Atomic Structure, Bonding, and Crystal Structures in Materials Science

atomic number (Z)

For a chemical element, the number of protons within the atomic nucleus.

isotope

Atoms of the same element that have different atomic masses.

atomic weight (A)

The weighted average of the atomic masses of an atom's naturally occurring isotopes.

atomic mass unit (amu)

A measure of atomic mass; 1/12 of the mass of an atom of C.

mole

The quantity of a substance corresponding to 6.022 × 10^23 atoms or molecules.

quantum mechanics

A branch of physics that deals with atomic and subatomic systems; it allows only discrete values of energy.

Bohr atomic model

An early atomic model in which electrons are assumed to revolve around the nucleus in discrete orbitals.

wave mechanical model

An atomic model in which electrons are treated as being wavelike.

quantum number

A set of four numbers, the values of which are used to label possible electron states.

electron state (level)

One of a set of discrete, quantized energies that are allowed for electrons.

Pauli exclusion principle

The postulate that for an individual atom, at most two electrons can occupy the same state.

ground state

A normally filled electron energy state from which an electron excitation may occur.

electron configuration

For an atom, the manner in which possible electron states are filled with electrons.

valence electron

The electrons in the outermost occupied electron shell, which participate in interatomic bonding.

periodic table

The arrangement of the chemical elements with increasing atomic number according to the periodic variation in electron structure.

electropositive

For an atom, having a tendency to release valence electrons.

electronegative

For an atom, having a tendency to accept valence electrons.

bonding energy

The energy required to separate two atoms that are chemically bonded to each other.

primary bonds

Interatomic bonds that are relatively strong and for which bonding energies are relatively large.

ionic bond

A coulombic interatomic bond that exists between two adjacent and oppositely charged ions.

coulombic force

A force between charged particles such as ions; the force is attractive when the particles are of opposite charge.

covalent bond

A primary interatomic bond that is formed by the sharing of electrons between neighboring atoms.

metallic bond

A primary interatomic bond involving the nondirectional sharing of nonlocalized valence electrons.

secondary bonds

Interatomic and intermolecular bonds that are relatively weak and for which bonding energies are relatively small.

van der Waals bond

A secondary interatomic bond between adjacent molecular dipoles that may be permanent or induced.

dipole (electric)

A pair of equal and opposite electrical charges separated by a small distance.

hydrogen bond

A strong secondary interatomic bond that exists between a bound hydrogen atom and the electrons of adjacent atoms.

polar molecule

A molecule in which there exists a permanent electric dipole moment.

crystalline

The state of a solid material characterized by a periodic and repeating three-dimensional array of atoms.

crystal structure

For crystalline materials, the manner in which atoms or ions are arrayed in space.

unit cell

The basic structural unit of a crystal structure.

face-centered cubic (FCC)

A crystal structure found in some common elemental metals.

coordination number

The number of atomic or ionic nearest neighbors.

atomic packing factor (APF)

The fraction of the volume of a unit cell that is occupied by hard-sphere atoms or ions.

body-centered cubic (BCC)

A common crystal structure found in some elemental metals.

hexagonal close-packed (HCP)

A crystal structure found for some metals.

polymorphism

The ability of a solid material to exist in more than one form or crystal structure.

allotropy

The possibility of the existence of two or more different crystal structures for a substance.

lattice parameters

The combination of unit cell edge lengths and interaxial angles that defines the unit cell geometry.

crystal system

A scheme by which crystal structures are classified according to unit cell geometry.

Miller indices

A set of three integers that designate crystallographic planes.

single crystal

A crystalline solid for which the periodic and repeated atomic pattern extends throughout its entirety.

grain

An individual crystal in a polycrystalline metal or ceramic.

polycrystalline

Crystalline materials composed of more than one crystal or grain.

grain boundary

The interface separating two adjoining grains having different crystallographic orientations.

anisotropic

Exhibiting different values of a property in different crystallographic directions.

Isotropic

Having identical values of a property in all crystallographic directions.

Diffraction (x-ray)

Constructive interference of x-ray beams scattered by atoms of a crystal.

Bragg's law

A relationship that stipulates the condition for diffraction by a set of crystallographic planes.

Noncrystalline

The solid state in which there is no long-range atomic order. Sometimes the terms amorphous, glassy, and vitreous are used synonymously.

