Ch2 Continued Examples: Ionic Bonding NaCl MgO CaF 2 CsCl Give up electrons Acquire electrons Chapter 2 - 1 Ionic Bonding Occurs between metals and non-metals Ionic bonding is non-directionalthat is, the magnitude of the bond is equal in all directions around an ion. Chapter 2 - 2 Covalent Bonding similar electronegativity share electrons bonds determined by valence s & p orbitals dominate bonding Example: CH4
C: has 4 valence e-, needs 4 more CH 4 H: has 1 valence e-, needs 1 more H Electronegativities are comparable. H C H shared electrons from carbon atom H shared electrons from hydrogen
atoms covalent bond is directionalthat is, it is between specific atoms and may exist only in the direction between one atom and another that participates in the electron sharing. Chapter 2 - Metallic Bonding Metallic Bond: -Delocalized as electron cloud -Non-directional Q: Explain why covalently bonded materials are less dense than mettalic or ionically bonded ones? Chapter 2 - 4 Mixed Bonding Ionic-Covalent Mixed Bonding % ionic character = (X A X B )2 4
1 e x (100%) where XA & XB are Pauling electronegativities Ex: MgO XMg = 1.2 XO = 3.5 % ionic character 1 e ( 3 .5 1 . 2 ) 2 4
x (100%) 73.4% ionic Chapter 2 - 5 Pauling Electronegativities % ionic character = (X A X B )2 4 1 e x (100%)
Calculate the %IC for TiO2 and the C-H bond. Chapter 2 - 6 SECONDARY BONDING Arises from interaction between dipoles Fluctuating dipoles asymmetric electron clouds + - secondary bonding + - ex: liquid H 2 H2 H2
H H H H secondary bonding Adapted from Fig. 2.13, Callister & Rethwisch 8e. Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer + - H Cl secon dary secondary bonding
+ secondary bonding H Cl bondi ng - Adapted from Fig. 2.15, Callister & Rethwisch 8e. secondary bonding Chapter 2 - 7 Summary: Bonding Comments Type Bond Energy
Secondary smallest Directional inter-chain (polymer) inter-molecular Chapter 2 - 8 Properties From Bonding: Tm Bond length, r Melting Temperature, Tm Energy r Bond energy, Eo ro Energy r smaller Tm
unstretched length ro r Eo = bond energy larger Tm Tm is larger if Eo is larger. Chapter 2 - 9 Chapter 2 - 10 2.20 Make a plot of bonding energy versus melting temperature for the metals listed in Table 2.3. Using this plot, approximate the bonding energy for: a. Copper, which has a melting temperature of 1084C. b. Molybdenum, which has melting temperature of 2617C. Hg Al Fe W
MeltingT Bonding emp. Energy (0C) (eV/atom) -39 0.7 660 3.4 1538 4.2 3410 8.8 Chapter 2 - 11 Properties From Bonding : a Coefficient of thermal expansion, a length, Lo coeff. thermal expansion unheated, T1 DL = a(T2 -T1)
Lo DL heated, T2 a ~ symmetric at ro Energy unstretched length ro Eo Eo r a is larger if Eo is smaller. larger a smaller a Chapter 2 - 12 Summary: Primary Bonds Ceramics (Ionic & covalent bonding):
Metals large Tm large E small a Variable bond energy (Metallic bonding): Polymers (Covalent & Secondary): secon d Large bond energy ary bondi ng moderate Tm moderate E moderate a
Directional Properties Secondary bonding dominates small Tm small E large a Chapter 2 - 13 Water (Its Volume Expansion Upon Freezing) https://www.youtube.com/watch?v=4i5r65QGUpw Chapter 2 - 14 Unusual Expansion of Water Most substances contract upon cooling. But, water expands while cooling from 4 0C until it freezes. Chapter 2 -
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