Crystals Essays - Crystallography, Phase Transitions, Free Essays

Crystals Essays - Crystallography, Phase Transitions, Free Essays Crystals Crystals What is a crystal? A crystal is a solid substance with definite geometric shapes and molecules that are arranged in a repeating pattern (Comptons ). Crystals have fixed angles between its faces, which have distinct edges. If the faces of a crystal can reflect light, then it will sparkle (Stangle). Crystals have constant angles because of the regular arrangement of its particles. There are seven types of crystals: cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic (Dr. Boyle). Crystals are classified by the shapes of their lattice or the regular, periodic configuration of particles (American Heritage). There are two different types of lattices. There are primitive lattices where there is only one lattice point per unit cell. There are also non-primitive lattices. They are the ones with more than one lattice point per unit cell. Crystals can also be classified by their physical/chemical properties. There are four types of crystals classified this way. They are covalent, metallic, ionic, and molecular (Dr. Boyle). A covalent crystal is a crystal which is one big molecule. They usually have extremely high melting points. An example of a covalent crystal is a diamond and zinc sulfide (Dr.Boyle). Metallic crystals have high melting points and densities. They are metal atoms which sit on lattice sites while the outer electrons from these atoms flow freely around the lattice (Dr. Boyle). Ionic crystals are crystals where the individual atoms do not have covalent bonds among themselves. These atoms are held together by electrostatic forces. They are also very hard and have a relatively high melting point. Sodium chloride (NaCl, salt) is an example of an ionic crystal (Dr. Boyle). Molecular crystals are crystals where there are recognizable molecules in the structure and the crystal is held together by non-covalnet interactions such as hydrogen bonding (Dr. Boyle). Sugar is an example of a molecular crystal. These crystals tend to have low melting points and are soft (Dr. Boyle). Crystals start growing by a process called nucleation. They are produced in solutions. The temperature and pH balance of the solutions must be controlled well. Crystals start growing one of two ways: unassisted nucleation, crystals that start with the molecules themselves or assisted nucleation, with the help of some solid matter already in the solution (Dr. Boyle). In unassisted nucleation, molecules of the solute are in the solution. Most of the time the solute molecules only see the solvent molecules around them. Sometimes the solute molecules can see other solute molecules and they become attracted to one another if the compound is solid and pure (Dr. Boyle). These molecules will stay together for a little while and will eventually be separated by other internal forces. However, sometimes the molecules will stay together long enough to meet up with a third, then a fourth, and then even a fifth solute molecule. When this happens the combined attraction force eventually becomes stronger than the other forces within the solution (Dr. Boyle). This attraction force tends to disrupt the formation of these aggregates. When this happens a protocrystal or a pre-crystal becomes a nucleation site (Dr. Boyle). As the protocrystal floats around in the solution, it encounters other solute molecules. These other molecules feel the attractive force of the protocrystal and decide to join the group of molecules (Dr. Boyle). This is when the crystal begins to grow. Crystals grow form the outside instead of the inside. The crystal molecule grows until it can no longer remain dissolved in the solution and falls out of the solution (Dr. Boyle). After this happens, the other solute molecules grow on the surface of the crystal. The crystal gets bigger until there is an equilibrium, or a state of a chemical reaction in which a forward and reverse reaction occur at equal rates so that the concentration of the reactant and product do not change with time (American Heritage). When there is equilibrium between the solute molecules in the crystal and the solute molecules in the solvent the crystal no longer get bigger (Dr. Boyle). In assisted nucleation the same process is followed as in unassisted nucleation. The only difference is that a solid surface such as a stone or brick acts as a meeting for all of the solute