Atomic structure and bonding
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Transcript of Atomic structure and bonding

1
MSE 280: Introduction to Engineering Materials
Reading: Chapter 2Atomic structure and Bonding
g p
Overview•Electrons, protons and neutrons in atoms (Bohr and QM models).•IP, EA, χ, and periodic trends.
MSE280© 2007, 2008 Moonsub Shim, University of Illinois1
•Bonding between atoms.•Intermolecular forces.•Relation to macroscopic properties.
Electrons in atomsorbital electrons:
n = principal quantum number
1
Nucleus: Z = # protons
Adapted from Fig. 2.1, Callister 6e.
Electrons in discrete orbitals.Bohr atom:
n=3 2 1
MSE280© 2007, 2008 Moonsub Shim, University of Illinois2
N = # neutrons
Atomic mass A ≈ Z + N
1) electrons are particles that revolve around the nucleus.
2) quantized angular momentum.Quantum Mechanics:
Wave or matrix mechanics→ Probability.

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Comparison of Bohr and QM models
MSE280© 2007, 2008 Moonsub Shim, University of Illinois3
Figs. 2.2 and 2.3 from Callister 6 ed.
Atomic orbitals
dyzdxy
MSE280© 2007, 2008 Moonsub Shim, University of Illinois4
s px py pz dz2dx2-y2dxz
• have discrete energy states (Quantized).• tend to occupy lowest available energy state.
Electrons...

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Quantum numbers• Principal: n = 1, 2, 3, 4…• Angular momentum: l = 0, 1, 2, 3…, n – 1 = s, p, d, f…
s = sharp, p = principal, d = diffuse, f = fundamental• Magnetic: ml = 0, ±1, ±2, ±3… , ±l
Determines the number of states in a given l subshell (2l +1 total)
• Spin: ms = ±1/2
e g
MSE280© 2007, 2008 Moonsub Shim, University of Illinois5
e.g.
1s n = 1, l = 0, ml = 0, ms = 1/2 n = 1, l = 0, ml = 0, ms = -1/2
2s n = 2, l = 0, ml = 0, ms = 1/2 n = 2, l = 0, ml = 0, ms = -1/2
Which atom is this? Be
Electron Configuration- Shorthand notation to represent which states electrons occupy in an atom (without specifying electron spin).
e.g. Carbon
1s
2s
Electron configuration: 1s22s22p2
2p
MSE280© 2007, 2008 Moonsub Shim, University of Illinois6
Note- each energy level can only hold two electrons of opposite spin (Pauli exclusion principle).- for degenerate levels (e.g. 2p-orbitals), each orbital is filled with one electron before electrons are paired up.

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Electron configuration1 electron in the s-orbital: Alkali metals
Li, Na, K, Rb…
2 electrons in the s-orbital: Alkaline earthsBe, Mg, Ca…
Filled s-orbital and 4 electrons in p-orbital: ChalcogensO, S, Se…
Filled s-orbital and 5 electrons in p-orbital: HalogensF Cl Br
MSE280© 2007, 2008 Moonsub Shim, University of Illinois7
F, Cl, Br…
Partially filled d-orbital: Transition metalse.g. Mn, Fe, Co…
Valence electrons determine which group atoms belong to.
Stable configuration
• have complete s and p subshellst d t b i t
Stable electron configurations...
• tend to be inert.
Z Element Configuration 2 He 1s 2 10 Ne 1s 22s 22p 6 18 Ar 1s 2 2s 22p 63 s 23p 6
Adapted from Table 2.2, Callister 6e.
MSE280© 2007, 2008 Moonsub Shim, University of Illinois8
36 Kr 1s 2 2s 22p 63 s 23p 63d 10 4 s 24p 6
Noble gases

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Valence electrons
2p
3s
2p
3s
Filled shell}
Valence electron
1s
2sp
Na
1s
2s
Na+
Filled shell leads to stability.
}Lose an electron
33p
3s3p Filled shell }
Valence electrons
MSE280© 2007, 2008 Moonsub Shim, University of Illinois9
Gain an electron
1s
2s2p
3s
Cl1s
2s2p
3s
Cl-
leads to stability.
How much energy does it require to take an electron out of an atom?
Energy of an electron in vacuum
Ene
rgy
Valence electron
IP
MSE280© 2007, 2008 Moonsub Shim, University of Illinois10
• Ionization potential (IP): Energy required to pull out a valence electron (in vacuum).
By convention, IP is positive (i.e. need to put in energy to pull out the electron).

