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