Mr. Bouyer
Day 1 | Day 2 | Day 3 | Lab | Skills Test         9 weeks review

  • Describe the arrangement of the particles in each of the seven crystal systems.
  • Describe two types of structural defects in crystals.
  • Describe how water molecules can be included in a crystal lattice.

Solids:
all true solid substances are crystalline.
link to an Internet website about Bucky Balls

According to the Kinetic Theory, the particles of a solid are so closely packed that they travel only a fraction of their diameter before colliding.

This table describes the 7 crystal systems and 14 unit cells.

Crystal System Unit Cell . . . . . Crystal System Unit Cell
1. Cubic simple 8. Orthorhombic body-centered
2. Cubic body-centered 9. Orthorhombic face-centered
3. Cubic face-centered 10. Orthorhombic simple
4. Tetragonal simple 11. Orthorhombic single face-centered
5. Tetragonal body-centered 12. Triclinic simple
6. Monoclinic simple 13. Rhombohedral simple
7. Monoclinic single face-centered 14. Hexagonal simple

rotating crystal lattice A study of solids must become a study of crystals. Geometry explains the macro-structure of crystals, including the angles at which the faces come together. However, the micro-structure of the crystals consists of atoms, which are too small to observe directly.

The atomic theory lets us make two helpful assumptions:

  1. Atoms have a spherical shape.
  2. Negative ions are larger than positive ions.
These two assumptions allow us to construct models of the unit cells making up the crystals of solid matter.

 

When a group of spheres are placed as close together as possible, we begin to have a model similar to the arrangement of atoms in a crystal. With the people at your table, ask your science facilitator for equipment to do the following:

Pack one layer of equal size marbles in a beaker. Now place a second layer on top of the first in an equally close arrangement. Continue this process until you have five layers of close-packed marbles. Like this model, crystals have no lines drawing off the geometric shape of the crystal lattice.

There are two possible lattice shapes in a close-packed arrangement of spheres:

  • hexagonal closest packing, aba   link to a local picture
  • cubic closest packing, abc   link to a local picture
    1. Which close-packed arrangement do you see in the model you have built?
    2. Show the arrangement to your science facilitator.

Day 2

  1. A compound unit cell has more than one type of atom. Crystals of sodium choloride are composed of sodium ions and chlorine ions. Positive sodium ions are smaller than negative chlorine ions. With the people at your table, ask your science facilitator for equipment to do the following:

    For this model, small styrofoam spheres will represent sodium ions and large styrofoam spheres will represent chlorine ions.
    1. Place 4 large spheres around a center large sphere to form the corners of a square. Glue these spheres in place.
    2. Make a second layer like the first.
    3. Place 4 large spheres around a center small sphere to form the corners of a square. Glue these spheres in place.
    4. To each of the three layers, glue small spheres into the spaces around the outer edge of the layer.
    5. Stack the layers on top of each other (do not glue the layers together).

  2. Use the model of the compound unit cell of NaCl that you have made to answer the following questions.
    1. Is the NaCl unit cell a simple cubic, body-centered cubic, or face-centered cubic structure?
    2. How many chlorine ions are arranged around each sodium ion?
    3. How many sodium ions are arranged around each chlorine ion?

Day 3

Important terms:
  • Elementary crystals - crystals bonded by electrostatic (ionic) forces or delocalized electrons.

  • Network crystals (macromolecules) - crystals with covalent bonds.
Explore this Internet Website about Chemistry

The molecules in the crystals of most molecular solids are held together by van der Waals forces. These "weak forces" are fairly easily overcome as the molecules vibrate faster when heated. These molecular solids have low melting points.
In network crystals, atoms in different molecules share electrons, forming covalent bonds between the molecules. This bonding arrangement makes the entire crystal behave as one giant molecule. The covalent bonds are much harder to overcome than van der Walls forces, giving the solid a very high melting point.

an edge dislocation

  • Types of crystal defects:

    1. Point defect - a change in a unit cell.
      • Missing atom
      • Foreign atom
    2. Dislocation (line defect) - unit cells are joined improperly.
      • Edge dislocation - an extra row.
      • Screw dislocation - planes within the crystal are twisted.
    3. Doped crystal - impurities are deliberately added to near perfect crystals. One current application of crystal doping is in the making of transistors and computer components. Impurities are added to pure crystals of semi-conducting elements to "paint" electric conducting pathways on them.

    Hydrated crystals:  (chem lab 084)

  • Liquid crystals:

  • Amorphous materials: link to an Internet website with useful information
    Homework Assignment 173:
    This assignment must be turned in by the beginning of class tomorrow to receive credit.
    Scoring criterialink to a local webpage

    1. Find the melting point of methane and the melting point of diamond. Based on their melting points, describe the bonding between molecules of these substances.

    2. How can crystals with defects useful?

    3. Some anhydrous substances, like silica gel, gain water so easily that they can be used to remove water from other substances. An anhydrous substance used in this way is called a desiccant. Silica absorbing water can be represented by the equation:

      SiO2 + H2O H2SiO3

      How many grams of water can be absorbed by one mole of silica gel?

    4. Compare smectic and nematic liquid crystals.

    5. Describe two ways you could prove that glass is an amorphous material.

    Research Links:

    Chemistry Class