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Atomic Structure and InteratomicBonding:
Callister Chapter 2
• What promotes bonding? • What types of bonds are there?
• What properties are conditional from bonding?
consider carbon: graphite and diamond
graphite – soft, "greasy"
diamond – hardest material known
• How are atoms arranged?
atomic structure
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2
Atomic Structure
• atom – electrons – 9.11 x 10-31 kgprotons neutrons
• atomic number = # of protons in nucleus of atom= # of electrons of neutral species
• A [=] atomic mass unit = amu = 1/12 mass of 12C
Atomic wt = wt of 6.023 x 1023 molecules or atoms
1 amu/atom = 1g/mol
CH
12.011
1.008 etc.Bohr model ofatomic structure
ElectronCloud
Nucleus
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3/15/201233
Atomic Structure
• Valence electrons determine all of thefollowing properties
1) Chemical
2) Electrical3) Thermal
4) Optical
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Electronic Structure
• Electrons have wavelike and particulateproperties.
– This means that electrons are in orbitals defined by aprobability.
– Each orbital at discrete energy level determined byquantum numbers.
Quantum # Designation
n = principal (energy level-shell) K , L, M , N , O (1, 2, 3, etc.)
l = subsidiary (orbitals) s , p , d , f (0, 1, 2, 3,…, n -1)
m l = magnetic 1, 3, 5, 7 (-l to +l)
m s = spin ½, -½
Bohr & Schrodinger
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Electron Energy States
1s
2s 2p
K -shell n = 1
L-shell n = 2
3s 3p M -shell n = 3
3d
4s
4p 4d
Energy
N -shell n = 4
• have discrete energy states • tend to occupy lowest available energy state.
Electrons...
Adapted from Fig. 2.4,Callister 7e.
I n c r e a s i n g
e n e r g y
l e v e l s
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The number of available electron in some of theelectron shell and sub shells
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Electron Configurations
• Valence electrons – those in unfilled shells• Filled shells more stable
• Valence electrons are most available forbonding and tend to control the chemical
properties
– example: C (atomic number = 6)
1s 2 2s 2 2p 2
valence electrons
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Electronic Configurations
ex: Fe - atomic # =
26
Adapted from Fig. 2.4,
Callister 7e.
1s
2s 2p
K -shell n = 1
L-shell n = 2
3s 3p M -shell n = 3
3d
4s
4p 4d
Energy
N -shell n = 4
1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2
Valenceelectrons
I n c r e a s i n g
e n e r g y
l e v e l s
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The Periodic Table• Columns: Similar Valence Structure
Adapted fromFig. 2.6,Callister 7e.
g i v e
u p 1 e
g i v e
u p
2 e
g i v e
u p
3 e
i n e r t g a s e s
a c c e p t 1 e
a c c e p t 2 e
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
ClS
Li Be
H
Na Mg
BaCsRaFr
CaK Sc
SrRb Y
Elements of Group IA have one valence electron.Elements in Group 4A have four valence electrons, etc
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Smaller electronegativity Larger electronegativityAdapted from Fig. 2.7, Callister 7e. (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.
Electronegativity
• Electronegativity is the ability of an atom toattract electrons to itself.
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Interactions between atoms
• Electrons in the outermost levels interact first.• When the outer shells are unfilled, atoms
gain, lose, or borrow electrons which is thebasis of bonding.
Eg NaCl
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BondingProperties
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Ionic bond – metal + nonmetal
donates acceptselectrons electrons
Dissimilar electronegativities
ex: MgO Mg 1s 2 2s 2 2p 6 3s 2 O 1s 2 2s 2 2p 4
[Ne] 3s 2
Mg2+ 1s 2 2s 2 2p 6 O2- 1s 2 2s 2 2p 6 [Ne] [Ne]
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• Occurs between + and - ions.
• Requires electron transfer. • Large difference in electronegativity require• Example: NaCl
Ionic Bonding
Na (metal)unstable
Cl (nonmetal)unstable
electron
+ -
Coulombic Attraction
Na (cation)stable
Cl (anion)stable
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Ionic Bonding
• Energy – minimum energy most stable – Energy balance of attractive and repulsive terms
Attractive energy E A
Net energy E N
Repulsive energy E R
Interatomic separation r
r A
n r B
E N = E A + E R = + -
Adapted from Fig. 2.8(b),Callister & Rethwisch 8e.
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• Predominant bonding in Ceramics
Adapted from Fig. 2.7, Callister 7e. (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.
Examples: Ionic Bonding
Give up electrons Acquire electrons
NaCl
MgO
CaF2CsCl
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Chapter 2 -
3/15/201217
C: has 4 valence e - ,needs 4 more
H: has 1 valence e - ,needs 1 more
Methane gas .
Adapted from Fig. 2.10, Callister 7e.
Covalent Bonding• similar electronegativity share electrons
• bonds determined by valence – s & p orbitalsdominate bonding
• Example: CH4shared electronsfrom carbon atom
shared electrons
from hydrogenatoms
H
H
H
H
C
CH4
Electronegativitiesare comparable.