Amorphous

Having a noncrystalline structure.

Imperfection

A deviation from perfection; normally applied to crystalline materials in which there is a deviation from atomic/molecular order and/or continuity.

Point defect

A crystalline defect associated with one or, at most, several atomic sites.

Vacancy

A normally occupied lattice site from which an atom or ion is missing.

Boltzmann's constant

A thermal energy constant having the value of J/atom K or eV/atom K.

Self-interstitial

A host atom or ion positioned on an interstitial lattice site.

Alloy

A metallic substance that is composed of two or more elements.

Solid solution

A homogeneous crystalline phase that contains two or more chemical species.

Solute

One component or element of a solution present in a minor concentration.

Solvent

The component of a solution present in the greatest amount.

Substitutional solid solution

A solid solution in which the solute atoms replace or substitute for the host atoms.

Interstitial solid solution

A solid solution in which relatively small solute atoms occupy interstitial positions between the solvent or host atoms.

Composition

The relative content of a particular element or constituent within an alloy, usually expressed in weight percent or atom percent.

Weight percent (wt%)

A concentration specification on the basis of weight of a particular element relative to the total alloy weight.

Atom percent (at%)

A concentration specification on the basis of the number of moles of a particular element relative to the total number of moles of all elements within an alloy.

Edge dislocation

A linear crystalline defect associated with the lattice distortion produced in the vicinity of the end of an extra half-plane of atoms within a crystal.

Dislocation line

The line that extends along the end of the extra half-plane of atoms for an edge dislocation.

Screw dislocation

A linear crystalline defect associated with the lattice distortion created when normally parallel planes are joined together to form a helical ramp.

Mixed dislocation

A dislocation that has both edge and screw components.

Burgers vector (b)

A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation.

Atomic vibration

The vibration of an atom about its normal position in a substance.

Microstructure

The structural features of an alloy subject to observation under a microscope.

Microscopy

The investigation of microstructural elements using some type of microscope.

Photomicrograph

A photograph made with a microscope that records a microstructural image.

Transmission electron microscope (TEM)

A microscope that produces an image by using electron beams that are transmitted through the specimen.

Scanning electron microscope (SEM)

A microscope that produces an image by using an electron beam that scans the surface of a specimen.

Scanning probe microscope (SPM)

A microscope that uses a very small and sharp probe to scan across the specimen surface.

Grain size

The average grain diameter as determined from a random cross section.

Diffusion

Mass transport by atomic motion.

Interdiffusion

Diffusion of atoms of one metal into another metal.

Self-diffusion

Atomic migration in pure metals.

Vacancy diffusion

The diffusion mechanism in which net atomic migration is from a lattice site to an adjacent vacancy.

Interstitial diffusion

A diffusion mechanism by which atomic motion is from interstitial site to interstitial site.

Diffusion flux (J)

The quantity of mass diffusing through and perpendicular to a unit cross-sectional area of material per unit time.

Fick's first law

The diffusion flux is proportional to the concentration gradient.

Diffusion coefficient (D)

The constant of proportionality between the diffusion flux and the concentration gradient in Fick's first law.

Steady-state diffusion

The diffusion condition for which there is no net accumulation or depletion of diffusing species.

Concentration profile

The curve that results when the concentration of a chemical species is plotted versus position in a material.

Concentration gradient (dC/dx)

The slope of the concentration profile at a specific position.

Driving force

The impetus behind a reaction, such as diffusion, grain growth, or a phase transformation.

Nonsteady-state diffusion

The diffusion condition for which there is some net accumulation or depletion of diffusing species.

Fick's second law

The time rate of change of concentration is proportional to the second derivative of concentration.

Carburizing

The process by which the surface carbon concentration of a ferrous alloy is increased by diffusion from the surrounding environment.

Activation energy (Q)

The energy required to initiate a reaction, such as diffusion.

Engineering stress

The instantaneous load applied to a specimen divided by its cross-sectional area before any deformation.

Engineering strain

The change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length.

Shear

A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other in a direction parallel to their plane of contact.

Modulus of elasticity (E)

The ratio of stress to strain when deformation is totally elastic; also a measure of the stiffness of a material.

Elastic deformation

Deformation that is nonpermanent—that is, totally recovered upon release of an applied stress.

Anelastic deformation

Time-dependent elastic (nonpermanent) deformation.