Chemistry of Engineering Materials: Metals PDF

Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals  LEARNING OBJECTIVE ► Introduce Metal ► Describe the o...

Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals  LEARNING OBJECTIVE ► Introduce Metal ► Describe the occurrence and abundance of metals in the Earth’s crust. ► Explain the processes involve in the metallurgy ► Explain the concept of the Band Theory of Electrical Conductivity ► Discuss the periodic trends of some metals and their reactivity.  Metal Metal is an element, compound or alloy that is a good conductor of both electricity and heat. Metal crystal structure and specific metal properties are determined by holding together the atoms of a metal. https://www.youtube.com/watch?v=vOuFTuvf4qk  Metal With the exception of hydrogen, all elements that form positive ions by losing electrons during chemical reactions are called metals. They are characterized by bright luster, hardness, ability to resonate sound and are excellent conductors of heat and electricity. Metals are solids under normal conditions except for Mercury. Metal  Physical Properties of Metal ✔ State: Metals are solids at room temperature with the exception of Hg, which is liquid at room temperature (Ga is liquid on hot days). ✔ Luster: Metals have the quality of reflecting light from their surface and can be polished e.g., Au, Ag and Cu. ✔ Malleability: Metals have the ability to withstand hammering and can be made into thin sheets known as foils. E.g., a sugar cube sized chunk of gold can be pounded into a thin sheet that will cover a football field.  Physical Properties of Metal ✔ Ductility: Metals can be drawn into wires. For example, 100 g of silver can be drawn into a thin wire about 200 meters long. ✔ Hardness: All metals are hard except sodium and potassium, which are soft and can be cut with a knife. ✔ Valency: Metals typically have 1 to 3 electrons in the outermost shell of their atoms.  Physical Properties of Metal ✔ Conduction: Metals are good conductors because they have free electrons. Silver and copper are the two best conductors of heat and electricity. Lead is the poorest conductor of heat. Bismuth, mercury and iron are also poor conductors. ✔ Density: Metals have high density and are very heavy. Iridium and osmium have the highest densities whereas lithium has the lowest density. ✔ Melting and Boiling Points: Metals have high melting and boiling points. Tungsten has the highest melting and boiling points whereas mercury has the lowest. Sodium and potassium also have low melting points.  Chemical Properties of Metal ✔ Electropositive Character: Metals tend to have low ionization energies, and typically lose electrons (i.e. are oxidized) when they undergo chemical reactions. They normally do not accept electrons. For example: Alkali metals are always 1+ (lose the electron in s subshell) Alkaline earth metals are always 2+ (lose both electrons in s subshell) Transition metal ions do not follow an obvious pattern, 2+ is common (lose both electrons in s subshell), and 1+ and 3+ are also observed Chemical Properties of Metal  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals Occurrence of Metal  Occurrence of Metal Clay (mineral of Aluminum) Bauxite (principal ore of Aluminum) Occurrence of Metal Manganese nodule Principal Types of Minerals  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals Metallurgy Principal steps Production of metal Production of metal Blast Furnace Production of metal Principal steps Metallurgy  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals Band Theory of Electrical Conductivity ► In solid-state physics, the band structure of a solid describes those ranges of energy, called energy bands, that an electron within the solid may have (“allowed bands”) and ranges of energy called band gaps (“forbidden bands”), which it may not have. ► Band theory models the behavior of electrons in solids by postulating the existence of energy bands.  Band Theory of Electrical Conductivity Band theory - a model use to study metallic bonding ⮚ states that delocalized electrons move freely through “bands” formed by overlapping molecular orbitals. This theory can also be applied to certain elements that are semiconductors. Semiconductors  Semiconductors Semiconductors are materials that have properties in between those of normal conductors and insulators; they are often produced by doping.  Semiconductors Semiconductors are materials that have properties of both normal conductors and insulators. Semiconductors fall into two broad categories: Intrinsic semiconductors - composed of only one kind of material; silicon and germanium are two examples. These are also called undoped semiconductors or i-type semiconductors.  