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How much energy does it require to place an electron in an atom?
Energy of electron in vacuum
Ener
gy
Valence electrons
EA
Lowest available state
MSE280© 2007, 2008 Moonsub Shim, University of Illinois11
• Electron Affinity (EA): Energy gained by putting an electron in (from vacuum).
By convention, EA is negative (i.e. electron goes from higher energy state in vacuum to lower energy state in atom).
How do we determine when an atom will accept an electron or give one up?
y EA IP
Vacuum level
• Electronegativity (χ): a measure of how likely an atom will take up or give up an electron
EAIP +
Ene
rgy
Valence electrons
EA
Lowest available state
IPχ
MSE280© 2007, 2008 Moonsub Shim, University of Illinois12
A simple (and intuitive) definition:
-When two atoms are brought together, the atom with larger χ will have higher electron density around its nucleus.
-Larger Δχ more ionic bond.
2~ EAIPx +

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MSE280© 2007, 2008 Moonsub Shim, University of Illinois13
BondingPrimary
Ionic ECovalentMetallic
SecondaryDipole-dipoleH bonds
E
MSE280© 2007, 2008 Moonsub Shim, University of Illinois14
H-bondsDipole-induced-dipoleFluctuating dipoles
Equilibrium bond length

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Ionic Bonding• Occurs between + and - ions.• Requires electron transfer.
Na (metal) unstable
Cl (nonmetal) unstable
electron
• Large difference in electronegativity required.• Example: NaCl
3s3p
MSE280© 2007, 2008 Moonsub Shim, University of Illinois15
electron
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
Na(χ = 0.9)
3s3p
Cl(χ = 3.0)
Ionic BondingNa (metal) unstable
Cl (nonmetal) unstable
electron
+Na (cation) Cl (anion)
ezzEA 4
221=
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable ro
A πε4
Since z1 = +1 for Na+ and z2 = -1 for Cl-
rA
reE
oA −=−=
πε4
2Negative energy means attraction only.Will the atoms collapse on themselves?
MSE280© 2007, 2008 Moonsub Shim, University of Illinois16
No, there is also repulsive energy (e.g. e-e repulsion)
nR rBE = B and n depend on atoms involved.
In many cases n ~ 8.

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Ionic BondingE
rAE
nR rBE =
Bond energy
MSE280© 2007, 2008 Moonsub Shim, University of Illinois17
rEA −=
Equilibrium bond length
gy
Note: Other types of bonds can also be described in a similar manner
Ionic Bonding: examples• Predominant bonding in Ceramics
M ONaCl
He -
Ne -
Ar -
Kr -
Xe -
F 4.0Cl
3.0Br
2.8I
2.5
Li 1.0Na 0.9K
0.8Rb 0.8
H 2.1
Be 1.5Mg 1.2Ca 1.0Sr 1.0
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
Cs Cl
MgOCaF 2
O 3.5
MSE280© 2007, 2008 Moonsub Shim, University of Illinois18
Give up electrons Acquire electrons
Rn -
At2.2
Cs 0.7Fr
0.7
Ba 0.9Ra 0.9
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 CornellUniversity.
From Callister 6e resource CD.

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Covalent Bonding
1s 1s
HH
Molecular orbitals
• “Sharing” of electrons• Why do some atoms want to share electrons?
• Example2: CH4
C: has 4 valence e,needs 4 more
• Example1: H2
shared electrons from carbon atomH
CH4
H HHH
Atomic orbitals
MSE280© 2007, 2008 Moonsub Shim, University of Illinois19
H: has 1 valence e,needs 1 more
Electronegativitiesare same or comparable.
Adapted from Fig. 2.10, Callister 6e.
shared electrons from hydrogen atoms
HH
H
C
an s-orbital three p orbitalsAtomic Orbitals
atomic orbitals for carbon:
1s
2s2p 4 valence electrons but
two different types orbitals.H’s on CH4 should be equivalent.
xy
zpx py pz
an s-orbital p
Hybridization sp3 hybridization for C in CH4
MSE280© 2007, 2008 Moonsub Shim, University of Illinois20x
y
z1s + 2p = sp2-orbitals 1s + 3p = sp3-orbitals
60°
60°x
y
z
y
xy
z1s + 1p = sp-orbitals
sp3 hybridization for C in CH4

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EXAMPLES: COVALENT BONDING
He -
H 2 1 SiC
C(diamond)
H2OH2
Cl2
F2
colu
mn
IVA
Ne -
Ar -
Kr -
Xe -
Rn -
F 4.0Cl
3.0Br
2.8I
2.5At
2.2
Li 1.0Na 0.9K
0.8Rb 0.8Cs 0.7Fr
0.7
2.1Be 1.5Mg 1.2Ca 1.0Sr
1.0Ba 0.9Ra 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
SiCC
2.5
Cl2
Si 1.8
Ga 1.6
GaAs
Ge 1.8
O 2.0
c
Sn 1.8Pb 1.8
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is
MSE280© 2007, 2008 Moonsub Shim, University of Illinois21
• Molecules with nonmetals• Molecules with metals and nonmetals• Elemental solids (RHS of Periodic Table)• Compound solids (about column IVA)
0.9 p g , ( gadapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.
% ionic character
Most bonds between two different types of atoms are somewhere in between ionic and covalent.between ionic and covalent.
% ionic character = ]})(25.0exp[1{ 2BA χχ −−−
χj = electronegativity of atom j
MSE280© 2007, 2008 Moonsub Shim, University of Illinois
χj electronegativity of atom j
KEY POINT: Larger electronegativity difference more ionic

