CHAPTER 2: BONDING AND PROPERTIES. What promotes bonding? What types of bonds are there? What properties are inferred from bonding?

CHAPTER 2: BONDING AND PROPERTIES • What promotes bonding? • What types of bonds are there? • What properties are inferred from bonding? 1 MATERIA...
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CHAPTER 2: BONDING AND PROPERTIES

• What promotes bonding? • What types of bonds are there? • What properties are inferred from bonding?

1

MATERIAIS Núcleos + Elétrons

Sólidos

Átomos Átomos Moléculas

Q: If you had only one sentence in which to pass on the next generation, the most important scientific knowledge we possess, what would that sentence be? Feynman: Everything is made of atoms!

Líquidos

Modelos Atômicos

• Plum pudding model • Rutherford model • Bohr model

Thomson’s experiments cathode ray tube

magnetic field

electrometer

Negative charges cannot be separated from the rays. http://www.aip.org/history/electron/electro.htm

Thomson’s experiments cathode ray tube

electric field

Cathode rays are negatively charged. http://www.aip.org/history/electron/jjappara.htm

Thomson’s experiment cathode rays

magnetic field

The charge-to-mass ratio was determined. http://www.aip.org/history/electron/jjemtube.htm

Millikan’s oil-drop experiment

The amount of charge carried by an electron was determined. http://www68.pair.com/willisb/millikan/experiment.html

Plum pudding model

Thomson proposed that the electrons are embedded in a positively charged `pudding’. http://www.physics.nus.edu.sg/einstein/

Rutherford scattering experiment

http://www.physics.nus.edu.sg/einstein/

Rutherford’s predicted result

It is like shooting bullets through tissue paper. http://www.physics.nus.edu.sg/einstein/

Rutherford’s observation

It is like shooting bullets through tissue paper and they come back and hit you! http://www.physics.nus.edu.sg/einstein/

Discovery of the atomic nucleus

There must be something that is small, dense and positively charged inside an atom. http://particleadventure.org/particleadventure/frameless/rutherfords_analysis.html

Solar system model of the atom

The atom consists of electrons orbiting around a small but dense nucleus. http://www.physics.nus.edu.sg/einstein/

Scale of the atom • Atom – Electrons & nucleus

• Nucleus – Protons & neutrons

• Proton & neutron – Quarks http://particleadventure.org/particleadventure/frameless/scale.html

Problem – stability issue

Rutherford’s model predicts that atoms are unstable! http://library.thinkquest.org/19662/low/eng/exp-rutherford.html

Problem – discrete spectra

Rutherford’s model cannot account for discrete spectra! http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

Bohr model – orbit quantization

Bohr proposed that only certain orbits for the electrons are allowed. http://www.physics.nus.edu.sg/einstein/

Energy level diagram

http://www.schoolisland.com/review/reference/phys5.htm

Quantum leap

http://www.physics.nus.edu.sg/einstein/

Photon absorption and emission

Electrons will only absorb or emit photons with precisely the right energies. http://www.physics.nus.edu.sg/einstein/

Hydrogen spectrum

http://www.physics.nus.edu.sg/einstein/

Emission & absorption spectra

http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

BOHR ATOM orbital electrons: n = principal quantum number 1 2 n=3

Adapted from Fig. 2.1,

Callister 6e.

Nucleus: Z = # protons = 1 for hydrogen to 94 for plutonium N = # neutrons Atomic mass A ≈ Z + N 2

FIGURE 2.2 (a) The first three electron energy states for the Bohr hydrogen atom. (b) Electron energy states for the first three shells of the wave-mechanical hydrogen atom. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

FIGURE 2.3 Comparison of the (a) Bohr and (b) wavemechanical atom models in terms of electron distribution. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

FIGURE 2.8 (a) The dependence of repulsive, attractive, and net forces as a function of interatomic separation for two isolated atoms. (b) The dependence of repulsive, attractive, and net potential energies as a function of interatomic isolated atoms. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

