ITB (Institut Teknologi Brunei) Engineering Materials Lecture 2 - Structure & Bonding PCE1ENM 2013

7/30/2019 ITB (Institut Teknologi Brunei) Engineering Materials Lecture 2 - Structure & Bonding PCE1ENM 2013

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ISSUES TO ADDRESS...

What promotes bonding?

What types of bonds are there?

What properties are inferred from bonding?

Atomic Structure and

Bonding in Solids

Study of Structure of Atoms The structure of atoms is important to Materials

Engineers because it influences the way atoms are

bonded together which in turn helps us to categorize

the materials that they form.

The atomic structure and bonding also allows us to

formulate some general conclusions on the

mechanical and physical properties of the material.

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Why Study Atomic Structure andInteratomic Bonding?

Bond types explain material properties!

=> Important in material selection/ design

Example: Graphite and Diamond

Example: Graphite and Diamond


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Atomic Structure

atom electrons 9.11 x 10-31 kgprotonsneutrons

atomic number = no of protons in nucleus of atom= no 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

C 12.011H 1.008 etc.

} 1.67 x 10-27 kg

How scientists represent atoms:Mass (Nucleon) number:number of neutrons and protons

Atomic Number: Number of protons.In a neutral atom this is also the same asthe number of electrons.

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Atomic Structure


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Atomic Structure

Valence electrons determineall of the following properties

1) Chemical

2) Electrical

3) Thermal

4) Optical

Q: Whats a valence electron?

Electrons in Atoms

The electrons form a cloud around the nucleus, ofradius of 0.05 2 nm.

Electron rotates at definite energy levels

Energy is absorbed to move to higher energy level

Energy is emitted during the transition to lower level.

EnergyAbsorbed

EnergyEmitted

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Electrons in Atoms (Quantum Numbers)

Subsidiary (Angular)Quantum Number l

Represents sub energylevels (orbital)

Represented by letterss, p, d and f

Principal QuantumNumber (n)

Represents main energylevels

Range 1 to 7

Larger the n the higherthe energy

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Electron Energy States

1s

2s2p

K-shell n= 1

L-shell n= 2

3s3p M-shell n= 3

3d

4s

4p4d

Energy

N-shell n= 4

have discrete energy states tend to occupy lowest available energy state.

Electrons...


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Electronic Structure of Atoms

Each orbital at discrete energy level determined by

quantum numbers The number of available electron states in some of the

electron shells and subshells

Principal

Quantum

Number, n

Shell

Designation

Subshells Number of

States

Electrons

per

Subshell

Electrons

per Shell

1 K s 1 2 2

2 Ls 1 2

8p 3 6

3 M

s 1 2

18p 3 6

d 5 10

4 N

s 1 2

32p 3 6

d 5 10

f 7 1411

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Survey of Elements

Why? Valence (outer) shell usually not filled completely.

Most elements: Electron configuration not stable.

Electron configuration

(stable)

...

...

1s22s22p63s23p6 (stable)...

1s22s22p63s23p63d10 4s24p6 (stable)

Atomic No.

18...

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Element1s11Hydrogen1s22Helium1s22s13Lithium1s22s24Beryllium1s22s22p15Boron1s22s22p26Carbon

...

1s22s22p6 (stable)10Neon1s22s22p63s111Sodium1s22s22p63s212Magnesium

1s22s22p63s23p113Aluminum...

Argon...

Krypton


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Example

The atomic structure of sodium (atomic number 11)showing the electrons in the K, L, and M quantum shells

2003Brooks/ColePublishing/ThomsonLearning

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Subshells by energy:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f14


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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 chemicalproperties

example: C (atomic number = 6)

1s2 2s2 2p2

valence electrons

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Electronic Configurations

ex: Fe - atomic no. =26

valenceelectrons

1s

2s2p

K-shell n= 1

L-shell n= 2

3s3p M-shell n= 3

3d

4s

4p4d

Energy

N-shell n= 4

1s2 2s2 2p6 3s2 3p6 3d6 4s2


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The Periodic Table

Columns: Similar Valence Structure

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

giveu

p1

e

giveu

p2

e

giveu

p3

einertgases

accept1e

accept2e

O

Se

Te

Po At

I

Br

He

Ne

Ar

Kr

Xe

Rn

F

ClS

Li Be

H

Na Mg

BaCs

RaFr

CaK Sc

SrRb Y

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Electronegativity Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

Large values: tendency to acquire electrons.


