Nature of Materials & Chemical Bonding PDF

**NATURE OF MATERIALS & CHEMICAL BONDING** **Material Science and Engineering** **Material Science** - Involves investigating the relationships that exist between the structures and properties of materials. **Materials Engineering** - Involves structure-property correlations, designing...

**NATURE OF MATERIALS & CHEMICAL BONDING** **Material Science and Engineering** **Material Science** - Involves investigating the relationships that exist between the structures and properties of materials. **Materials Engineering** - Involves structure-property correlations, designing or engineering the stricture of a material to produce a predetermined set of properties. **Structure** - Nebulous term that deserved some explanation. - Relates to the arrangement of its internal components - May be classified on the basis of size. - Several levels: subatomic structure, atomic structure, nanostructure, microstructure, macrostructure. **Classification of Structural Elements** 1. **Subatomic Structure** - Involves electrons within the individual atoms - Their energies and interactions with the nuclei 2. **Atomic Structure** - Organization of atoms to yield molecules or crystals 3. **Nanostructure** - Deals with aggregates of atoms that form particles (nanoparticles) that have nanoscale dimensions (less that about 100 nm). 4. **Microstructure** - Direct observation using some type of microscope ( dimensions between 100nm and several millimeters) 5. **Macrostructure** - Viewed with the naked eye (with scale range between several millimeters and on the order of a meter). **Atomic Structure** - consist of an extremely small, positively charged nucleus surrounded by a cloud of negatively charged electrons. **Chemical Bonding** - attraction between two or more atoms, and is what forms a chemical - Atoms form bonds with other atoms in order to have a full outer shell of electrons like the noble gases. **Ionic Bonding** - electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion. - Metals form positive ions because they lose electrons to become stable. - Often solids, forms crystals - Example: NaCl, Silver iodide, NaF **Covalent Bonding** - sharing of electrons to form electron pairs- shared pairs or bonding pairs, between atom. - The stable balance of attractive and repulsive forces between atoms when they share electrons. - Example: H2O, CO2, NH3, O3, C6H12O6, CH4 **Metallic Bonding** - sharing of free electrons among a structure of positively charged ions (cations). - It accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and lustre. **Crystal Structure** - highly ordered and symmetrical chemical bonds - has a basic component called the unit cell, which is the smallest portion of the structure that contains all of its geometric properties. **Lattice and Unit Cells** - used in the context of crystal structures; - three-dimensional array of points coinciding with atom positions (or sphere centers) **Metallic Crystal Structures** 1. **Face-Centered Cubic (FCC)** - 8 corner atoms, 6 face atoms, and no interior atoms. - Ex: copper, aluminum, silver, and gold 2. **Body-Centered Cubic (BCC)** - 8 corner atoms, no face atoms and 1 center atom. - Ex: Chromium, iron, tungsten 3. **Hexagonal Closed Pack (HCP)** - has a unit cell that is hexagonal. - top and bottom faces of the unit cell consist of 6 atoms that form regular hexagons and surround a single atom in the center. **Non-Crystalline Structures** - lack a systematic and regular arrangement of atoms over relatively large atomic distances. - also called amorphous (meaning literally "without form"), or supercooled liquids, **Classification of Engineering Materials** 1. **Metals** - composed of one or more metallic elements and often also nonmetallic elements in relatively small amounts. - atoms in metals and their alloys are arranged in a very orderly manner. - relatively dense in comparison to the ceramics and polymers. - relatively stiff and strong, yet are ductile and are resistant to fracture which accounts for their widespread use in structural applications. 2. **Ceramics** - compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. - relatively stiff and strong--- stiffnesses and strengths are comparable to those of the metals, they are typically very hard. - extreme brittleness (lack of ductility) and are highly susceptible to fracture. 3. **Polymers** - plastic and rubber materials - organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements - have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms - have low densities - extremely ductile and pliable **Material Selection for Engineering Materials** - Availability of the materials - Suitability of the materials for the working conditions - The cost of the materials