1. Ch2 Continued… Examples: Ionic Bonding
• Predominant bonding in Ceramics
NaCl
MgO
Give up electrons
Acquire electrons
CaF 2
CsCl

2. Covalent Bonding similar electronegativity  share electrons
bonds determined by valence – s & p orbitals dominate bonding
Example: CH4
shared electrons
from carbon atom
from hydrogen
atoms H C CH 4
C: has 4 valence e-,
needs 4 more
H: has 1 valence e-,
needs 1 more
Electronegativities
are comparable.
Adapted from Fig. 2.10, Callister & Rethwisch 8e.

3. Metallic Bonding Metallic Bond: -Delocalized as electron cloud
-Non-directional

4. Mixed Bonding Ionic-Covalent Mixed Bonding % ionic character =
where XA & XB are Pauling electronegativities %)
100 ( x
Ex: MgO XMg = XO = 3.5

5. Pauling Electronegativities
For TiO2, XTi = 1.5 and XO = 3.5, and therefore,

6. SECONDARY BONDING + - Arises from interaction between dipoles
• Fluctuating dipoles
asymmetric electron
clouds + -
secondary
bonding H 2
ex: liquid H
Adapted from Fig. 2.13, Callister & Rethwisch 8e.
• Permanent dipoles-molecule induced + -
-general case:
secondary
bonding
Adapted from Fig. 2.15,
Callister & Rethwisch 8e. Cl Cl
-ex: liquid HCl
secondary H H
bonding
secondary bonding
-ex: polymer
secondary bonding

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

8. Properties From Bonding: Tm
• Bond length, r
• Melting Temperature, Tm r o
Energy r
• Bond energy, Eo Eo =
“bond energy”
Energy r o
unstretched length
smaller Tm
larger Tm
Tm is larger if Eo is larger.

10. 2.20 Make a plot of bonding energy versus melting temperature for the metals listed in Table Using this plot, approximate the bonding energy for copper, which has a melting temperature of 1084C.

11. Properties From Bonding : a
• Coefficient of thermal expansion, a D L
length, o
unheated, T 1
heated, T 2
coeff. thermal expansion D L = a ( T - T ) 2 1 L o
• a ~ symmetric at ro r o
smaller a
larger a
Energy
unstretched length Eo
a is larger if Eo is smaller.

12. Summary: Primary Bonds
Ceramics
Large bond energy
large Tm
large E
small a
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Variable bond energy
moderate Tm
moderate E
moderate a
Polymers
(Covalent & Secondary):
Directional Properties
Secondary bonding dominates
small Tm
small E
large a
secondary bonding

13. Water (Its Volume Expansion Upon Freezing)

14. Unusual Expansion of Water
Most substances contract upon cooling. But, water expands while cooling from 4 0C until it freezes.