1. Atomic structure & bonding
Materials Science
Atomic structure & bonding

2. Understanding materials
Properties
Structure
Processing
“Materials Science”
“Materials Engineering”
Electronic level (subatomic)
Atomic (molecular level, chemical composition)
Crystal (arrangement of atoms or ions wrt one another)
Microstructure (can study with microscopes)
Macrostructure (can see with naked eye)

3. Structure of the Atom Niels Bohr model of the atom
Valence electrons
(participate in bonding)
Niels Bohr model of the atom
Electrons –ve orbit nucleus in discrete orbital shells
Electrons have quantized positions, and specific energies
Stable configurations have full outer shell d p s
Nucleus contains Protons
(1 Hydrogen – 94 Plutonium)
and Neutrons.
Atomic mass ~ Protons + Neutrons

4. Electron energy states
Lowest energy state is filled first
Electrons are characterized by quantum numbers:
Principal (1,2,3…) relates to distance from nucleus
Second (s,p,d,f) relates to shape of subshell
Max. number of electrons per subshell: s=2, p=6, d=10, f=14

5. Stable elements Stable electron configurations:
Have complete s and p sub-shells (octet)
Tend to be very unreactive.

6. Survey of elements Most elements are not stable… Why?
Adapted from Table 2.2, Callister 6e.
Most elements are not stable… Why?
Valence electrons determine physical and chemical properties (bonding).

7. The periodic table – History
Credited to Dmitri Mendeleev (1834 – 1907).
Russian chemist and inventor
Recognized periodicity amongst the elements
We now know that the atomic structure of elements determines the properties observed

8. The periodic table Columns have similar valence structure
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
Adapted from Fig. 2.6, Callister 6e.

9. The periodic table – properties
Elements are grouped into columns, that have similar numbers of valence electrons… and hence properties.
Electron donors (left) are metals and electron acceptors (right) non-metals.
Through bonding, atoms can achieve a full outer electron shell, with lower energy and more stability.

10. Bonding in solids Primary bonding Secondary bonding (weaker)
Ionic (ceramics – some covalent)
Covalent (polymer C=C bonds)
Metallic (metals)
Secondary bonding (weaker)
Van der Waals (polymers)
Hydrogen (similar to VdW)
We will consider the mechanisms and characteristics.

11. Ionic bonding – origin - + Na (metal) Cl (nonmetal) Unstable unstable
Compounds of metallic and non-metallic elements (e.g. NaCl, Al2O3, MgO, many ceramics)
Requires electron transfer (+ve and –ve ions) and large difference in electronegativity.
Atomic number of Na = 11 (1 valence electron)
Atomic number of Cl = 17 (7 valence electrons)
Na (metal)
Unstable
Cl (nonmetal)
unstable
electron + -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)

12. Ionic bonding – examples
Give up electrons
Acquire electrons

13. Ionic bonding – characteristics
Bonding is:
Non-directional
Relatively strong
Material properties:
Often ceramic (e.g. Alumina, Al2O3)
High melting points (e.g. 2,200°C)
High elastic modulus (e.g. E=400 GPa)
Brittle (difficult for atoms to slide/ rearrange)
Electrical and thermal insulators (no free electrons)

14. Covalent bonding – origin
Stable electron configurations by sharing electrons between atoms
Shared electrons belong to both atoms
Typically non-metal compounds (polymers C-C & C-H bonds)
E.g. Methane (CH4)
C – has 4 valence e, needs 4 more
H – has 1 valence e, needs 1 more.
Electronegatvities are comparable

15. Covalent bonding – examples

16. Covalent bonding – examples
Compounds containing elements on right side of table (GaAs, Si3N4)
Non-metallic molecules (H2, Cl2)
Some elemental solids (Si, C)
C – Diamond
Prevalent in polymers
Occurs in ceramics

17. Covalent bonding – characteristics
Bonding is:
Directional (exists in specific orientation)
Very strong
Material properties:
Often polymers, glasses and ceramics (e.g. Diamond)
Less dense than ionic/metallic bonded materials (directional bonding makes is harder to ‘pack’ atoms)
High elastic modulus (e.g. E~1000 GPa)
High melting point (e.g. 3,550°C)
Brittle (strong, directional atomic bonds)
Electrical and thermal insulators
…But polymers have low melting point and stiffness?...

18. Metallic bonding – origins
Metals and alloys
Low number of valence electrons (1, 2, 3 from each atom)
Valence electrons become delocalized
Electrons are not bound to any particular atom
Electrons are free to drift throughout the metal
‘Sea of electrons’ around positive ion cores
High electrical conductivity

19. Metallic bonding- characteristics
Bonding is:
Non-directional
Intermediate strength
Material properties:
Metals (e.g. Aluminium – Al, Tungsten – W)
Intermediate melting point (Al~660°C, W~3,410°C)
Intermediate elastic modulus (Al~70, W~400 GPa)
Close packing of atoms (high density)
High electrical and thermal conductivity (free electrons)
Ductile (planes of atoms can slide over each other).

20. Van der Waals – origins
Secondary bond is weak in comparison to Primary bonds
It arises from atomic or molecular dipoles, e.g. asymmetric molecules
Secondary bonding in inert gases or between covalently bonded molecules

21. Van der Waals – examples
• Fluctuating dipoles
• Permanent dipoles-molecule induced
Adapted from Fig. 2.14,
Callister 6e.
-general case:
-ex: liquid HCl
Adapted from Fig. 2.14,
Callister 6e.
-ex: polymer

22. Van der Waals – characteristics
Bonding is:
Weak
Directional
Material properties:
Polymers (between covalent chains)
Low stiffness (E<5 GPa)
Low melting point (<400°C)
Very ductile

23. Hydrogen bonding Weak, secondary bond (H-H)
Occurs from interaction and delocalisation of hydrogen electrons
Not significant for this course

24. Atomic bonding - Summary
Type
Ionic
Covalent
Metallic
Secondary
Bond Energy
Large!
Variable
large-Diamond
small-Bismuth
large-Tungsten
small-Mercury
smallest
Comments
Nondirectional (ceramics)
Directional
(semiconductors, ceramics
polymer chains)
Nondirectional (metals)
inter-chain (polymer)
inter-molecular

25. Bonding energies
The forces result from the energy potential between the atoms
Force is the space differential of energy
This energy equilibrium reveals fundamental properties of materials

26. Secondary bonding dominates
Bonding in materials
Ceramics
Large bond energy
large Tm
large E
(Ionic & covalent bonding):
Metals
Variable bond energy
moderate Tm
moderate E
(Metallic bonding):
Polymers
Secondary bonding dominates
small Tm
small E
(Covalent & Secondary):

27. Typical bond properties
Example
eV / atom
M’Pt (°C)
E (GPa)
Ionic
MgO 5
2,800
250
Covalent
C (diamond) 7
3,550+
1,000
Metallic Al 3
660 70 W 8
3,410
400
VdW
PVC
0.5
210
Bond strength determines fundamental properties (MPt & stiffness)
But: strength of materials is dependent on defects within materials (e.g. chalk). We will consider defects later…

28. Exercise: Bonding in materials…
What bonding would you expect in…
CaF2 ?
Bronze (Cu-Sn alloy) ?
Polyethylene ?
SiC ?
Solid Xe ?
Which has…
The highest melting point ?
Most ductile ?
The greatest electrical conductivity ?