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2 Estrutura Atomica e Ligacao Interatomica

2 Estrutura Atomica e Ligacao Interatomica. Introducao Propriedades dependem dos arranjos geometricos dos atomos e das interacoes (entre atomos ou moleculas). Atomo de Bohr. Nucleo: Z = # protons. N = # neutrons. Massa Atomica A ≈ Z + N. ELECTRON ENERGY STATES. Electrons.

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2 Estrutura Atomica e Ligacao Interatomica

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  1. 2 Estrutura Atomica e Ligacao Interatomica • Introducao Propriedades dependem dos arranjos geometricos dos atomos e das interacoes (entre atomos ou moleculas) Atomo de Bohr Nucleo: Z = # protons N = # neutrons Massa Atomica A ≈Z + N

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

  3. Tabela Periodica • Columns: Similar Valence Structure

  4. Ligacao Atomica • Forcas e Energias de Ligacao

  5. Energia de Ligacao • Resistencia Mecanica • Temperatura de Fusao • Modulo de Elasticidade • Coeficiente de Expansao termica Distancia de Equilibrio • Energia Total e minima • Forcas de Atracao e repulsao sao nulas

  6. Ligacoes Atomicas • Ionica • Covalente • Metalica • Van der Waals Moleculas

  7. Ligacao Ionica • Ocorre entre ions + e – • E necessario transferencia de eletrons • Exemplo: NaCl _ +

  8. 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.

  9. SECONDARY BONDING Arises from interaction between dipoles • Fluctuating dipoles • Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer

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

  11. 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

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

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

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

  15. 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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