2 Estrutura Atomica e Ligacao Interatomica

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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
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ELECTRON ENERGY STATES
Electrons...
have discrete energy states tend to occupy
lowest available energy state.
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Tabela Periodica
Columns Similar Valence Structure
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Ligacao Atomica

• Forcas e Energias de Ligacao

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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

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Ligacoes Atomicas

• Ionica
• Covalente
• Metalica
• Van der Waals

Moleculas
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Ligacao Ionica

• Ocorre entre ions e

E necessario transferencia de eletrons
Exemplo NaCl
_

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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.
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SECONDARY BONDING
Arises from interaction between dipoles
Fluctuating dipoles
Permanent dipoles-molecule induced
-general case
-ex liquid HCl
-ex polymer
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METALLIC BONDING
Arises from a sea of donated valence
electrons (1, 2, or 3 from each atom).
Primary bond for metals and their alloys
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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
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PROPERTIES FROM BONDING TM
Bond length, r
Melting Temperature, Tm
Bond energy, Eo
Tm is larger if Eo is larger.
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PROPERTIES FROM BONDING E
Elastic modulus, E
E curvature at ro
E is larger if Eo is larger.
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PROPERTIES FROM BONDING a
Coefficient of thermal expansion, a
a symmetry at ro
a is larger if Eo is smaller.
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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)