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 have py and pz orbitals
occupied 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
Hybridized Bonds

• Elemental carbon (no other elements)

sp2 hybridization
?
?
?
?
trigonal symmetry
one s two p orbitals ? 3 (x 2) electron
states (resulting orbital is a combination)
5
Hybridized Bonds
sp1 hybridization
rare for elemental C
linear bonding

-
?
unchanged
one s one p orbitals ? 2 (x 2) electron states
6
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

7
Metallic Structures

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

1-D
2-D
8

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

9
mirror
glide
a2
a1
10
B
A
11
C
A
12
AB
BABABA.
A
13
AC
CACACA.
A
14
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15
AB
ABCABC.
C
16
Close-packed Structures

• Metallic materials have isotropic bonding
• In 2-D close-packed spheres generate a hexagonal
  array
• In 3-D, the close-packed layers can be stacked in
  all sorts of sequences
• Most common are
• ABABAB..
• ABCABCABC

Hexagonal close-packed
Cubic close-packed
17
AB
BABABA.
A
18
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