Semiconductors Extrinsic Semiconductors - are intrinsic semiconductors with other substances added to alter their properties — that is to say, they have been doped with another element. There are two types of extrinsic semiconductors that result from doping: 1. n-type for negative, from group V, such as phosphorus 2. p-type for positive, from group III, such as boron.  Extrinsic semiconductors N-Type Semiconductors are a type of extrinsic semiconductor in which the dopant atoms are capable of providing extra conduction electrons to the host material (e.g., phosphorus in silicon). This creates an excess of negative (n-type) electron charge carriers. P-Type Semiconductors are a type of extrinsic semiconductor in which the atoms have one fewer electron (e.g., boron).  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals  Alkali metals ⮚ Chemical elements found in Group 1 of the periodic table. They appear silvery and can be cut with a plastic knife. ⮚ The alkali metals include: lithium, sodium, potassium, rubidium, cesium, and francium. ⮚ Hydrogen is not technically an alkali metal since it rarely exhibits similar behavior. ⮚ The word "alkali" received its name from the Arabic word "al qali," meaning "from ashes", which since these elements react with water to form hydroxide ions, creating alkaline solutions (pH>7). Alkali metals  Common properties of Alkali metals ⮚ The most electropositive or the least electronegative elements ⮚ Common oxidation state +1 ⮚ Found dissolved in seawater due to geologic erosion of minerals ⮚ All the discovered alkali metals occur in nature. ⮚ These metals have a BCC structure with low packing efficiency. ⮚ Low melting point. Lithium - lightest known metal and has great chemical reactivity. Do not occur free in elemental form, are combined in halides, sulfates, carbonates and silicates Alkaline Earth Metals  Alkaline Earth Metals ⮚ Less electropositive and less reactive than Group IA ⮚ Common oxidation state +2 ⮚ IIA Metals attain stable electron configuration of the preceding noble gases ⮚ Have much higher melting points than the alkali metals, harder metals than the Group 1A elements, but are soft and lightweight compared to many of the transition metals. ⮚ The chemistry of radium is not well established due to its radioactivity.  Alkaline Earth Metals Emerald is a variety of beryl, a mineral that contains the alkaline earth metal beryllium. Beryllium only occurs naturally in combination with other elements in minerals.  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals  Aluminum ✔Most abundant metal and the 3rd most plentiful element in the Earth’s crust. ✔Elemental form doesn’t occur in nature ✔Principal ore: Bauxite (Al2O3 H2O) ✔Other minerals containing aluminum are orthoclase (KAlSi3O8), beryl (Be3Al2Si6O18), cryolite (Na3AlF6), and corundum (Al2O3). ✔Considered a precious metal until Hall developed a method of Aluminum Charles Hall, pioneer of production development of Aluminum production  Preparation of Aluminum Anhydrous aluminum oxide (Al2O3 or corundum) is reduced to aluminum by the Hall process. The cathode is also made of carbon and constitutes the lining inside the cell. The key to the Hall process is the use of cryolite, or Na3AlF6 (melting point is 1000°C), as the solvent for aluminum oxide (melting point is 2045°C). The mixture is electrolyzed to produce aluminum and oxygen gas. Oxygen gas reacts with the carbon anodes to form carbon monoxide, which escapes as a gas.  Preparation of Aluminum The liquid aluminum metal (melting point is 660.2°C) sinks to the bottom of the vessel, from which it can be drained from time to time during the procedure.  Recycling of Aluminum ⮚ Aluminum is one of the most recycled and most recyclable materials on the market today. Nearly 75% of all aluminum produced in the U.S. is still in use today. ⮚ Aluminum can be recycled directly back into itself over and over again in a true closed loop. ⮚ Recycling aluminum costs 95% less energy compared to producing primary aluminum. Recycling of Aluminum  Chemistry of Engineering Materials: Metals Introduction to Metals Occurrence of Metals Metallurgy Band Theory of Electrical Conductivity The Alkali and Alkaline Earth Metals Aluminum Transition Metals  Transition Metals Transition Metal – any of various chemical elements that have valence electrons—i.e., electrons that can participate in the formation of chemical bonds—in two shells instead of only one. ̶ Transition elements are the elements that are found in Groups 3-12 (old groups IIA-IIB) on the periodic table. ̶ Transition metals typically have incompletely filled d subshells or readily give rise to ions with incompletely filled d subshells. ̶ Many transition element compounds are brightly colored due to the inner-level d electron transitions.  