Metallic Bonding

1
Chapter 4

• Metallic Bonding

2
Metallic Bonding

• Free movement of electrons throughout the metal
  structure
• electrical and thermal conductivity
• reflectivity
• Lack of directional bonding
• malleability and ductility
• MO theory best explains these properties

3
Molecules of Metal Atoms
4
Band Structure
5
Metals, Insulators, and Semiconductors
6
Doped Semiconductors
7
Structure of Metals

• crystal lattice
• arrangement of metals atoms
• 4 main types of arrangements
• simple cubic
• body-centered cubic
• hexagonal close-packed
• cubic close-packed

8
Simple Cubic (sc)

• coordination number of 6
• not very common
• polonium

9
Body-Centered Cubic (bcc)

• coordination number of 8

10
Close Packing

• Most efficient way of filling space
• Built up from hexagonal arrays of atoms

vs.
11
Hexagonal Close-Packed (hcp)

• coordination number of 12
• abab arrangement

12
Cubic-Close Packed (ccp) or Face-Centered Cubic
(fcc)

• coordination number of 12
• abcabc arrangement

13
Packing Efficiency

• Most metals adopt one of the more compact
  arrangements
• As the number of valence electrons increases,
  arrangements generally move from bcc to hcp then
  to fcc

14
Common Arrangements
15
Unit Cells

• Simplest arrangement of atoms which will
  reproduce the crystal structure
• Can calculate how many atoms are present in each
  unit cell
• Each atom at the corner is 1/8 inside the cell
• Each atom at the face is 1/2 inside the cell
• Each atom on the edge is 1/4 inside the cell
• Knowing the arrangement and the density allows
  calculation of the metallic radius

16
SC Unit Cell

• 8(1/8) 1 total atom per unit cell

17
BCC Unit Cell

• 1 8(1/8) 2 atoms per unit cell

18
FCC Unit Cell

• 6(1/2) 8(1/8) four atoms per unit cell

19
Alloys

• Combination of two or more metals
• held together by metallic bonds
• Two types
• solid solutions
• alloy compounds

20
Solid Solution Alloys

• Molten metals blend to form a homogeneous mixture
• atoms have to be about the same size
• crystals have to be of the same arrangement
• metals must have similar properties

21
Alloy Compounds

• Formation of precise stoichiometric phases
• CuZn, Cu5Zn8, and CuZn3