ELECTRON ENERGY STATES

Increasing energy

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

4p

n=4

4s

n=3

3s

n=2 n=1

2s 1s

3d

3p 2p Adapted from Fig. 2.5,

Callister 6e. 3

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

Z 2

Element Configuration Adapted from Table 2.2, 2 Callister 6e. He 1s 10 Ne 1s22s 22p6 18 Ar 1s2 2s22p63s23p6 36 Kr 1s2 2s22p63s23p63d10 4s24p6

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SURVEY OF ELEMENTS • Most elements: Electron configuration not stable. Element Atomic # Hydrogen 1 Helium 2 Lithium 3 Beryllium 4 Boron 5 Carbon 6 ... Neon 10 Sodium 11 Magnesium 12 Aluminum 13 ... Argon 18 ... ... Krypton 36

Electron configuration 1s 1 1s 2 (stable) 1s 22s 1 1s 22s 2 1s 22s 22p 1 Adapted from Table 2.2, Callister 6e. 1s 22s 22p 2 ... (stable) 1s 22s 22p 6 1s 22s 22p 63s 1 1s 22s 22p 63s 2 1s 22s 22p 63s 23p 1 ... 1s 22s 22p 63s 23p 6 (stable) ... 1s 22s 22p 63s 23p 63d 10 4s 24 6 (stable)

• Why? Valence (outer) shell usually not filled completely. 5

give up 1e give up 2e give up 3e

• Columns: Similar Valence Structure

H

Li Be

Metal Nonmetal Intermediate

accept 2e accept 1e inert gases

THE PERIODIC TABLE

He

Ne

O

F

Na Mg

S

Cl Ar

K Ca Sc

Se Br Kr

Rb Sr

Te

Y

Cs Ba

I

Adapted from Fig. 2.6,

Callister 6e.

Xe

Po At Rn

Fr Ra

Electropositive elements: Readily give up electrons to become + ions.

Electronegative elements: Readily acquire electrons to become - ions. 6

ELECTRONEGATIVITY • Ranges from 0.7 to 4.0, • Large values: tendency to acquire electrons. H 2.1 Li 1.0 Na 0.9

Be 1.5 Mg 1.2

K 0.8 Rb 0.8

Ca 1.0

Cs 0.7

Ba 0.9

Fr 0.7

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Sr 1.0

Smaller electronegativity

Ni 1.8

Zn 1.8

As 2.0

F 4.0

He Ne -

Cl 3.0 Br 2.8

Ar Kr -

I 2.5 At 2.2

Xe Rn -

Larger electronegativity

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

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IONIC BONDING • • • •

Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Na (metal) unstable

Cl (nonmetal) unstable electron

Na (cation) stable

-

+ Coulombic Attraction

Cl (anion) stable

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EXAMPLES: IONIC BONDING • Predominant bonding in Ceramics NaCl MgO CaF2 CsCl

H 2.1 Li 1.0

Be 1.5

Na 0.9

Mg 1.2

K 0.8

Ca 1.0

Rb 0.8

He O F 3.5 4.0

Ne -

Cl 3.0

Ar -

Br 2.8

Kr -

Sr 1.0

I 2.5

Xe -

Cs 0.7

Ba 0.9

At 2.2

Rn -

Fr 0.7

Ra 0.9

Ti 1.5

Cr 1.6

Give up electrons

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

Acquire electrons

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

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FIGURE 2.9 Schematic representation of ionic bonding in sodium chloride (NaCl). (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

COVALENT BONDING • Requires shared electrons • Example: CH4 H

C: has 4 valence e, needs 4 more

CH4

H: has 1 valence e, needs 1 more

H

Electronegativities are comparable.

C

H

shared electrons from carbon atom

H shared electrons from hydrogen atoms

Adapted from Fig. 2.10, Callister 6e.

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EXAMPLES: COVALENT BONDING H2

column IVA

H2O C(diamond)

H 2.1 Li 1.0 Na 0.9

Be 1.5

K 0.8

Mg 1.2 Ca 1.0

Rb 0.8

Sr 1.0

Cs 0.7

Ba 0.9

Fr 0.7

Ra 0.9

• • • •

SiC Ti 1.5

Cr 1.6

Fe 1.8

F2 He O 2.0

C 2.5

Ni 1.8

Zn 1.8

Ga 1.6

Si 1.8 Ge 1.8

As 2.0

Sn 1.8 Pb 1.8

F 4.0 Cl 3.0

Ne -

Br 2.8

Ar Kr -

I 2.5

Xe -

At 2.2

Rn -

Cl2

GaAs

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA)

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

+

+

+

+

+

+

+

+

+

Adapted from Fig. 2.11, Callister 6e.

