Lecture 3: Atomic Bonding & Crystal Structures PDF

Properties and Testing of Electromechanical Materials MEC 147N Lecture 3 Atomic Bonding & Crystal Structures Dr. Mohamed M. AbdelKader Hassan Outline Atomic structure and electron configurations Interatomic bonding: primary and secondary o Ionic...

Properties and Testing of Electromechanical Materials MEC 147N Lecture 3 Atomic Bonding & Crystal Structures Dr. Mohamed M. AbdelKader Hassan Outline Atomic structure and electron configurations Interatomic bonding: primary and secondary o Ionic o Covalent o Metallic o Secondary WHY STUDY Atomic Structure and Interatomic Bonding? Some of the properties in solid materials strongly depend on the type of atomic structure and interatomic bonding. Atomic Structure Some of the important properties of solid materials depend on geometrical atomic arrangements, and also the interactions that exist among constituent atoms or molecules. This lecture consider several fundamental and important concepts—namely, atomic structure, electron configurations in atoms and the various types of primary and secondary interatomic bonds that hold together the atoms comprising a solid. Bohr atomic model Electrons are assumed to revolve around the atomic nucleus in discrete orbitals. Each atom consists of a very small nucleus composed of protons and neutrons, which is surrounded by moving electrons. Both electrons and protons are electrically charged, which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Periodic table 1- IONIC BONDING EXAMPLE 2- COVALENT BONDING 3- METALLIC BONDING  SEA OF ELECTRONS Free floating electrons act like a glue and hold the structure in place. COMPARISON For Metallic bonding Metallic Crystal Structures FCC (Face Centered Cubic) BCC (Body Centered Cubic) HCP (Hexagonal Close Packed) FCC (Face Centered Cubic) Examples:γ-iron, Cu,Au,Al, Ni  ATOMIC PACKING FACTOR (APF) Packing factor is the fraction of the volume of a unit cell that is occupied by "hard sphere" atoms or ions. It is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume. It is dimensionless and always less than unity. UNIT CELL  In these structure, there are 8 corner atoms and 6 atoms at center of the face Therefore, n = no. of atoms per unit cell. Nc= total no. of corner atom in unit cell. Nf= total no. of face atom in unit cell. Ni = center or interior atom Nc = 8, Nf=6, Ni=0 n= (Nc/8)+(Nf/2)+(Ni/1) =(8/8)+(6/2)+(0/1) Simple cube model =4 Each corner atom is shared by 8 unit cells BCC (Body Centered Cubic) Examples: V, Ta, Cr UNIT CELL  In these structure, there are 8 corner atoms and one atom at in the interior i.e. in the centre of the unit cell with no atom on face. Therefore, n = no. of atoms per unit cell. Nc= total no. of corner atom in unit cell. Nf= total no. of face atom in unit cell. Ni = center or interior atom Nc = 8, Nf=0, Ni=1 n= (Nc/8)+(Nf/2)+(Ni/1) =(8/8)+(0/2)+(1/1) =2 HCP (Hexagonal Close Packed) Examples: Mg, Zn, Cd, Ti HCP in 2-D c/a=1.633  For hexagonal structure, the corner atoms are shared by 6 cells (3 from below and 3 from above), face atoms are shared by adjacent 2 cells, and atoms in the interior are shared by only one cell. n= (Nc/6)+(Nf/2)+(Ni/1) For HCP structure , there are 12 corner atoms, 2 atoms at the centers of the above two faces and 3 atoms in the interior of the unit cell.  Nc = 12, Nf=2, Ni=3 n= (Nc/8)+(Nf/2)+(Ni/1) =(12/6)+(2/2)+(3/1) =6