Properties of Transition metal Transition metals have similar properties, and some of these properties are different from those of the metals in group 1. Physical properties ̶ they are good conductors of heat and electricity ̶ they can be hammered or bent into shape easily ̶ they have high melting points (but mercury is a liquid at room temperature) ̶ they are usually hard and tough ̶ they have high densities  Properties of Transition metal Chemical properties The transition metals have the following chemical properties in common: ̶ they are less reactive than alkali metals such as sodium ̶ they form colored ions of different charges ̶ some are very unreactive (silver and gold) ̶ many are used as catalysts  Uses of transition metals Transition metals have a wide range of uses. Their properties are very similar but not identical. It is important to choose the right transition metal for the required purpose. GOLD  Uses of transition metals SILVER COPPER  Uses of transition metals IRON Iron is usually too soft to be used as the metal alone. It is usually mixed with small amounts of other elements to make steels, which are harder and stronger than iron, but easily shaped. However, iron and steel react slowly with water and air to produce rust. They must be protected with, for example, a layer of paint.  Uses of transition metals CHROMIUM  Supplementary videos ► https://www.youtube.com/watch?v=vOuFTuvf4qk ► https://www.youtube.com/watch?v=kCM2mSb4qIU ► https://www.youtube.com/watch?v=CmiitvJiCPc ► https://www.youtube.com/watch?v=8qh5myTmcRs  Chemistry of Engineering Materials: Polymers Properties and Characterization of Polymers The Chemistry of Polymer Molecules Molecular Structure of Polymers Common Polymeric Materials Molecular Weight and Degree of Polymerization  LEARNING OBJECTIVE ► Describe the properties and structure of polymers and know the common polymeric materials. ► Determine the average molecular weights of polymers and degree of polymerization. ► Cite the differences in behavior and molecular structure of thermoplastic and thermoset ting polymers. ► Describe the sequencing arrangements along polymer chains and crystalline state in polymeric materials.  Polymer ⮚ Molecular compound that can be distinguished by a high molar mass, ranging into thousands and even millions of mass and they are made up of many repeating units. ⮚ The roots of the word polymer are actually very descriptive of a polymer. The root ‘mer’ means unit, and poly means many. Taken together, the word polymer can be deconstructed as many units. Typically, ‘mer’ is referred to as a monomer. https://www.youtube.com/watch?v=UwRVj9rz2QQ Polymer https://www.youtube.com/watch?v=UwRVj9rz2QQ  Natural Polymer Natural polymers have been around since life itself began. - occur in nature and can be extracted. - water-based Examples: cellulose, starch (and other complex carbohydrates), natural rubber, and DNA.  Synthetic Polymer ⮚ Synthetic polymers were first developed in the early 20th century, and these polymers remarkably transformed our world as different materials can be created with properties that are ideal for different applications. Examples are nylon, polyethylene, polyester, Teflon, and epoxy.  Synthetic Polymer ⮚ Presently, crude oil is the starting material for many synthetic polymers in plastics, pharmaceuticals, fabrics, and other carbon-based products. Natural and Synthetic Polymers  Homopolymer ⮚ If a polymer is made up of only type of monomer (e.g. polyethylene), then it is known as homopolymer. Other homopolymer that are synthesized by the radical mechanism are TeflonTM (polytetrafluoroethylene PTFE) and poly-vinyl chloride (PVC).  Polymer Molecules The molecules in polymers are gigantic and because of their size they are often referred to as macromolecules. The backbone of each of a carbon-chain polymer is a string of carbon atoms and within each molecule, the atoms are bound together by covalent interatomic bonds. Many times each carbon atom singly bonds to two adjacent carbon atoms on either side which is represented as follows:  Chemistry of Engineering Materials: Polymers Properties and Characterization of Polymers The Chemistry of Polymer Molecules Molecular Structure of Polymers Common Polymeric Materials Molecular Weight and Degree of Polymerization  Polyethylene (PE) Ethylene (C2H4) is a gas at ambient temperature and pressure. Under appropriate conditions, ethylene gas will react and it will transform to polyethylene (PE) which is a solid polymeric material. Ethylene is a stable molecule with two carbon atoms connected by a double bond. Polyethylene is made by the reaction of multiple ethylene molecules in the presence of catalyst. Polyethylene (PE) Polyethylene (PE)  Polyetetrafluoroethylene (PTFE) Other chemistry of polymer structure such as tetrafluoroethylene monomer to form polytetrafluoroethylene (PTFE) is shown below: PTFE (having the trade name Teflon) belongs to a family of polymers called the fluorocarbons.  