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Example problem
• Order the following semiconductors from t l t t t i imost covalent to most ionic.
1) ZnS, GaP, CuCl2) ZnS, ZnSe, ZnO
MSE280© 2007, 2008 Moonsub Shim, University of Illinois23
What’s so important about ionicity of bonds?
• Chemical propertiesNaCl (highly ionic solid) dissolves readily in water but– NaCl (highly ionic solid) dissolves readily in water but Si (covalent solid) does not.
• Electronic properties– Ionicity of the bonds will have a strong influence on
the band gap and other electronic properties.• All properties of materials are largely determined
MSE280© 2007, 2008 Moonsub Shim, University of Illinois24
All properties of materials are largely determined by the types and strength of bonds between the constituent atoms.

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Metallic Bonding• Arises from a sea of donated valence electrons
Fixed ion cores (nuclei+ + +
+ + +
Fixed ion cores (nuclei and inner electrons)
“sea” of electrons
MSE280© 2007, 2008 Moonsub Shim, University of Illinois25
• Primary bond for metals and their alloys.• Large atomic radius and small IP will more likely lead to metallic bonding.
+ + + Adapted from Fig. 2.11, Callister 6e.
Secondary Bonds: Intermolecular Forces
• Dipole-dipole interaction: secondary bond between molecules with permanent dipole moments
Van der Waals
+ - secondary bonding + -
H Cl H Clsecondary bonding
secondary bonding
-general case:
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.14,Callister 6e.
Adapted from Fig. 2.14,Callister 6e.
From Callister 6e resource CD.
MSE280© 2007, 2008 Moonsub Shim, University of Illinois26
• Hydrogen bonding
HO
H
HO H
HO
H

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• Dipole-induced-dipole interaction: secondary bond between molecules with permanent dipole moments
+
-
+
- +
-
• Fluctuating dipoles
H2 H2ex: liquid H 2asymmetric electron
clouds
secondary bondingpolar
Nonpolar (e.g. atom)
MSE280© 2007, 2008 Moonsub Shim, University of Illinois27
HH HH
H2 H2
secondary bonding
clouds
+ - + -secondary
bondingAdapted from Fig. 2.13, Callister 6e.
SUMMARY: BONDING
TypeIonic
Bond EnergyLarge!
CommentsNondirectional (ceramics)Ionic
Covalent
Large!
Variablelarge-Diamondsmall-Bismuth
Variable
Nondirectional (ceramics)
Directional(semiconductors, ceramics
polymer chains)
MSE280© 2007, 2008 Moonsub Shim, University of Illinois28
Metallic
Secondary
large-Tungstensmall-Mercury
smallest
Nondirectional (metals)
Directionalinter-chain (polymer)
inter-molecularFrom Callister 6e resource CD.

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• Bond length, r
F F
• Melting Temperature, Tm
Energy (r)
PROPERTIES FROM BONDING: TM
• Bond energy, Eo
r
Energy (r)
unstretched length
r
larger T
smaller T m
r o
MSE280© 2007, 2008 Moonsub Shim, University of Illinois29
Eo= “bond energy”
r o r
u st etc ed e gt larger T m
Tm is larger if Eo is larger.
From Callister 6e resource CD.
• Elastic modulus, E cross sectional area A o length, Lo
undeformed ΔLF
Elastic modulus
PROPERTIES FROM BONDING: E
• E ~ curvature at ro
ΔL
F
undeformed
deformed
ΔLFAo
= E Lo
Energy
unstretched length E is larger if curvature at
MSE280© 2007, 2008 Moonsub Shim, University of Illinois30
r
larger Elastic Modulus
smaller Elastic Modulus
r o unstretched length E is larger if curvature at
ro is larger.
From Callister 6e resource CD.

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Ceramics(Ionic & covalent bonding):
Large bond energylarge Tm
large Esmall α
SUMMARY: BONDING and Materials’ properties
Metals(Metallic bonding):
Polymers
small α
Variable bond energymoderate Tm
moderate Emoderate α
Directional Properties
MSE280© 2007, 2008 Moonsub Shim, University of Illinois31
Polymers(Covalent & Secondary):
secondary bonding
Directional PropertiesSecondary bonding dominates
small Tsmall Elarge α
From Callister 6e resource CD.