1. Atomic Structure and Bonding
CHAPTER 2
Atomic Structure
and Bonding 1

2. Why Study Atomic Structure and Interatomic Bonding?

3. Types of Atomic Bonds Atomic Bonding Strong Primary Bonds
Weak Secondary Bonds
Ionic Bond
Covalent Bond
Metallic Bond
Fluctuating Dipoles
Permanent Dipoles

4. Ionic Bond A primary bond formed by the transfer of one or
Types of Atomic Bonds
Ionic Bond
A primary bond formed by the transfer of one or
more electron from an electropositive atom to an
electronegative one.
The ions are bonded together in a solid crystal by electrostatic forces.
Example: NaCl crystal (see animation NaCl4 & 7)

5. Ionic Bonding of NaCl

7. The atoms in the bonded state are in a more stable energy condition than when they are unbonded
Ionic bonds form between oppositely charged ions because there is a net decrease in potential energy of the ions after bonding
The lattice energies and melting points of ionically bonded solids are relatively high

8. A primary bond resulting from the sharing of electrons.
Types of Atomic Bonds
Covalent Bond
A primary bond resulting from the sharing of
electrons.
Its involves the overlapping of half-filled orbitals of two atoms.
Example: Diamond, H2, H2O, methane

9. Covalent Bonding - Examples
F F
F F
F F
Bond Energy=160KJ/mol
O + O
O O
O = O
Bond Energy=28KJ/mol
N N
N N
N N
Bond Energy=54KJ/mol 9

10. Arrangement of water in solid ice and liquid water

11. Table 2.6

12. Metallic Bond A primary bond resulting from the sharing of
Types of Atomic Bonds
Metallic Bond
A primary bond resulting from the sharing of
delocalized outer electrons in the form of an
electron charged cloud by an aggregate of metal
atoms.
Example elemental Zinc

13. Metallic Crytal

14. Metallic Bond of Zinc

15. Metallic Bond Types of Atomic Bonds Positive Ion
Engineering Materials
Types of Atomic Bonds
Metallic Bond
Positive Ion
Valence electron charge cloud

16. Metallic bonding, particularly the outer electrons of the atom, accounts for many physical properties of metals, such as strength, malleability, ductility, thermal and electrical conductivity, opacity and luster
As the number of bonding electrons increases, the more attraction thus the bonding energies and melting points of the metals also increases

18. The high thermal and electrical conductivities of metals support the theory that some electrons are free to move through the metal crystal lattice.
Most metals can be deformed a considerable amount without fracturing because the metal atoms can slide past each other without completely disrupting the metallically bonded structure

20. The bond energies and melting points of different metals vary greatly.
In general, the fewer valence electrons per atom involve in metallic bonding, the more metallic the bonding is. That is, the valence electrons are freer to move
Metals with fewer delocalized electrons has low bonding energies and melting points ( bec electrons are less attracted in the ion so less amount of energy needed to break away the bond of ions )

21. Physical Properties explained through the Bonding
Melting points and boiling points
Metals tend to have high melting and boiling points because of the strength of the metallic bond. The strength of the bond varies from metal to metal and depends on the number of electrons which each atom delocalises into the sea of electrons and on the packing.

22. Electrical conductivity
Metals conduct electricity. The delocalised electrons are free to move throughout the structure in 3-dimensions.
Thermal conductivity
Metals are good conductors of heat. Heat energy is picked up by the electrons as additional kinetic energy (it makes them move faster). The energy is transferred throughout the rest of the metal by the moving electrons (free electrons )

23. Density Metal and ceramics are heavier than polymer because they are
Crystal structure for Ionic
Crystal structure for Covalent
Crystal structure of Metallic
Metal and ceramics are heavier than polymer because they are
densely packed. Metals and ceramics are non-directional in
nature thus can packed tightly.