Generalized form Some polymers may be represented using the following generalized form: where the R represents either an atom or an organic group such as CH3 (methyl), C2H5 (ethyl), and C3H7 (phenyl).  Chemistry of Engineering Materials: Polymers Properties and Characterization of Polymers The Chemistry of Polymer Molecules Molecular Structure of Polymers Common Polymeric Materials Molecular Weight and Degree of Polymerization  Polymer structure Molecular weight and shape of a polymer is not the only basis of its physical characteristics, the difference in the structure of the molecular chains must also be considered.  Polymer structure: Linear ⮚ Linear polymers are those in which the repeat units are joined together end to end in single chains. These long chains are flexible where each circle represents a unit. ⮚ There may be extensive van der Waals and hydrogen bonding between the chains. ⮚ Some of the common polymers that form with linear structures are polyethylene, polyvinyl chloride, polystyrene, polymethyl methacrylate (PMMA), nylon, and the fluorocarbons.  Polymer structure: Branched ⮚ The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density. ⮚ For example, high-density polyethylene (HDPE) is primarily a linear polymer, whereas low- density polyethylene (LDPE) contains short- chain branches.  Polymer structure: Cross-linked ⮚ Adjacent linear chains are joined one to another at various positions by covalent bonds. ⮚ The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction. ⮚ Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains. ⮚ Many of the rubber elastic materials are crosslinked. Polymer structure: Cross-linked  Polymer structure: Network ⮚ Multifunctional monomers forming three or more active covalent bonds making three-dimensional networks. ⮚ A polymer that is highly crosslinked may also be classified as a network polymer. ⮚ These materials have distinctive mechanical and thermal properties; the epoxies, polyurethanes, and phenol- formaldehyde belong to this group Polymer structure: Network  Chemistry of Engineering Materials: Polymers Properties and Characterization of Polymers The Chemistry of Polymer Molecules Molecular Structure of Polymers Common Polymeric Materials Molecular Weight and Degree of Polymerization  Six Common Polymers Presently, there are more than 60,000 synthetic polymers known, with this, six types of polymers account for roughly 75% of those used in both Europe and the United States.  Six Common Polymers Polyethylene terephthalate (PETE or PET) Properties: transparent, strong, shatter- resistant. Impervious to acids and atmospheric gases. Most costly of the six. Uses: Soft-drink bottles, clear food containers, beverage glasses, fleece fabrics, carpet yarns, fiber-fill insulation.  Six Common Polymers Polyethylene (PE) Properties: Similar to LDPE but more rigid, tougher, and slightly more dense. Uses: Opaque milk, juice, detergents, and shampoo bottles. Also used in buckets, crates, and fencing materials.  Six Common Polymers Polyvinyl chloride (PVC) Properties: Variable. Rigid if not softened with a plasticizer. Clear and shiny, but often pigmented. Resistant to most chemicals, including oils, acids, and bases. Uses: Rigid: Plumbing pipe, house siding, charge cards, and hotel room keys. Softened: Garden hoses, waterproof boots, shower curtains, and IV tubing.  Six Common Polymers Polyethylene (PE) Properties: Translucent if not pigmented. Soft and flexible. Unreactive to acids and bases. Strong and tough. Uses: Bags, films, sheets, bubble wrap, toys, wire insulation.  Six Common Polymers Polypropylene (PP) Properties: Opaque, very tough, good weatherability. Has high melting point. Resistant to oils. Uses: Bottle caps. Yogurt, cream, and margarine containers. Carpeting, casual furniture, luggage.  Six Common Polymers Polystyrene (PS) Properties: Crystal form is transparent, sparkling, somewhat brittle. Expandable form is lightweight. Both forms are rigid and degrades in many organic solvents. Uses: Crystal form: Food wrap, CD cases, transparent cups. Expandable form: Foam cups, insulated containers, food packaging trays, and egg cartons.  Chemistry of Engineering Materials: Polymers Properties and Characterization of Polymers The Chemistry of Polymer Molecules Molecular Structure of Polymers Common Polymeric Materials Molecular Weight and Degree of Polymerization  Polymerization Polymers with very long chains has extremely large molecular weights but during polymerization process, not all polymer chains will grow to the same length. Usually, an average molecular weight is specified, which can be determined by the measurement of various physical properties such as viscosity and osmotic pressure.  