Primary

1
Types of Chemical Bonds

• Primary
• Ionic
• Electronegative/Electropositive
• Metallic
• Electropositive give up electrons
• Colavent
• Electronegative want electrons
• Shared electrons along bond direction
• Secondary
• Fluctuating/instantaneous dipoles
• Permanent dipoles (H-bonds)

Isotropic, filled outer shells
2
Covalent Bonds

• Locally well-defined orbitals
• Elements with electrons up to 2p or 3p states
• Filled outer shell ? octet rule (s p ? 8
  states)
• Rule 8-N electrons n bonds
• Example carbon (C)
• 6 electrons 1s22s22p2
• 2s22p2 ? N 4 ? n 4

how can carbon atoms fill px, py and pz orbitals
if the other element is also electronegative?
s orbital
p orbitals
?
?
?
?

• solution sp2 or sp3 hybrization

http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
orbitals.gif
3
Hybridized Bonds

• Elemental carbon (no other elements)

sp3 hybridization
?
?
?
?
also methane CH4
one s three p orbitals ? 4 (x 2) electron
states (resulting orbital is a combination)
4
Properties from Bond-Energy Curve
Mechanical Properties
E
R (interatomic distance)
R0
E0
F dE/dR
The bond force curve
Elastic constants relate stress to strain
Stress related to force Strain related to
displacement
at R0 no net force (equilibrium bond distance)
attractive
F k Dx
F
k
stressarea
strainlength
R0
R (interatomic distance)
stress ? k strain
repulsive
Elastic constants given by slope of B.F. curve at
R0 given by curvature
of B.E. curve at R0
5
Chemical Bonding

• Covalent between electronegative elements
• Metallic between electropositive elements
• Ionic between different elements with differing
  electronegativities
• Clear distinction between metallic non-metallic
• Ionic covalent somewhat qualitative boundary
• ionic chararacter 1 exp( -¼ (xA xB)2)
• xA, xB electronegativities

6
Metallic Structures

• Goal Achieve as many bonds as possible
• Treat as the close-packing of hard spheres

1-D
2-D
7

• Definitions
• Lattice point
• Unit cell
• Lattice parameters
• Atoms per unit cell
• Fractional coordinates
• Symmetry operations
• Coordination number

8
mirror
glide
a2
a1