Title: 2 Estrutura Atomica e Ligacao Interatomica
12 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
2ELECTRON ENERGY STATES
Electrons...
have discrete energy states tend to occupy
lowest available energy state.
3Tabela Periodica
Columns Similar Valence Structure
4Ligacao Atomica
- Forcas e Energias de Ligacao
5Energia 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
6Ligacoes Atomicas
- Ionica
- Covalente
- Metalica
- Van der Waals
Moleculas
7Ligacao Ionica
E necessario transferencia de eletrons
Exemplo NaCl
_
8COVALENT 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.
9SECONDARY BONDING
Arises from interaction between dipoles
Fluctuating dipoles
Permanent dipoles-molecule induced
-general case
-ex liquid HCl
-ex polymer
10METALLIC BONDING
Arises from a sea of donated valence
electrons (1, 2, or 3 from each atom).
Primary bond for metals and their alloys
11SUMMARY 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
12PROPERTIES FROM BONDING TM
Bond length, r
Melting Temperature, Tm
Bond energy, Eo
Tm is larger if Eo is larger.
13PROPERTIES FROM BONDING E
Elastic modulus, E
E curvature at ro
E is larger if Eo is larger.
14PROPERTIES FROM BONDING a
Coefficient of thermal expansion, a
a symmetry at ro
a is larger if Eo is smaller.
15SUMMARY 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)