Basic Concepts of Crystal Structure - SCI 401

SCI 401 GENERAL CHEMISTRY Lecturer: Ms. Cristine Joy M. Dalida, LPT Chemistry of Engineering Materials Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS Crystal Structures Crystal Structures  Crystal Structures A crystal is...

SCI 401 GENERAL CHEMISTRY Lecturer: Ms. Cristine Joy M. Dalida, LPT Chemistry of Engineering Materials Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS Crystal Structures Crystal Structures  Crystal Structures A crystal is a solid whose atoms are arranged in a "highly ordered" repeating pattern. These patterns are called crystal systems.  Crystal Structures Lattice points are the positions an atom, ion, or molecule can occupy in a crystal giving it its shape and characteristics. If any of these particles go missing or are not in a correctly ordered arrangement, it can introduce a defect in the crystal. CATEGORIES OF SOLIDS ceramics and polymers highly defined and repeatable arrangements of molecular chains of atoms no periodic arrangement (random order) CATEGORIES OF SOLIDS  Crystalline structure Have highly defined and repeatable arrangements of molecular chains of atoms These materials tend to have sharp melting points. common example: diamonds  Noncrystalline Structure Noncrystalline materials have atoms with no periodic arrangement (random order)  Chemistry of Engineering Materials: Basic Concepts of Crystal CRYSTAL Structures STRUCTURES UNIT CELLS DENSITY COMPUTATIONS Unit Cells  Simple Cubic Crystal Structure The possibility of a unit cell that consists of atoms placed only at the corners of a cube do exist and it is called the simple cubic (SC) crystal structure. ( only one lattice point at each corner of the cube-shaped unit cell )  Simple Cubic Crystal Structure In the simple cubic structure there is only one lattice point at each corner of the cube-shaped unit cell. They mark the position of either a single atom, or the same group of atoms, known as the motif, which is repeated across the lattice. Body-Centered Cubic Crystal StructurE The body-centered cubic (BCC) contains two atoms. It has atoms at each of the eight corners of a cube plus one atom in the center of the cube. Face-Centered Cubic Crystal Structure atoms are arranged at the corners and center of each cube face of the cell Face-Centered Cubic Crystal Structure The face-centered cubic (FCC) has a coordination number of 12 and contains 4 atoms per unit cell.  Chemistry of Engineering Materials: Basic Concepts of Crystal CRYSTAL Structures STRUCTURES UNIT CELLS DENSITY COMPUTATIONS  Density Computations A theoretical density (ρ) can be computed with a knowledge of the crystal structure of a metallic solid through the relationship Wher n = number of atoms associated with each e unit cell A = atomic weight VC = volume of the unit cell NA = Avogadro’s number (6.022 x 1023 atoms/mol) Problem No. 1  Problem No. 2 Strontium (Sr) has an FCC crystal structure, an atomic radius of 0.215 nm and an atomic weight of 87.62 g/mol. Calculate the theoretical density for Sr.  Problem No. 3 Calculate the theoretical density, given that Ta has a BCC crystal structure and has an atomic weight of 180.9 g/mol. Given with radius of 8.23 x 10-9 nm.