• Primary bond for metals and their alloys 12

SECONDARY BONDING Arises from interaction between dipoles • Fluctuating dipoles ex: liquid H2 asymmetric electron H2 H2 clouds

+

- secondary + bonding

-

H H

Adapted from Fig. 2.13, Callister 6e.

H H

secondary bonding

• Permanent dipoles-molecule induced -general case: + -ex: liquid HCl -ex: polymer

-

H Cl s e c on dary

secondary bonding

+

secondary bonding

H Cl

bond in

-

Adapted from Fig. 2.14,

Callister 6e.

Adapted from Fig. 2.14,

Callister 6e.

g 13

FIGURE 2.12 Schematic illustration of van der Waals bonding between two dipoles. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

FIGURE 2.13 Schematic representations of (a) an electrically symmetric atom and (b) an induced atomic dipole. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

FIGURE 2.14 Schematic representation of a polar hydrogen chloride (HCl) molecule. (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

FIGURE 2.15 Schematic representation of hydrogen boding in hydrogen fluoride (HC). (William D. Callister, JR. Materials Science and Engineering an Introduction, John Wiley & Sons, Inc.)

SUMMARY: BONDING Type

Bond Energy

Comments

Ionic

Large!

Nondirectional (ceramics)

Covalent

Variable Directional large-Diamond semiconductors, ceramics small-Bismuth polymer chains)

Metallic

Variable large-Tungsten small-Mercury

Nondirectional (metals)

smallest

Directional inter-chain (polymer) inter-molecular

Secondary

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PROPERTIES FROM BONDING: TM • Bond length, r F

• Melting Temperature, Tm Energy (r)

F

r ro

• Bond energy, Eo

r

Energy (r) unstretched length ro

smaller Tm

r

Eo= “bond energy”

larger Tm

Tm is larger if Eo is larger.

15

PROPERTIES FROM BONDING: E • Elastic modulus, E length, Lo

cross sectional area Ao

undeformed ∆L

deformed

F

Elastic modulus F ∆L =E Ao Lo

• E ~ curvature at ro Energy unstretched length ro

r

E is larger if Eo is larger.

smaller Elastic Modulus larger Elastic Modulus

16

PROPERTIES FROM BONDING: α • Coefficient of thermal expansion, α length, Lo

unheated, T1

coeff. thermal expansion ∆L Lo

∆L

heated, T2

= α (T2-T1)

• α ~ symmetry at ro Energy ro

r

α is larger if Eo is smaller.

larger α smaller α 17

SUMMARY: PRIMARY BONDS Ceramics

Large bond energy

(Ionic & covalent bonding):

Metals

Variable bond energy

(Metallic bonding):

Polymers

moderate Tm moderate E moderate α

Directional Properties

(Covalent & Secondary): s e c on dary

large Tm large E small α

bond in

g

Secondary bonding dominates small T small E large α

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J.J.J.J.Thomson Thomson (1897) (1897)

P. P.Drude Drude (1900) (1900)

H. H.A. A.Lorentz Lorentz (1905) (1905)

Lei empírica de Wiedeman-Franz

...

Calor Calorespecífico específico

(Dulong (DulongeePetit, Petit,3R 3R?) ?)

Coeficiente CoeficienteHall Hall positivo? positivo?

22T k/ σ =2(k /e) k/σ=2(kBB/e) T

Efeito EfeitoSeebeck Seebeck EE==Q Q∇∇TT??

...

(MECÂNICA QUÂNTICA) W. Pauli 1926 nµ2/kBT

Sommerfeld Peirls

Fermi Dirac

F. Bloch 1928

Wilson

Solução do problema está bem estabelecida Mecânica Quântica

^ Ψ=EΨ H

Interações bem conhecidas Forças Forças eletromagnéticas eletromagnéticas

N N N Zα ^ 1 ___ 2 ∑ ) + ___ H = ∑ ( -½∇ i ) + ∑ ( -∑ i

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