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

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.

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

Mixed Bonding Ionic-Covalent Mixed Bonding % ionic character = where XA & XB are Pauling electronegativities %) 100 ( x Ex: MgO XMg = 1.2 XO = 3.5

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

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

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

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.

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 copper, which has a melting temperature of 1084C.

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.

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

Water (Its Volume Expansion Upon Freezing) https://www.youtube.com/watch?v=4i5r65QGUpw

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