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ISSUES TO ADDRESS...

CHAPTER 2: Atomic Structure and Interatomic Bonding. ISSUES TO ADDRESS. • What promotes bonding?. • What types of bonds are there?. • What properties are inferred from bonding?. BOHR ATOM. Adapted from Fig. 2.1, Callister 6e. Nucleus: Z = # protons.

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ISSUES TO ADDRESS...

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

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

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

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

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

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

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

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

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

  10. COVALENT BONDING • Requires shared electrons • Example: CH4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Electronegativities are comparable.

  11. Calculating the degree of ionic character % ionic character = [1-e(-.25xDen2)]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. EXAMPLES: COVALENT BONDING • Molecules with nonmetals • Molecules with metals and nonmetals • Elemental solids (RHS of Periodic Table) • Compound solids (about column IVA)

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

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

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

  16. PROPERTIES FROM BONDING: TM • Bond length, r • Melting Temperature, Tm • Bond energy, Eo Tm is larger if Eo is larger.

  17. PROPERTIES FROM BONDING: E • Elastic modulus, E • E ~ curvature at ro • E ~ curvature at ro E is larger if Eo is larger.

  18. PROPERTIES FROM BONDING: a • Coefficient of thermal expansion, a • a ~ symmetry at ro a is larger if Eo is smaller.

  19. Properties of common Metals

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

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