Atomic Structure and Crystalline Solids Vocabulary

These vocabulary flashcards cover the fundamental concepts of atomic structure, interatomic bonding types, crystal structures, and crystallographic indexing methods based on the provided engineering materials lecture notes.

Atomic Number

The number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral species.

Atomic Mass Unit (amu)

A unit of mass defined as $1/12$ the mass of $^{12}\text{C}$, where $1\text{ amu/atom} = 1\text{ g/mol}$. atomic weight is the weight of $6.022 \times 10^{23}$ molecules or atoms.

Quantum Numbers

A set of four values ($n$, $\text{l}$, $m_\text{l}$, $m_s$) that determine the shape, size, and orientation of an electron's probability density.

Principal Quantum Number ($n$)

An integer designation (1, 2, 3, 4, etc.) representing the electron shell (K, L, M, N, O).

Valence Electrons

Electrons located in the outermost unfilled shells that are available for bonding and determine an atom's chemical properties.

Electronegativity

A value ranging from $0.7$ to $4.0$ that represents the tendency of an atom to acquire electrons; larger values indicate a stronger tendency.

Ionic Bonding

Primary bonding occurring between metals and nonmetals with large differences in electronegativity, requiring electron transfer and resulting in coulombic attraction.

Covalent Bonding

A type of bonding where atoms with similar electronegativities share electrons, often involving $s$ and $p$ orbitals.

Metallic Bonding

Bonding in which valence electrons are delocalized to form an "electron cloud" surrounding the ion cores.

Secondary Bonding (van der Waals)

Weak attractive forces arising from atomic or molecular dipoles, which can be fluctuating or permanent.

Crystalline Materials

Materials in which atoms are arranged in periodic, 3D arrays; typical of metals, many ceramics, and some polymers.

Noncrystalline (Amorphous) Materials

Materials where atoms have no periodic arrangement, often occurring due to complex structures or rapid cooling.

Coordination Number

The number of nearest-neighbor or touching atoms in a crystal structure.

Atomic Packing Factor (APF)

The ratio of the volume of atoms in a unit cell to the total volume of the unit cell, calculated as $\frac{\text{Volume of atoms in unit cell}}{\text{Volume of unit cell}}$, assuming hard spheres.

Simple Cubic (SC) Crystal Structure

A rare crystal structure where atoms are located only at the eight corners of a cube; it has a coordination number of 6 and an APF of $0.52$. Polonium (Po) is an example.

Body-Centered Cubic (BCC) Structure

A crystal structure with atoms at the 8 cube corners and a single atom at the cube center, featuring a coordination number of 8 and an APF of $0.68$. Examples include Cr, W, and $\text{Fe}(\text{α})$.

Face-Centered Cubic (FCC) Structure

A crystal structure with atoms at 8 cube corners and at the centers of the 6 faces, featuring a coordination number of 12 and the maximum achievable APF of $0.74$. Examples include Al, Cu, and Au.

Hexagonal Close-Packed (HCP) Structure

A crystal structure with an ABAB… stacking sequence of close-packed planes, featuring a coordination number of 12 and an APF of $0.74$. Examples include Cd, Mg, Ti, and Zn.

Polymorphism (Allotropy)

The phenomenon where a material can exist in two or more distinct crystal structures depending on temperature and pressure, such as iron ($\text{α-Fe}$, $\text{γ-Fe}$, $\text{δ-Fe}$).

Anisotropy

The condition where the value of a property (e.g., modulus of elasticity) depends on the crystallographic direction of measurement, typically observed in single crystals.

Isotropy

The condition where properties are independent of crystallographic orientation, often found in polycrystalline materials with randomly oriented grains.

Miller Indices

A system of indices $(hkl)$ used to specify crystallographic planes based on the reciprocals of the intercepts with the axes.

Linear Density (LD)

The number of atoms centered on a specific direction vector divided by the length of that direction vector.

Planar Density (PD)

The number of atoms centered on a specific crystallographic plane divided by the area of that plane.

Bragg's Law

An equation used in X-ray diffraction to determine interplanar spacing: $n\text{λ} = 2d\text{sin}(\text{θ})$, where $\text{λ}$ is the X-ray wavelength and $d$ is the distance between parallel planes of atoms.