Bonds and crystals

Types of Materials Metals strong, ductile Ceramics and glasses resistant, brittle Polymers ductile, inexpensive Composites...

Types of Bonds Primary bonds outer orbital electrons are transferred or shared between/amongst atoms. Secondary bonds subtle attraction between positive and negative charges. No sharing of electrons.

Primary Bonds Ionic Covalent Metallic

Ionic Bonds Electron transfer from one atom to another. e.g. transfer electron from sodium to chlorine Bond forms from Coulombic attraction: the oppositely charged ions are drawn together.

Opposite charges attract each other. The closer they are, the lower the energy of the bond. However, two objects can’t occupy the same position. There is a repulsive force to prevent this. Bonding

Bonding Force Attractive Force Repulsive Force Net Bonding Force

Force Position Energy Position F = dE/dx Equilibrium: force=0, E is minimum

Covalent Sharing of valence electrons Valence electrons are outer orbital electrons that take part in bonding

Non-directional electron sharing “Delocalized” electrons electron cloud leads to high electrical conductivity Metallic

Similar to ionic bonding (Coulombic) No electrons are transfered. From assymetric distribution of + and - charges - “dipole” Hydrogen bridge is most common (water) Secondary (VanDerWaals) Bonds

Secondary are typically less than 1% of primary bond strength. Primary vs Secondary

EnergyPosition

(a) C 60 molecule, or buckyball. (b) Cylindrical array of hexagonal rings of carbon atoms, or buckytube. (Courtesy of Accelrys, Inc.)

Arrangement of polymeric chains in the unit cell of polyethylene. The dark spheres are carbon atoms, and the light spheres are hydrogen atoms. The unit-cell dimensions are nm × nm × nm. (Courtesy of Accelrys, Inc.)