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• Classification of materials includes metals, ceramics, and polymers.

Metals

• Metals include metallic elements like iron, aluminum, copper, titanium, gold, and nickel.
• Metals may contain small amounts of nonmetallic elements like carbon, nitrogen, and oxygen.
• Metals have a higher density than ceramics and polymers.
• Metals are stiff and strong materials.
• Metals are ductile, which means they can deform without fracturing.
• Metals have a resistance to fracture, also known as fracture toughness.
• Metals are good conductors of electricity and heat.
• Metals are not transparent to visible light.
• Some metals, such as Fe, Co, and Ni, possess desirable magnetic properties.

Ceramics

• Ceramics consist of metallic and non-metallic elements.
• Oxides, nitrides, and carbides, are the forms ceramics often take
• Aluminum oxide (alumina, Al2O3), silicon dioxide (silica, SiO2), silicon carbide (SiC), and silicon nitride (Si3N4) are all ceramics.
• Traditional ceramics include clay minerals (i.e., porcelain), cement, and glass.
• Ceramic materials have relative stiffness and strengths comparable to those of metals.
• Ceramics are known to be very hard.
• Ceramics are brittle and prone to fracturing, with newer ceramics designed for enhanced resistance to breaking.
• Cookware, cutlery, and automotive engine components utilize upgraded ceramic materials.
• Ceramics are typically insulative to the passage of heat and electricity, having low electrical conductivities.
• Ceramics are more resistant to high temperatures and harsh environments than metals and polymers.
• Some oxide ceramics, such as Fe3O4, exhibit magnetic behavior.

Polymers

• Polymers are familiar plastic and rubber materials.
• Polymers are organic compounds chemically based on carbon, hydrogen, and other nonmetallic elements such as O, N, and Si.
• Polymers have very large chainlike molecular structures with a backbone of carbon atoms.
• Common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber.
• Polymers have a low density, and their stiffness and strength are lower than metals and ceramics.
• Many polymers are extremely ductile and pliable and can be easily formed into complex shapes.
• Generally, polymers are chemically inert and unreactive in many environments.
• A major drawback to polymers is their tendency to soften and/or decompose at modest temperatures which limits their use.
• Polymers have low electrical conductivities and are nonmagnetic.

Bohr atomic model

• Electrons revolve around the atomic nucleus in discrete orbitals.
• The position of any particular electron is more or less well-defined in terms of its orbital.

Atomic Bonding in Solids

• Understanding the interatomic forces that bind atoms together enhances the comprehension of material properties.

Valence electron

• Valence electrons are those in the outermost shell.

Electropositive Elements

• These elements can give up their few valence electrons to become positively charged ions.

Electronegative Elements

• These elements readily accept electrons to form negatively charged ions, or sometimes they share electrons with other atoms.

Primary Interatomic Bonds (Chemical Bonds) (Valence Electrons)

• Ionic Bonding
• Covalent Bonding
• Metallic Bonding

Ionic Bonding

• Ionic bonding occurs in compounds composed of both metallic and nonmetallic elements.
• Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms.
• Sodium chloride (NaCl) exemplifies an iconic material.
• All sodium and chlorine exist as ions in sodium chloride.
• Ionic bonding is non-directional, meaning the bond's magnitude is equal in all directions around an ion.
• For ionic materials to be stable, positive ions must have negatively charged ions as nearest neighbors in a three-dimensional scheme, and vice versa.
• Ionic materials are characteristically hard and brittle, and furthermore, electrically and thermally insulative.

Covalent bonding

• In covalent bonding, stable electron configurations are achieved by sharing electrons between adjacent atoms.
• Each of two covalently bonded atoms contributes at least one electron to the bond, with shared electrons considered belonging to both atoms.
• In methane (CH4), the carbon atom has four valence electrons, while each of the four hydrogen atoms has a single valence electron.
• The number of covalent bonds possible for an atom is determined by its number of valence electrons.

Metallic Bonding

• In metallic bonding, valence electrons form a "sea of electrons" dispersed around metal ion cores, acting like glue.
• Materials exhibiting this type of bonding are metallic materials.
• Remaining nonvalenced electrons and atomic nuclei form ion cores, possessing a net positive charge equal in magnitude to the total valence electron charge per atom.
• Metals are good conductors of both electricity and heat because of their free electrons, unlike ionically and covalently bonded materials, which are typically electrical and thermal insulators because of the absence of large numbers of free electrons.

Fundamental Concepts of Crystal Structures

• Crystalline materials are ones, in which atoms are situated in a repeating, periodic array.
• Metals, ceramic materials, and certain polymers form crystalline structures under normal solidification conditions.

Crystal Structure Representation

• Atomic hard sphere model (spheres having well-defined diameters).
• Lattice (three-dimensional array of points coinciding with atom positions (or sphere centers).
• Unit Cells (smallest repeat unit).

Metallic Crystal Structures

• The Simple Cubic Crystal Structure (SC) APF for a simple cubic structure = 0.52
• 1 Atom is associated with each SC united cell

Face-Centered Cubic Crystal Structure (FCC)

• Atoms located at each of the corners and the centers of all the cube faces.
• Copper, aluminum, silver, nickel, lead, and gold are familiar metals having this crystal structure.
• 4 Atoms are associated with each FCC unit cell.

Atomic Packing Factor (APF)

• The APF includes the sum of the sphere volumes of all atoms within a unit cell divided by the unit cell volume.

Body-Centered Cubic Crystal Structure (BCC)

• Atoms located at all eight corners and a single atom at the cube center.
• Chromium, tungsten, Iron (a), Tantalum, Molybdenum exhibit a BCC structure.
• 2 Atoms are associated with each BCC unit cell.

Hexagonal Close-Packed Crystal Structure (HCP)

• The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center.
• Another plane provides three additional atoms to the unit cell, situated between the top and bottom planes.
• Cadmium, Magnesium, Zinc, and Titanium have this crystal structure.
• Unit cell has two lattice parameters a and c with an ideal ratio c/a = 1.633.
• Atomic packing factor, APF = 0.74 (same as in FCC).

Theoretical density for metals

• Metallic solid's crystal structure makes it possible to compute its theoretical density p through the relationship
• Knowledge of a metallic solid's crystal structure permits computation of its theoretical density p through the relationship

Polymorphism

• This is the ability of a solid material to exist in more than one crystal structure.
• For example, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at 912°C.

Allotropy

• This is the property of some chemical elements to exist in two or more different forms.

Crystal system

• Lattice parameters: Axial lengths (a, b, & c) and Inter axial angles (α, β, & γ).