Atomic Bonding 2

Phases (or state)diagrams represent the different phases that a substance can go through.
Example H20 Solid phase is ice
Liquid phase is water, the gaseous phase of H2O is steam.

Solids occur 2 forms:
Crystalline solids and
Amorphous solids

They possess an ordered three-dimensional geometric arrangment that repeats itself.
The majority of engineering and structural materials are crystaline ex. metals

Study of crystalline structures

They are used to describe the basic building blocks of basic geometric arrangement of atoms in a crystal.

The points on the unit cell represents atoms, ions or molecules of differing sizes.

The lines (labeled a, b & c) on the unit cell represent the lattice parameters in the x,y, & z directions. They are also known as intercepts.

The angles represent the angle measured in degrees between different planes. ?

Cubic
tetragonal
Orthohombic
Monoclinic
Triclinic
Hexagonal
Rhombohedral

Cubric-simple,body centered, face centered
Tetragonal-simple, body centered,
Orthohombic-simple, base centered, body centered, face centered
Monoclinic-simple, body centered
Triclinic-simple
Hexagonal
Rhombohedral-simple

see previous question

Coordination number (CN) is the number of neighboring atoms that each atom has immediately surrounding it.

The type of bonding and the realtive size of the atoms or ions involved.

Simple cubic CN=6

Body centered cubic CN=8

Face centered cubic CN=12

A allotrophic material can, after changing from one crystal structure to another reverse the phenomenon and return to its previous structure A polymorhic material CANNOT,

Represent defects in the perfect atomic arrangement and should not suggest that the material itself is defective.
Defects are often intentionally added to produce a desired set of mechanical or physical properties.

Crystal impurities and crystal defects.

Involve impure atoms
Crystal impurities occur when a small percentage of one material is added to another material.

An example of an impurity refers to the small percentage of copper in sterling silver that distinguishes sterling from pure silver. An example whould be alloys

In a mixture the two crystal structures remain distinct.

In a solid solution the two crystal structures of copper and zinc are said to diffuse to create one crystal structure.

Steel is a mixture of iron with a bcc structure and cementite with a fcc structure.
Brass is a solid solution (copper and zinc) because its sigle phase structure is all fcc.

Point defects
Line defects
Area defects

Point Defect


(absence of an atom at a lattice site in an otherwise regular crystal)

aka dislocations.
A dislocation is a linear array of atoms along which there is some imperfection in the bonding of the atoms.

Edge dislocation and Screw dislocation

Edge-when an extra row or plane of atoms is either added or eliminated
Screw-when a torsional force is applied to the lattice.

Plane on which the displacement of the atom occurs.

Area defects are surface defects

They do not have anyt repetitious patterns of atoms

Polymers are examples of amorphous solids and they have four basic structures

Linear polymers, branched polymers,
cross-linked polymers,
ladder polymers.

Linear Polymers (PVC)

Cross-Linked Polymers (Glass)