CLASSIFICATION OF MATERIALS

Metals

Valence  electrons are detached  from  atoms, and spread in  an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.

Semiconductors

 The bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

 Ceramics

 Atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Examples: glass, porcelain, many minerals.

 Polymers

 Are bound by covalent forces  and also by weak  van  der  Waals  forces, and usually based on H,  C and other non-metallic elements.  They  decompose  at moderate temperatures (100 - 400 C), and are lightweight.  Other  properties vary greatly. Examples: plastics (nylon, Teflon, polyester) and rubber.