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• NonMetallic ElementalMolecules; e.g. F2
• Hydrogen Compounds;e.g., HF, HNO3
• Elemental Solids; e.g.,C, Si, Ge
• Near Group-IVA SolidCompounds; e.g. GaAs
Covalent bonding
Covalent bonds are formed by sharing of the valence electronsCovalent bonds are very directional
Covalent bond model: an atom can have at most 8-N’ covalent
bonds, where N’ = number of valence electrons
Covalent bonds can be very strong, eg diamond, SiC, Si, etc,also can be very weak, eg Bismuth
Polymeric materials exhibit covalent type bonding.
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• Electrons Shared By All Atoms
– “sea of electrons” around “ion cores”
• Ion Cores
– Atom Gives Up e- to the “sea”, leaving remaining
Atom with a Positive Ionic Charge
• The Ion contains the Large & Heavy Nucleusand is thus FIXED in Space
• Generally Apply to Electro-Positive Elements
– e.g.; Transition Metals such as Ti, Ni, Zn
Metallic bonding
A metallic bond is non-directional(bonds form in any direction)
atoms pack closely high density.
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20http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/metallicblue.gif
Metallic bonds may be weak or strong.
Bonding energies: range from 68 kJ/mol (0.7 eV/atom) forHg to 850 kJ/mol (8.8 eV/atom) for W.Melting temperatures: -39 C for Hg and 3410 C for W.
Metallic bonding
Metals can be reshaped high melting points
hardness electrical conductivity
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Mixed Ionic+Covalent Bonding• Many Compounds Exhibit
Ionic-Covalent Mixed Bonding• semiconducting compounds such as GaAs, ZnSe
2
4100%
%Ionic 1 1
A B X X
e
--
- – where X A & X B are Pauling ElectroNegativities
• Example MgO: X Mg
= 1.2, X O
= 3.5
ionic73.4%(100%)xe1characterionic% 4
)2.15.3(
2
-
--
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Determine how ionic the bonds between thefollowing atoms would be
Sodium andchlorine
Na E0 0.9Cl E0 3.0
Δ=2.168% ionic
Carbon andnitrogen
C E0 2.5
N E0 3.0
Δ=0.5
10% ionic
90%covalent
Potassium andsuphur
K E0 0.8
S E0 2.5
Δ=2.1
50% ionic
2
4100%
%Ionic 11
A B X X
e
--
-
XA XB % Ionic
electronegativity
chargesfrom
periodic table
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Covalent vs Ionic Bonding
• The polarity of a covalentbond is the partial ioniccharacter
• Real bonds liesomewhere between ionic
and covalent• The difference in
electronegativitycorresponds directly to thepercent ionic character ofthe bond
Very few materials have pure ionic or covalent bonding;electronegativity inpart defines how much time electronsspend between ion cores…
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Type Substance Energy(eV/atom)
Melt. Temp(°C)
IonicNaCl 3.3 801
MgO 5.2 2800
CovalentSi 4.7 1410
C 7.4 >3550
Metallic
Hg 0.7 -39
Al 3.4 660Fe 4.2 1538
W 8.8 3410
Bonding Energies and MeltingTemperatures
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Dipole Forces• Electrostatic interactions
between adjacentmolecules are called
Dipole Forces• The most extreme form of
dipole forces result fromthe interaction of a
hydrogen atom with ahighly electronegativeatom resulting inhydrogen bonding.
Secondary bonding – Dipole Forces
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Dispersion Forces (or Van der Waalsforces)
• Interactions resulting frommomentary concentration
variations in the electronclouds of adjacent atoms
• The interactions are briefand weak, but largemolecules (such as
polymers) haveopportunities for manysimultaneous interactions
http://www.elmhurst.edu/~chm/vchembook/122Adensityice.html
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Hydrogen Bonding
Hydrogen bonding in HF
solid ice
liquid water
Special type of secondary
bonding, exists between somemolecules that have hydrogenas one of constituents
The increase in volume of ice is about9%. This increase causes enoughforce to break most rigid containers.
This is the same force, repeated on a daily basis, that creates "pot holes" in the roads inthe winter time.
p y
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28Adapted from Fig. 2.14,Callister 6e.
•The dipole moment of water provides a "handle" forinteraction with microwave electric fields in a microwaveoven.
• Microwaves can add energy to the water molecules, theyvibrate and heat the compounds.
Secondary Bonding, Polar
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C Bonding-Type → Properties
Diamond:covalent (directional) bonds
Graphite:covalent bonds within layersvan der Waals b/w layers
layers slide soft, greasy (lubricant)
hardest known material
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• Bond energy, Eo
• Bond length, r
FF
r
• Melting Temperature, Tm
if bonding energy Eo is largemelting temperature Tmis large.
Properties from Bonding: TM
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LF
Ao = E
Lo
Elastic modulus
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
r o
unstretched length
Properties from Bonding: E• Elastic modulus, E
• E ~ curvature at r o if slope r=ro is steepModulus is high
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• Coefficient of thermal expansion, a
= a(T2-T1)
L
Lo
coeff. thermal expansion
Properties from Bonding: a
• a ~ symmetry at ro Eg. Thermostats for central
heating
a is larger if E o is smaller.
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Bonding Energies and MeltingTemperatures for Various Substances
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Summary: Primary Bonds
Ceramics (Ionic & covalent bonding):
Metals (Metallic bonding):
Polymers
(Covalent & Secondary):
Large bond energylarge Tm large Esmall a
Variable bond energymoderate Tm moderate Emoderate a
Directional Properties
Secondary bonding dominatessmall Tm
small Elarge a
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