Polymerization The number-average molecular weight Mn is obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size range. The number-average molecular weight is expressed as: where Mi represents the mean (middle) molecular weight of size range i, and Xi is the fraction of the total number of chains within the corresponding size range.  Polymerization A weight-average molecular weight Mw is based on the weight fraction of molecules within the various size ranges. It is calculated according to: where, again, Mi is the mean molecular weight within a size range, and Wi denotes the weight fraction of molecules within the same size interval.  Polymerization The length of polymer chains has affected many polymer properties. As molecular weight of a polymer increases, its melting or softening temperature also increases. MW: 100g/mol MW: 1000g/mol MW: >10000g/mol Very short chain High polymers polymers Liquids at room Waxy solids (e.g. Exist as solid temperature paraffin wax)  Degree of Polymerization Degree of Polymerization (DP) is an alternative way of expressing average chain size of a polymer. DP represents the average number of repeat units in a chain and it is related to the number-average molecular weight Mn by the equation: where m is the repeat unit molecular weight. Polymerization Polymerization Polymerization  Chemistry of Engineering Materials: Polymers Thermoplastic and Thermosetting Polymers Copolymers Polymer Crystallinity  Thermoplastics and Thermosets Molecular structure has a great effect on how polymers react to mechanical forces at elevated temperatures. Indeed, one classification for these materials is according to behavior with rising temperature. Thermoplastics and thermosets are the two subdivisions.  Thermoplastics Thermoplastics or thermoplastic polymers soften upon heating and later liquefy, then it hardens when cooled. This process is reversible and can be repeated. Exposure of a molten thermoplastic polymer to a very high temperature results to an irreversible degradation. Examples of common thermoplastic polymers are PE, PS, PETE and PVC.  Thermosets Thermosets or thermosetting polymers are network polymers, they do not soften upon heating and they become permanently hard during their formation. Network polymers have covalent crosslinks between adjacent molecular chains. Excessive heating temperatures will cause severance of these crosslink bonds and polymer degradation.  Thermosets As compared to thermoplastics, these thermoset polymers are generally harder and stronger and have better dimensional stability. Examples of these thermosets are vulcanized rubbers, epoxies, phenolics, and some polyester resins. Thermosets  Chemistry of Engineering Materials: Polymers Thermoplastic and Thermosetting Polymers Copolymers Polymer Crystallinity  Copolymers A copolymer is composed of two repeat units. It is possible that there are different sequencing arrangements along the polymer chains which depends on the polymerization process and the relative fractions of these repeat unit types. Synthetic rubbers are usually copolymers. Copolymers  Copolymers: Alternating Two repeat units alternate chain positions  Copolymers: Random Two different units are randomly dispersed along the chain  Copolymers: Block Identical repeat units are clustered in blocks along the chain  Copolymers: Graft Also called homopolymer, side branches of one type may be grafted to homopolymer main chains that are composed of a different repeat unit  Chemistry of Engineering Materials: Polymers Thermoplastic and Thermosetting Polymers Copolymers Polymer Crystallinity  Polymer Crystallinity In crystalline state, the atomic arrangement in polymer materials are more complex as compared to metals because in polymers it involves molecules instead of just atoms or ions. Polymer crystallinity is the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex.  Polymer Crystallinity An example of a unit cell is shown for polyethylene and its relationship to the molecular chain structure (unit has orthorhombic geometry). The chain molecules also extend beyond the unit cell.  