1. ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What properties are inferred from bonding? CHAPTER 2: Atomic Structure and Interatomic Bonding

2. Adapted from Fig. 2.1, Callister 6e. BOHR ATOM Nucleus: Z = # protons = 1 for hydrogen to 94 for plutonium N = # neutrons Atomic mass A ≈ Z + N

3. have discrete energy states tend to occupy lowest available energy state. Electrons... ELECTRON ENERGY STATES

4. have complete s and p subshells tend to be unreactive. Stable electron configurations... STABLE ELECTRON CONFIGURATIONS

5. Why? Valence (outer) shell usually not filled completely. Most elements: Electron configuration not stable. SURVEY OF ELEMENTS

6. Columns: Similar Valence Structure Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. THE PERIODIC TABLE

7. Ranges from 0.7 to 4.0, Smaller electronegativityLarger electronegativity Large values: tendency to acquire electrons. ELECTRONEGATIVITY

8. Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl IONIC BONDING

9. Predominant bonding in Ceramics Give up electronsAcquire electrons EXAMPLES: IONIC BONDING

10. Requires shared electrons Example: CH 4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Electronegativities are comparable. COVALENT BONDING

11. Calculating the degree of ionic character % ionic character = [1-e (-.25x  en2) ]x 100 Example: en(Mg) = 1.2 en(O) = 3.5 %ionic character = {1-e [-.25*(3.5-1.2)^2] } x 100 = 73.3%

12. Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA) EXAMPLES: COVALENT BONDING

13. Arises from a sea of donated valence electrons (1, 2, or 3 from each atom). Primary bond for metals and their alloys METALLIC BONDING

14. Arises from interaction between dipoles Permanent dipoles-molecule induced Fluctuating dipoles -general case: -ex: liquid HCl -ex: polymer Adapted from Fig. 2.13, Callister 6e. Adapted from Fig. 2.14, Callister 6e. Adapted from Fig. 2.14, Callister 6e. SECONDARY BONDING

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

16. Bond length, r Bond energy, E o Melting Temperature, T m T m is larger if E o is larger. PROPERTIES FROM BONDING: T M

17. Elastic modulus, E E ~ curvature at r o E is larger if E o is larger. PROPERTIES FROM BONDING: E E ~ curvature at r o

18. Coefficient of thermal expansion,   ~ symmetry at r o  is larger if E o is smaller. PROPERTIES FROM BONDING: 

19. MaterialMelting Temp ( o C)Elastic Modulus (GPa) Thermal Expansion Coefficient , (10 -6 m/m/ o C) Tungsten, W34104104.5 Iron, Fe153820012 Aluminum, Al6606923 Magnesium, Mg6494514.4 Properties of common Metals

20. Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Polymers (Covalent & Secondary): Large bond energy large T m large E small  Variable bond energy moderate T m moderate E moderate  Directional Properties Secondary bonding dominates small T small E large  SUMMARY: PRIMARY BONDS