CLASSIFICATION OF MATERIALS,TYPES OF BONDING

 1 CLASSIFICATION OF MATERIALS

1Metals

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.

2 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.

 

3Ceramics

 

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.

 

4.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.

 

2 TYPES OF BONDING

1 Ionic Bonding

 

This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl- and less around Na, forming Na+. Ionic bonds are the strongest bonds.

1.2.2 Covalent Bonding

In covalent bonding, electrons are shared between the   molecules, to saturate the valency. The simplest example is the   H2 molecule,   where the electrons spend more time in between the nuclei than outside, thus producing bonding.

 

3 Metallic Bonding

 

In the metallic bond encountered in pure metals and metallic alloys, the atoms contribute their outer-shell electrons to a generally shared electron cloud for the whole block of metal.

 

Ø Secondary Bonding (Van der Waals)

Ø Fluctuating Induced Dipole Bonds Polar

Ø Molecule-Induced Dipole Bonds

Ø Permanent Dipole Bonds