24. Crystal and Amorphous Structure in Materials
CHAPTER 3
Crystal
and
Amorphous Structure
in Materials

25. Why study the structures of metals and ceramics?

26. A crystal's structure and symmetry play a role in determining many of its physical properties thus the behavior of a material 26

27. Crystal and Amorphous Structure in Materials
The physical structure of solid materials mainly depends on the arrangements of the atoms, ions, or molecules that make up the solid and the bonding forces between them.
IF the atoms or ions of solid are arranged in pattern that repeats itself in three dimensions. Forming solid that has long range order (LRO)  crystalline solid or crystalline materials
CRYSTAL STRUCTURE a solid composed of atoms, ions, or molecules arranged in a pattern that is repeats in three dimensions.
Examples metals, alloys and some ceramic materials
Amorphous or non crystalline materials. If the atoms or ions of solid are NOT arranged in a long range and repeatable manner, then forming solid that has short range order (SRO) Example liquid water has a short range order

28. Atomic Packing Most materials are crystalline – have a regularly
repeating pattern of structural units
Atoms often behave as if they are
hard and spherical
Layer A represents the close-packed
layer – there is no way to pack the atoms
more closely than this
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

29. Atomic structures are close-packed in three dimension
Close-packed hexagonal: ABABAB stacking sequence
Face-centered cubic: ABCABC stacking sequence
Packing fraction for CPH and FCC structures is 0.74 – meaning
spheres occupy 74% of all available space
Figure 4.8
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

30. Non Close-Packed Structures
Figure 4.9
Body-centered cubic:
ABABAB packing sequence
Packing fraction = 0.68
Figure 4.10
Amorphous structure:
Packing fraction ≤ 0.64
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

31. Atomic Packing in Ceramics
Figure 4.13
(a): Hexagonal unit cell with a W-C atom pair associated
with each lattice point
(b): Cubic unit cell with a Si-C atom pair associated with each
lattice point
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

32. Atomic Packing in Glasses
Amorphous silica is the bases of most glasses
Rapid cooling allows material to maintain amorphous
structure achieved after melting
Figure 4.14
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

33. Atomic Packing in Polymers
Figure 4.15
Atomic Packing in Polymers
Figure 4.16
Polymers have a
carbon-carbon
backbone with
varying side-groups
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

34. Polymer chains bond to each other through weak hydrogen bonds
Figure 4.17
Polymer chains bond to each other through weak hydrogen
bonds
Red lines indicate strong cross-linked carbon-carbon bonds
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

35. Polymer Structure (a): No regular repeating pattern of
Figure 4.18
(a): No regular repeating pattern of
polymer chains – results in a
glassy or amorphous structure
(b): Regions in which polymer chains
line up and register – forms
crystalline patches
(c): Occasional cross-linking allowing
they polymer to stretch – typical
of elastomers
(d): Heavily cross-linked polymers
exhibit chain sliding – typical of
epoxy
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

36. Crystal Structure in Materials
Space lattice
A three dimensional array of points each of which has identical
surroundings
Unit cell
Is a repeating unit of a space lattice. The axial lengths (a, b, c) and the axial angles
(alfa, beta, gamma) are the lattice constants of the unit cell
Unit

37. Unit cell
Is a repeating unit of a space lattice. The axial lengths (a, b, c) and the axial angles (alfa, beta, gamma) are the lattice constant of the unit cell

38. Crystal and Amorphous Structure in Materials
There are 7 different crystal classes according to the values of the axial lengths
and the axial angles (lattice constants, a, b, c, alfa, beta, and gamma).
Cubic
Tetragonal
Orthorhombic
Rhombohedral
Hexagonal
Monoclinic
Triclinic
Also there are 4 basic types of unit cell
Simple
Body centered
Faced centered
Side centered b c a

39. Materials Engineering

40. Principal metallic crystal structures
1. Simple Cubic crystal structure
2. Body- Centered Cubic (BCC) crystal structure
3. Faced- centered Cubic (FCC) crystal structure
4. Hexagonal Close-Packed (HCP) crystal structure
Coordination number - the number of closes neighbors to which the atom is bonded

41. Body- Centered Cubic (BCC) crystal structure
Examples are lithium, sodium, potassium, chromium, barium, vanadium and tungsten
Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure

42. The atoms in the BCC unit cell contact each other across the cube diagonal, so the relationship between the length of the cube side (a) and the atomic radius (R) is;
The central atom in the unit cell is surrounded by 8 nearest neighbor, therefore, the BCC unit cell has a coordination number of 8.

