Bonds and crystals

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Bonds and crystals
Types of Materials
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•
•
•
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.
Bonding
•
•
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 Force
Attractive Force
Net Bonding Force
Repulsive Force
Force
Position
F = dE/dx
Energy
Position
Equilibrium:
force=0, E is minimum
Covalent
•
Sharing of valence electrons
•
Valence electrons are outer orbital
electrons that take part in bonding
Metallic
•
•
•
Non-directional electron sharing
“Delocalized” electrons
•
electron cloud
leads to high electrical conductivity
Secondary
(VanDerWaals) Bonds
•
•
•
•
Similar to ionic bonding (Coulombic)
No electrons are transfered.
From assymetric distribution of + and charges - “dipole”
Hydrogen bridge is most common (water)
Primary vs Secondary
•
Secondary are typically less than 1% of
primary bond strength.
Energy
Position
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 0.255 nm × 0.494 nm × 0.741 nm. (Courtesy of Accelrys,
Inc.)
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