatures shown in the diagram.
            Form of iron:          alpha-ferrite                gamma-ferrite                delta-ferrite
         Crystal structure:     body-centered cubic          face-centered cubic          body-centered cubic

                                    Forms of iron at different temperatures:
form:      alpha-ferrite                     gamma-ferrite                       delta-ferrite

T (°C)                        940                                      1410

For each of the following temperatures, list the form (alpha, gamma, or delta) and the coordination
      number of the crystal structure of iron:
         25°C                       500°C                     1000°C                       1500°C

3. In the sodium chloride structure, which ions most closely surround Na+ ions?
   Which ions most closely surround Cl- ions?

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