44. Atomic packing factor (APF)
The APF for the BCC unit cell is 68%; which means that 68% of the volume of the BCC unit cell is occupied by atoms and the remaining 32% is empty space.
Many metals (iron, chromium, vanadium) have the BCC crystal structure at room temperature.

45. The number of atoms in the BCC unit cell = 2
1 (at the center) + 8 * (1/8) = 2 atoms per unit cell
Example 1
Iron at 20 C is BCC with atoms of atomic radius nm. Calculate the lattice constant (a) for the cube edge of the iron unit cell.
Example 2
Calculate the atomic packing factor (APF) for the BCC unit cell, assuming the atoms to be hard spheres.

46. Faced- Centered Cubic (FCC) crystal structure
Examples : aluminum, copper, gold, iridium, lead, nickel, platinum and silver.

47. The atoms in the FCC unit cell contact each other across the cube face diagonal, so the relationship between the length of the cube side (a) and the atomic radius (R) is;
x2 = a2 + a2 a x
90 deg a
Each atom is surrounded by 12 other atoms, therefore, the FCC unit cell has a coordination number of 12.

49. Atomic packing factor (APF);
The APF for the FCC unit cell is 74%; which means that 74% of the volume of the FCC unit cell is occupied by atoms and the remaining 26% is empty space.
Many metals (copper, lead, nickel) have the FCC crystal structure at elevated temperatures (912 to 1394 C).

50. The number of atoms in the FCC unit cell = 4
8 * (1/8) + 6 * (1/2) = 4 atoms per unit cell
Example 3
Calculate the atomic packing factor (APF) for the FCC unit cell, assuming the atoms to be hard spheres.

51. Hexagonal close-packed (HCP) crystal structure
A bigger cell of HCP
A unit cell of HCP
Space lattice HCP

52. Each atom is surrounded by 12 other atoms, therefore, the HCP unit cell has a coordination number of 12.

54. 4 * (1/6) + 4 * (1/12) + 1 (center) = 2 atoms per unit cell
The number of atoms in the HCP unit cell = 2
4 * (1/6) + 4 * (1/12) + 1 (center) = 2 atoms per unit cell c a
Example 4
Calculate the volume of the zinc crystal structure unit cell by using the following data: Pure zinc has the HCP crystal structure with lattice constants a= nm and c= nm.

56. Atomic packing factor (APF)
The APF for the HCP unit cell is 74%; and equal to that of FCC.

57. Volume density unit-cell calculation
Example 5:
Copper has an FCC crystal structure and an atomic radius of nm. Assuming the atoms to be hard sphere that touch each other along the face diagonals of the FCC unit cell, calculate a theoretical value for the density of copper in megagrams per cubic meter. The atomic mass of copper is g/mol.
1g = 1x10 -6 Mg
1 nm = 1x10-9 m

58. Planar density unit-cell calculation
Fig 3.13 : Cubic Crystal planes

59. Planar density unit-cell calculation
Example 6
Calculate the planar atomic density on the (110) plane of the α iron BCC
lattice in atoms per square milimeters. The lattice constant of α is nm.

60. Linear density unit-cell calculation
Example 7
Calculate the linear atomic density in the (110) direction in the copper crystal lattice in atoms milimeters. Copper is FCC and has a lattice constant of nm

61. No. of atomic diameters intersected by the
Length of line are =2 atoms. a
(110)
Length of line= length of face diagonal=

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