SCI 401 GENERAL CHEMISTRY Engr. Anamarie C. Daño Guest Lecturer Chemistry of Engineering Materials Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS TYPES OF CRYSTALS AMORPHOUS SOLIDS  LEARNING OBJECTIVES ► Describe the basic structural unit or building block of the crystal structure. ► Determine to compute the density of a solid given its unit cell. ► Classify the four types of crystals. ► Describe the characteristics of amorphous solids. CATEGORIES OF SOLIDS CATEGORIES OF SOLIDS CRYSTAL STRUCTURES  Chemistry of Engineering Materials: Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS TYPES OF CRYSTALS AMORPHOUS SOLIDS UNIT CELLS https://www.youtube.com/watch?v=qAeaHYSX0hs  The 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. Polonium, a metalloid or a semi-metal is the only simple-cubic element that has a relatively low atomic packing factor. SEVEN TYPES OF PRIMITIVE UNIT CELLS SEVEN TYPES OF PRIMITIVE UNIT CELLS The Face-Centered Cubic Crystal Structure  The Face-Centered Cubic Crystal Structure Example 1. Calculate the volume of an FCC unit cell in terms of atomic radius R. The Face-Centered Cubic Crystal Structure The Face-Centered Cubic Crystal Structure The Face-Centered Cubic Crystal Structure Important Characteristics of a Crystal Structure Important Characteristics of a Crystal Structure For FCCs, the coordination number is 12. Front face atoms has four nearest neighboring atoms around it, four face atoms that are link from behind, and four other equivalent face atoms positioned in the next unit cell to the front which is not shown. Atomic Packing Factor Atomic Packing Factor The Body-Centered Cubic Crystal Structure A body-centered cubic (BCC) is another common metallic crystal structure that also has a cubic unit cell with atoms located at all eight corners and a single atom at the center of the cube. Corner atoms and center touch one another along with the diagonal of the cube, and unit cell length a and atomic radius R are related by the way of The Body-Centered Cubic Crystal Structure Edge Length and Atomic Radius Relationships The Hexagonal Close-Packed Crystal Structure The final common metallic crystal structure is the hexagonal close-packed (HCP). The top and bottom faces of the unit cell consist six atoms that form regular hexagons and surround a single atom in the center. Between the top and bottom planes, there is another plane that provides three additional atoms to the unit cell. The atoms in this midplane have as nearest neighbors atoms in both of the adjacent two planes. The Hexagonal Close-Packed Crystal Structure The Hexagonal Close-Packed Crystal Structure To compute the number of atoms per unit cell for HCP crystal structure, the formula is shown below: One-sixth of each corner atom is designated to a unit cell instead of 8 as with the cubic structure. This is because, HCP has 6 corner atoms in each of the top and bottom faces for a total of 12 corner atoms, 2 face center atoms (one from each of the top and bottom faces), and 3 midplane interior atoms. Using Equation 5, the value of N for HCP can be found. The Hexagonal Close-Packed Crystal Structure Crystal Structure  Chemistry of Engineering Materials: Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS TYPES OF CRYSTALS AMORPHOUS SOLIDS  Density Computations A theoretical density (ρ) can be computed with a knowledge of the crystal structure of a metallic solid through the relationship Where n = number of atoms associated with each unit cell A = atomic weight VC = volume of the unit cell NA = Avogadro’s number (6.022 x 1023 atoms/mol) Density Computations Density Computations Density Computations  Chemistry of Engineering Materials: Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS TYPES OF CRYSTALS AMORPHOUS SOLIDS  Types of Crystals In determining the structures and properties of crystals, such as melting point, density, and hardness it is important to consider the kinds of forces that hold the particles together. The classification of any crystal has four types: ionic, covalent, molecular, or metallic.  Ionic Crystals There are two important characteristics of ionic crystals and they are as follows: (1) They are composed of charged species (2) anions and cations are generally quite different in size. The radii of the ions must be known because it is helpful in understanding the structure and stability of these compounds. It is hard to measure the radius of an individual ion but sometimes it is possible to come up with an estimation.  Ionic Crystals For example, the crystal which has a FCC lattice shows that the edge length of the unit cell of NaCl is twice the sum of the ionic radii of Na+ and Cl-. Getting the values of ionic radius given in some references: Na+=95pm Cl-=181pm We can calculate the length of the edge to: 2*(95+181)pm = 552pm  Ionic Crystals The edge length shown was determined experimentally and has a value of 564pm. Ionic Crystals Ionic Crystals Ionic Crystals Covalent Crystals Covalent Crystals Covalent Crystals Molecular Crystals Molecular Crystals Molecular Crystals SO2 Molecular Crystals Metallic Crystals Metallic Crystals Metallic Crystals Metallic Crystals General Properties of Crystals  Chemistry of Engineering Materials: Basic Concepts of Crystal Structures CRYSTAL STRUCTURES UNIT CELLS DENSITY COMPUTATIONS TYPES OF CRYSTALS AMORPHOUS SOLIDS Amorphous Solids Amorphous Solids Amorphous Solids Amorphous Solids