Transcript lecture 2 powerpoint

Types of Primary Chemical Bonds
Isotropic, filled outer shells
• Metallic
– Electropositive: give up electrons
• Ionic
– Electronegative/Electropositive
• Colavent
– Electronegative: want electrons
+
+
+
e-
+
e-
+
+
+
+
+
-
+
-
+
-
+
-
+
+
e-
– Shared electrons along
bond direction
Close-packed
structures
Metallic Structures
• Goal: Achieve as many bonds as possible
• Treat as the close-packing of hard spheres
1-D
2-D
Definitions
• Lattice point
• Unit cell
• Lattice parameters
• Atoms per unit cell
• Fractional coordinates
• Symmetry operations
• Coordination number
mirror plane (glide plane)
Definitions
• Lattice point
• Unit cell
• Lattice
parameters
• Atoms per
unit cell
• Symmetry
operations
• Fractional
coordinates
• Coordination
number
unit cell
not a unit cell
2-fold rotation axes
a2
a1
6-fold rotation axis
3-fold
rotation
axes
high symmetry locations are good candidates for lattice points
B
A
C
A
AB ABABABA….
ACACACACA….
ABCABCABC….
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
ABABABABA….
Hexagonal close-packed structure
AB ABABABA….