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Chapter 1
The Atomic Structures of Materials
Metal-
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Those materials that occupy the left side of the periodic table are characterized by having
one, two, or three valence electrons, and bond with the metallic bond.
Ceramics-
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A family of materials that are compounds, traditionally consisting of a metal and an oxide, but
they may also be carbides, sulfides, nitrides, and intermetallic compounds.
Ceramics generally have an ionic bond, are very hard and brittle, and can withstand high
temperatures.
Valence-
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The capacity of an atom to combine with other atoms to form a molecule.
The inert gases have zero valence.
Valence considers positive and negative properties of atoms, as determined by the gain or
loss of valence electrons.
Lattice-
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A term that is used to denote a regular array of points in space.
The points of the three-dimensional space lattice are constructed by the repeated application
of the basic translations that carry a unit cell into its neighbor.
Thermoplastic-
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Capable of softening or fusing when heated and of hardening again when cooled.
Thermosetting-
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Capable of becoming permanently rigid when cured by heating; will not soften by reheating.
Polymer-
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A compound or compounds, usually hydrocarbons, that have been polymerized to form a
long chain repeating unit structures.
Polymerization-
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A chemical reaction in which two or more small molecules combine to form larger molecules
that contain repeating structural units of the original molecules.
Polycrystalline-
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The term used to describe the crystalline nature of most metals encountered, i.e., they are
made up of more than one metallic crystal, as opposed to being single crystals.
Grain
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In metals, a structure containing atoms of one crystalline orientation.
Grains form during the solidification (or crystallization) of the metal; they may be re-formed
during recrystallization.
Allotropy-
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The ability of a material to exist in several crystalline forms.
Nucleus-
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The positively charged central part of an atom containing the protons and neutrons, and
therefore most of the atom's mass.
Atomic bonding-
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The joining together of atoms with ionic, covalent, or metallic bonds; the molecular or van der
Waals bond is a weak bond seen in inert gases, or a secondary bond in polymers.
Metallic bond-
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In metals, the attractive force between their positive nuclei and inner electron shells (with a
net positive charge), and a negatively charged cloud of valence electrons.
This type of bonding provides free electrons for electrical and thermal conductivity and
permits plastic deformation or cold working.
Ionic bond-
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A type of atomic bonding in which atoms with one or more valence electrons donate or give
away their valence electrons to elements that lack one or more electrons to fill their valence
shell; each atom thus becomes an ion; the one donating becomes positive, the one accepting
becomes negative.
The ionic bond commonly forms compounds between elements widely separated on the
periodic table, e.g., Na + and CI- combine to form NaCI.
Covalent bond-
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A type of atomic bonding that requires the sharing of valence electrons to complete the outer
shell; appears principally in gases, liquids, and polymers.
van der Waals bond-
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A relatively weak bond between molecules, for example, in inert gases.
It is a weak secondary bond between adjacent chains of a polymer.
The strength of the bond is directly related to the size of the molecules, inversely related to
the distance between the molecules, and easily weakened by heat.
Solidification-
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The process in which a liquid metal changes to a solid; in this process heat is removed and
the atoms have to fit into an atom lattice.
Dendrite-
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A crystal characterized by a treelike pattern that is usually formed by the solidification of a
metal.
Dendrites generally grow inward from the surface of a mold.
Grain-boundary-
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The outer perimeter of a single grain where it is in contact with an adjoining grain; because
atoms are not at their ideal distance apart (and therefore at their lowest energy level), it is a
region of higher energy.
Isomerism-
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Compounds are said to be isomeric when they have the same elementary composition (i.e.,
their molecules contain the same numbers and kinds of atoms) but different structures, and
hence, properties.
It is believed that differences are due to the arrangement of atoms in each molecule.
Cross-linking-
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Primary, i.e., ionic or covalent, bonds between chains of a polymer, so that the polymer takes
on a three-dimensional structure; this normally occurs in thermoset polymers.
Crystalline, Crystallinity-
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In solid metals, having a repeating geometric arrangement of atoms.
Polymers are said to be crystalline, or exhibit crystallinity, when their chains become aligned
into a pattern.
The opposite of amorphous, having a random structure.
Vulcanization-
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The process of treating crude or synthetic rubber or similar plastic material chemically to give
it useful properties, such as elasticity, strength, and stability.