Material Structure & Properties Flashcards
Ionic Bonding Description
Description: Found in compounds composed of metallic and nonmetallic elements; involves atoms at the extremes of the periodic table.
Ionic Bonding Process
Process: Metallic atoms give up their valence electrons to nonmetallic atoms; atoms achieve stable inert gas configurations (filled orbital shells) and acquire electrical charges (become ions).
Ionic Bonding Characteristics
Characteristics: Ionic bonding is nondirectional; stability requires a three-dimensional arrangement of positive and negative ions as nearest neighbors; bonding energies range between 600 and 1500 kJ/mol, reflecting high melting temperatures; typical materials include ceramics, which are hard, brittle, and electrically and thermally insulative.
Covalent Bonding Description
Description: Found in materials with small differences in electronegativity (near each other in the periodic table).
Covalent Bonding Process
Process: Atoms achieve stable electron configurations by sharing electrons; each atom contributes at least one electron to the bond; shared electrons belong to both atoms.
Covalent Bonding Characteristics
Characteristics: Directional bonding; common in nonmetallic molecules (e.g., Cl2, F2) and compounds (e.g., CH4, H2O, HF); found in elemental solids like diamond, silicon, and compounds such as gallium arsenide (GaAs).
Covalent Bonding Strength
Bonding Strength: Varies widely; very strong in diamond (high melting temperature of 3550°C), weak in bismuth (melting temperature around 270°C); many covalently bonded materials are electrical insulators or semiconductors.
Metallic Bonding Description
Description: Found in metals and their alloys.
Model: Valence electrons are not bound to any particular atom, forming a “sea of electrons.”
Metallic Bonding Characteristics
Characteristics: Nonvalence electrons and atomic nuclei form ion cores with a net positive charge; free electrons shield positively charged ion cores from repulsive forces, resulting in nondirectional bonding.
Conductivity: Metals are good conductors of electricity and heat due to free electrons; most metals and alloys fail ductilely at room temperature.
Metallic Bonding Strength
Bonding Strength: Varies; energies range from 62 kJ/mol for mercury to 850 kJ/mol for tungsten.
Van Der Waals Forces
Weak electric forces attract neutral molecules.
Present in gasses, liquefied/solidified gasses, and most organic liquids/solids.
Named after Dutch physicist Johannes Diderik van der Waals (1873).
Postulated while developing a theory on real gas properties.
Solids held by van der Waals forces have lower melting points and are softer compared to those held by ionic, covalent, or metallic bonds.
Mixed Bonding
A chemical bond where both ionic and covalent character are present within a single compound, meaning that electrons are partially transferred between atoms (ionic) and partially shared (covalent) due to a difference in electronegativity between the atoms involved; essentially, a combination of the two primary bonding types
Unit Cell
The atomic order in crystalline solids shows that small groups of atoms form a repetitive pattern. To describe crystal structures, the structure is divided into small repeating units called unit cells. For most crystal structures, these unit cells are parallelepipeds or prisms with three sets of parallel faces. A unit cell is selected to represent the symmetry of the crystal structure, and all atom positions in the crystal can be generated by translating the unit cell along its edges by integral distances. The unit cell acts as the basic structural unit or building block of the crystal structure, defining the structure through its geometry and the arrangement of atoms within it.
There are four main types of crystal structures that are covered in Material Science: FCC, BCC, HCP, and SC.
Face Centered Cubic
Also known as cubic packed
Atoms are located at each of the 8 corners as well as in the centers of each of the 6 faces
Follows an ABCABC close packing pattern - there are 3 repeating layers, where the atoms of the third layer are located above holes in the first and second layers
Densest of the cubic packing arrangements, with an atomic packing factor of 0.74
Each unit cell contains 4 atoms and has a side length of A = 4R√2
Each atom in the matrix has a coordination number of 12
Body Centered Cubic
Atoms are located at each of the 8 corners as well as in the center of the cubic cell
Less dense than FCC, with an atomic packing factor of 0.68
Each unit cell contains 2 atoms and has a side length of A = 4R√3
Each atom in the matrix has a coordination number of 8
Hexagonal Close Packing
Another close-packed arrangement
Composed of two hexagons of 6 atoms each, an additional atom in the center of each hexagon, and a triangle of atoms in between the two hexagons
Differs from FCC in that HCP follows an ABAB packing pattern - there are only 2 repeating layers, where the atoms of the third layer are located above the atoms of the first layer, not above gaps
Has an atomic packing factor of 0.74, the maximum possible
Each unit cell contains 6 atoms and has two parameters, A (side length) and B (height)
Each atom in the matrix has a coordination number of 12
Simple Cubic
Simple cubic is a very basic arrangement, only containing atoms at each corner of the unit cell
SC is the least dense, with an atomic packing factor of 0.52
Each unit cell contains 1 atom and has a side length of A = 2R
Each atom in the SC matrix has a coordination number of 6
Atomic Packing Factor (APF)
The atomic packing factor (APF) is a measure of how efficiently atoms are packed within a unit cell of a crystal structure. It is defined as the ratio of the total volume of atoms within a unit cell to the volume of the unit cell itself. To find the APF, one assumes that atoms are hard spheres and calculates the volume occupied by these spheres within the unit cell. The formula for APF is: APF = (volume of atoms in a unit cell) / (total unit cell volume). For example, in the face-centered cubic (FCC) structure, with four atoms per unit cell, the APF is 0.74, meaning that 74% of the unit cell’s volume is occupied by atoms, while the rest is empty space.
Coordination Number
The coordination number refers to the number of nearest-neighbor atoms surrounding a given atom in a crystal structure. It indicates how many other atoms are in direct contact with an atom. To find the coordination number, look at the arrangement of atoms in the unit cell and count the nearest neighbors. In the FCC structure, for example, each atom has 12 nearest-neighbor atoms, which can be visualized as four atoms surrounding it on the same plane, four atoms in the plane above, and four in the plane below. In contrast, the body-centered cubic (BCC) structure has a coordination number of 8, as the central atom is in contact with the eight corner atoms of the unit cell.
Crystallinity
Crystallinity refers to the degree of structural order in a solid, indicating how well the atoms or molecules are arranged in a repeating pattern. Ceramics vary in crystallinity, from highly crystalline, vitrified fired ceramics to amorphous glasses.
Crystalline
Crystalline solids are composed of atoms, molecules, or ions arranged in an ordered pattern extending in all three spatial dimensions
Large crystals are identifiable by their macroscopic geometrical shape, with flat faces and specific, characteristic orientations
Crystal structures are formed by repeating units called unit cells
Poly-Crystalline
Semi-crystalline structures have both crystalline and amorphous properties, also known as semi-crystalline structures
These structures contain true crystal portions with mixed sizes and orientations
Semi-crystalline solids are heavily bonded but lack the rigidity and constant structure of fully crystalline solids
Almost all metals and many ceramics are polycrystalline
Amorphous
Amorphous structures have little to no crystal properties
They possess short-range order but have significantly less chain linkage compared to crystalline structures
Common types of amorphous solids include gels, thin films, and glass
