Lesson 7 Crystal Structures, Unit Cells, X-ray diffraction by crystals, types of crystals and amorphous solids.

Crystalline solids

has rigid and long-range order and its atoms, molecules, or ions occupy specific positions.

the arrangement of particles is such that the net attractive intermolecular forces are at their maximum.

Amorphous solids

observes absence of a well-defined arrangement and long-range molecular order.

Atoms or ions

are thought of as being solid spheres having well-defined diameters known as the atomic hard-sphere model, in which spheres representing nearest-neighbor atoms touch one another.

Lattice

is used in the context of crystal structure.
“three-dimensional array of points coinciding with atom positions or sphere centers”.

Unit cell

is the basic structural (repeating) unit of a crystalline solid.

Lattice point

For each sphere, it denotes an atom, ion, or molecule and it is called a

Face centered cubic

is a crystal structure with atoms located at each of the corners and the center of all the cube faces. Some familiar metals having this crystal structure are copper, aluminum, silver, and gold .

the coordination number (for metals)

wherein each atom has the same number of nearest-neighbor or touching atoms

atomic packing factor (APF)

which is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume.

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.

Simple cubic (SC) 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 parking factor.

Polonium

a metalloid or a semi-metal is the only simple-cubic element that has a relatively low atomic packing factor.

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.

X-ray diffraction

refers to the scattering of X-rays by the units of a crystalline solid. The patterns produced by scattering, or diffraction are used to deduce the particle arrangement in the solid lattice.

electron-density contour map

basically tells us the relative electron densities at various locations in a molecule. The densities reach a maximum near the center of each atom and in this manner we can determine the positions of the nuclei and thus the molecular geometric parameters.

Ionic Crystals Characteristics

They are composed of charged species, anions and cations are generally quite different in size.

High melting points

most ionic crystals have ___________which is an indication of the strong cohesive forces holding the ions together.

The ions are fixed in position.

therefore these solids do not conduct electricity. The ions are free to move when in the molten state (melted) or dissolved in water, thereby the resulting liquid is conducting electricity.

the lattice energy;

A measure of the stability of ionic crystals is ___________, the higher the lattice energy, the more stable the compound.

Covalent Crystals

exists when atoms are held together in an extensive three-dimensional network by covalent bonds.

Graphite

exists when atoms are held together inis considered as a good conductor of electricity in directions along the planes of carbon atoms, this is because electrons are free to move around in this extensively delocalized molecular orbital.an extensive three-dimensional network by covalent bonds.

Molecular Crystals

The lattice points in a molecular crystal are occupied by molecules which has a van der Waals forces and/or hydrogen bonding. The molecules in a molecular crystals (except for ice) are packed together as closely as their size and shape allow.

Amorphous Solids

lack a regular three-dimensional arrangement of atoms.

Glass

commonly refers to an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing.