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Chapter 2Chapter 2
Atomic BondingAtomic Bonding
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Bonding Forces and Energy
y Ions attract as they come closer, but eventually repulse as
electron charge clouds start to overlap.
y Equilibrium is achieved when there is no net force between ions.
Fnet = Fattractive + Frepulsive
y Net energy is minimum when ions at equilibrium spacing.
y
Bond length, r = interatomic distance.
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y Energy (E) Force (F)
RAnet
r r
RAnet
r
netnet
EEE
drFdrFE
drFE
FdrE
!
!
!
!
g g
g
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Force
Potential
energy
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y Two important parameters from these relationships:
y Equilibrium interatomic distance, ro (or lattice constant, ao).
y Equilibrium distance at which forces balance each other.
y Minimum potential energy.
y Bonding energy, Eo.
y Energy per atom required to dissociate from the solid.
y A measure of strength.
y The strength varies, depending on the nature of bonding.
y
a
E
(e.g. Liquid)y a E (e.g. Gas)
y Types of bonding strength
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Eo =
bond energy
Energy
ro r
unstretched length
Bond energy, Eo
ror
Energy
Melting Temperature, Tm
Tm is larger ifEo is larger.ror
largerE
smallerE
Energy
unstretched length
E
o
Eo
Coefficient of thermal expansion, a
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Chemical bonds between atoms are divided into 2
categories:
a) Primary atomic bonds
- Ionic
- Covalent- Metallic
b) Secondary atomic and molecular bonds
-Van der Waals bond
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Ionic Bonding
y Occurs between + and ions.
y Requires electron transfer and large difference in electro
negativity.
yExample: NaCl
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Ions are attracted by strong coulombic forces.
Ionic bonds are non-directional, i.e. ions may be attractedby one another in any direction.
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Repulsive energy between ion pair , ER
r = interatomic separation distance.n and B = constants (n usually ranges from 7 to 9)
nR
r
BE !
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Example:
Calculate the coulombic attractive energy between Na+
andCl- ions.
Repulsive energy = +8.34 x 10-19J
JE
mNmC
CE
r
eE
mrrr
nmrnmr
o
ClNao
ClNa
19
102212
219
2
21
10
10x34.8
)10x76.2](/10x85.8[4
)10x60.1)(1)(1(
4
10x76.2
181.0;095.0
!
!
!
!!
!!
T
TI
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Covalent Bonding
y Sharing of outer s and p electrons between two or more atoms.
Example: CH4C: has 4 valence e-,
needs 4 more.
H: has 1 valence e-,
needs 1 more.
Electronegativities
are comparable.
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y Single or multiple covalent bond.
F + F F F F F
O + O O O O = O
N + N N N N N
y Many non-metallic elemental molecules (H2, Cl2).
y Molecules containing dissimilar atoms (CH4, H2O, HNO3, HF)
y Other elemental solids: diamond (carbon), Si, Ge.
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y Number of covalent bonds is determined by number of valence
electrons.
y Highly directional type of bonding.
y Very few compounds exhibit pure covalent bonding (or ionic
bonding).
y Possibility of having partially ionic and partially covalent.
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y Pauling equation:
% ionic character = (1 e(-)(XA XB)2)x100%
XA and XB are electro-negativities of atoms A and B in the
compound.
% covalent = e(-)(XA XB)2
Ex: MgO XMg = 1.3
XO = 3.5
Therefore, % covalent = 100% - 70.2%
ionic70.2%(100%)xe1characterionic% 4)3.15.3(
2
!
!
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Metallic Bond
Metallic elements give up their valence
electrons to form a sea of electrons.
Valence electrons are attracted to the
positively charge nucleus of theirneighbours.
Electrons not associated with any
particular nucleus, forming a electron
charge cloud.
Weakly bonded to positive-ion cores
and can move freely.
Non-directional in nature.
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Van der Waals Bond
y Weak electrostatic attractionbetween molecules or groups of
atoms.
y But atoms within molecules or groups of atoms are joined by
strong covalent or ionic bonds.y Results from interaction of atomic or molecular dipoles.
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y Fluctuating dipole
- Asymmetrical distribution of electron charge distribution inatoms create electric dipoles.
- Electron charge cloud changes with time, thus creating
fluctuating dipole.
- Fluctuating dipoles of nearby atoms attract.
- Creates weak inter-atomic non-directional bonds.
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y Permanent dipole
- Dipole moment is not induced.- It exists by virtue of the direction of bonds or asymmetrical
arrangement of positively and negatively charged regions.
- Example: HCl molecule.
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y There are 3 types of van derWaals interactions:
(a) London forces interaction between two induced dipoles.
asymmetric electron
clouds
+ - + -secondarybonding
HH HH
H 2 H 2
secondarybonding
ex: liquid H
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(b) Debye interaction when an induced dipole interacts with a
molecule that has a permanent dipole (e.g. forces between water
molecules and those of CCl4).
(c) Keesom interaction (Hydrogen bonding) - bonding between
molecules that have permanent dipole moment.
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Comparison of Bonding Energies of Different Bonding Types
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Summaryy Electronic structure determines nature of bonding and hence,
physical and mechanical properties of materials.
y Primary atomic bonding is categorized by ionic, covalent,
metallic and mixed bonding.
y Secondary bonding includes weak van derWaals interactions
between induced and/or permanent dipoles.
y Bond energy relates to bond strength and hence, physical
properties of materials.
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y Describe ionic, covalent, metallic and van der waals bonds
and note what materials exhibit each of these types of
bonding.
y Calculate the percent ionic character of a mixed ionic
covalent bond.
y Calculate the attractive, repulsive or net bonding energy for
two atoms or ions.
y Relate bonding energy to material properties.
Study Checklist
After this chapter, you should be able to: