11

2 Estrutura Atomica e Ligacao 2 Estrutura Atomica e Ligacao InteratomicaInteratomica

IntroducaoIntroducaoPropriedades dependem Propriedades dependem dos arranjos dos arranjos geometricos dos atomos geometricos dos atomos e das interacoes (entre e das interacoes (entre atomos ou moleculas)atomos ou moleculas)

orbital electrons: n = principal quantum number

n=3 2 1

Nucleo: Z = # protons

N = # neutrons

Massa Atomica A ≈ Z + N

Atomo de Bohr

22

• have discrete energy states• tend to occupy lowest available energy state.

Incr

easi

ng e

nerg

y

n=1

n=2

n=3

n=4

1s2s

3s2p

3p

4s4p

3d

Electrons...ELECTRON ENERGY STATESELECTRON ENERGY STATES

33

He

Ne

Ar

Kr

Xe

Rn

inert

gases

accept

1e

accept

2e

giv

e u

p 1e

giv

e u

p 2e

giv

e u

p 3e

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

• Columns: Similar Valence Structure

Tabela PeriodicaTabela Periodica

44

Ligacao AtomicaLigacao Atomica Forcas e Energias de LigacaoForcas e Energias de Ligacao

55

Energia de LigacaoEnergia de Ligacao

Resistencia MecanicaResistencia Mecanica Temperatura de FusaoTemperatura de Fusao Modulo de ElasticidadeModulo de Elasticidade Coeficiente de Expansao termicaCoeficiente de Expansao termica

Distancia de EquilibrioDistancia de Equilibrio Energia Total e minimaEnergia Total e minima Forcas de Atracao e repulsao sao Forcas de Atracao e repulsao sao

nulasnulas

66

Ligacoes AtomicasLigacoes Atomicas

IonicaIonica CovalenteCovalente MetalicaMetalica Van der WaalsVan der Waals

MoleculasMoleculas

77

Ligacao Ionica Ligacao Ionica

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

• Ocorre entre ions + e – • E necessario transferencia de eletrons

• Exemplo: NaCl

+_

88

• Requires shared electrons• Example: CH4

C: has 4 valence e, needs 4 more

H: has 1 valence e, needs 1 more

Electronegativities are comparable.

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH4

COVALENT BONDINGCOVALENT BONDING

99

Arises from interaction between dipoles

• Permanent dipoles-molecule induced

• Fluctuating dipoles

+ - secondary bonding + -

H Cl H Clsecondary bonding

secondary bonding

HH HH

H2 H2

secondary bonding

ex: liquid H2asymmetric electron clouds

+ - + -secondary bonding

-general case:

-ex: liquid HCl

-ex: polymer

SECONDARY BONDINGSECONDARY BONDING

1010

• Arises from a sea of donated valence electrons (1, 2, or 3 from each atom).

• Primary bond for metals and their alloys

+ + +

+ + +

+ + +

METALLIC BONDINGMETALLIC BONDING

1111

Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

Comments

Nondirectional (ceramics)

Directional(semiconductors, ceramics

polymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)

inter-molecular

SUMMARY: BONDINGSUMMARY: BONDING

1212

• Bond length, r

• Bond energy, Eo

F F

r

• Melting Temperature, Tm

Eo=

“bond energy”

Energy (r)

ro r

unstretched length

r

larger Tm

smaller Tm

Energy (r)

ro

Tm is larger if Eo is larger.

PROPERTIES FROM BONDING: PROPERTIES FROM BONDING: TTMM

1313

• Elastic modulus, E

• E ~ curvature at ro

cross sectional area Ao

L

length, Lo

F

undeformed

deformed

L F Ao

= E Lo

Elastic modulus

r

larger Elastic Modulus

smaller Elastic Modulus

Energy

ro unstretched length

E is larger if Eo is larger.

PROPERTIES FROM BONDING: PROPERTIES FROM BONDING: EE

1414

• Coefficient of thermal expansion,

• ~ symmetry at ro

is larger if Eo is smaller.

L

length, Lo

unheated, T1

heated, T2

= (T2-T1) L Lo

coeff. thermal expansion

r

smaller

larger

Energy

ro

PROPERTIES FROM BONDING: PROPERTIES FROM BONDING:

1515

Ceramics(Ionic & covalent bonding):

Metals(Metallic bonding):

Polymers(Covalent & Secondary):

secondary bonding

Large bond energylarge Tm

large Esmall

Variable bond energymoderate Tm

moderate Emoderate

Directional PropertiesSecondary bonding dominates

small Tsmall Elarge

SUMMARY: PRIMARY SUMMARY: PRIMARY BONDSBONDS