What is a Crystal? A crystal is a solid substance made of atoms, molecules or ions that form regular repeating patterns called a crystal lattice. The regular patterns of atoms in crystals cause crystals to grow or cleave in geometric shapes. The atoms in glass are not arranged in regular patterns.
Crystals formed from Sublimation Phenol CrystalsIodine Crystals
Crystals From Solution Nickel (II) sulfate Ferrous Ammonium Sulfate Chrome Alum
Atoms, Ions, Molecules and the Unit Cell The unit cell is the smallest portion of the entire crystal that contains all of the identifying characteristics of the crystal. Crystals form as a result of bonds (attractions) between atoms, ions and molecules. The particles align themselves to maximize the number of attractions (for example, Cl – and Na + attract each other).
Symmetry of Crystals Crystals form in seven geometric forms called crystal systems. The crystal systems are named cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral. Each crystal system has a unique shape.
Cubic Cubic crystals have all edges of equal length. Cubic crystals have all angles at 90 0. These crystals also have 3-fold axes. Pyrite Crystals Halite, NaCl
Tetragonal Crystals that form the tetragonal crystal structure have 2 of their 3 edges having the same length. All of the angles are 90 0. These crystals also have one 4-fold axis.
Hexagonal Crystals that form a hexagonal shape have 2 of 3 edges that are the same lengths. The angles are of different degrees. One angle is 90 0, another at 60 0, and the third at 120 0.
Orthorhombic In a crystal showing the orthorhombic formation, all three sides have different lengths however all angles are 90 0.
Monoclinic A crystal exhibiting a monoclinic structure will have different lengths for each side, will have two angles equaling 90 0 and one angle that does not.
Triclinic A crystal that exhibits a triclinic structure will have all sides of different lengths and all of the angles are also different.
Rhombohedral Crystal showing this structure will have all sides equal however, none of the angles are equal.
Optical Properties of Crystals Fluorescence – the crystal will “glow in the dark” Above: Crystals in white light. Left: Crystals in UV light.
Optical Properties of Crystals Interference – the making of rainbows Above: A bismuth crystal that shows color due to the accumulation of oxides. Left: A portion of mica that forms rainbows as light goes through it.
Optical Properties of Crystals Birefringence – the splitting of light to cause “double vision.”
Setting Up the Experiment Measure 50 mL of water into a graduated cylinder. Make sure the meniscus, that’s the bottom of the curve at the top of the liquid, is touching the 50 mL line.
Setting Up the Experiment To start the experiment, you will need to obtain a graduated cylinder and a plastic cup. You will be making a device for a standard measurement.
Setting Up the Experiment Pour the 50 mL of water into the plastic cup. Using a permanent marker, draw a line around the plastic cup at the top of the water level. You will now have your standard liquid measurement for most of the crystal labs.
Setting Up the Experiment Put spoonfuls of solute into the styrofoam cup. The amount of spoonfuls are given in the lab write up. Pour water from clear plastic cup into styrofoam cup. Stir water and solute until all solute is dissolved.
Setting Up the Experiment Cut a piece of string slightly longer than the height of the cup. Tie one end of the string to the plastic spoon. Place the other end of the string into the solution. Rest the spoon across the cup as shown. Cover the cup with a piece of paper or paper towel and leave undisturbed overnight to one week.
Results of the Experiment Remove string from solution and examine crystals.
Crystals Made of Two Compounds Chrome Alum is a crystal that is formed from the simultaneous growing of potassium chromium sulfate and potassium aluminum sulfate. Here the chromium and the aluminum are distributed throughout the crystal based on the concentration of the potassium chromium sulfate.
Chrome Alum By examining the crystals, a color difference is seen. The intensity of the purple increases with increased concentration of KCrSO 4.
Substitution of Cr for Al in Alum Crystals The deep purple dots stand for Cr, the pale dots stand for Al