EP104 Sen Lnt 002b Atomic Bonding May11

8/6/2019 EP104 Sen Lnt 002b Atomic Bonding May11

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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:

EP104 Sen Lnt 002b Atomic Bonding May11

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