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Chemical Bonding

A chemical bond is an electrostatic interaction between

areas of positive and negative charges present in the

binding atoms and/or molecules

Primary Bonds: Ionic

Covalent

Metallic

Secondary Bonds: Hydrogen Bonds

Van de Waals Forces

Primary Bonding

Primary

Ionic Covalent Metallic

For each type, the bonding necessarily involves

the valence electrons.

In general, each of these types arises from the

tendency of the atoms to assume stable electron

structures, like those in inert gases by completely

filling the outermost shell

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Some of the electronic structure-chemicalproperty relationships for metals and non-

metals:

Metals Non-metals

1. Have few electrons inouter shells usually threeor less

1. Have four or moreelectrons in outer shells

2. Form cations by losingelectrons

2. Form anions by gainingelectrons

3. Have lowelectronegativities

3. Have highelectronegativities

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Ionic Bonding

Found in compounds that are composed of bothmetallic and non-metallic.

Occurs by electron transfer Metal: Donates electrons

Non-metal: Accepts electrons

Ions are attracted by strongcoulombic attraction Oppositely charged atoms attract

An ionic bond is non-directional (ions

may be attracted to one another in anydirection)

Note relative sizes of ions:

Na (metal) shrinks and Cl (nonmetal)expands

CoulombicAttraction

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Ionic bond metal + nonmetal

donates accepts

electrons electrons

Dissimilar electronegativities

ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4

[Ne] 3s2

Mg2+

1s2

2s2

2p6

O2-

1s2

2s2

2p6

[Ne] [Ne]

Bonding Forces and Energies

Interatomic forces determine the physical propertiesof materials. There are two types of forces:

1. the attractive force: Oppositely charged ions attractedby Coulombic forces (nucleus of one ion will attract theelectron charge cloud of the other ion and vice versa)

2. the repulsive forces: as the ions closer together, theirelectron charge clouds will interact and repulsive forceswill arise.

The net force between a pair of oppositely chargedions:

Fnet = Fattractive +Frepulsive

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The attractive force between the ion pair is theCoulombic force that results when the ions areconsidered as point charges.

Where Z1,Z2 = number of electrons removed oradded

from the atoms

e = electron charge

a = interionic distance0 = permittivity of free space = 8.85 10

-12

C2/(N.m2)

Fattractive = (Z1e)(Z2e)

40a2

= Z

1Z

2e2

40a2

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The repulsive force between an ion pair is found byexperiment to be inversely proportional to the interionicseparation distance a and can be described as:

Where a = interionic distance

n, b = constants (n usually ranges from 7-9)

The net force between ion pair:

Frepulsive = nb

an+1

Fnet = Z

1Z

2e2

40a2

nb

an+1

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Energy versus separation distance for a pair of oppositely

charged ions. The equilibrium interionic separationdistance a0 is reached when the net potential energy is aminimum.

Attractive energy EA

Net energy EN

Repulsive energy ER

Interatomic separation r

a0 Enet = +Z

1

Z2

e2

40a+

b

an

Attractiveenergy

Repulsiveenergy 27

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Examples: Ionic Bonding

Predominant bonding in Ceramics

Give up electrons Acquire electrons

NaCl

MgO

CaF2CsCl


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Properties From Bonding: Tm

Bond length, r

Bond energy, Eo

Melting Temperature, Tm

Tm is larger if Eo is larger.

ror

Energy

r

larger Tm

smaller Tm

Eo =

bond energy

Energy

ror

unstretched length

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Properties From Bonding :

Coefficient of thermal expansion,

~ symmetry at ro

is larger if Eo is smaller.

= (T2 -T1)L

Lo

coeff. thermal expansion

L

length, Lo

unheated, T1

heated, T2

ror

larger

smaller

Energy

unstretched length

E

oE

o


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Ionic Bonding (examples)

He-

Ne-

Ar-

Kr-

Xe-

Rn

-

F4.0

Cl

3.0

Br2.8

I2.5

At2.2

Li1.0

Na0.9

K0.8

Rb0.8

Cs0.7

Fr0.7

H2.1

Be1.5

Mg

1.2

Ca1.0

Sr1.0

Ba0.9

Ra0.9

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

CsCl

MgO

CaF2

NaCl

O3.5

Give up electrons Acquire electrons

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Primary Bonding Ionic-Covalent Mixed Bonding

% ionic character =

where XA & XB are Pauling electronegativities

%)100(x1e

(XAXB)2

4

ionic70.2%(100%)xe1characterionic% 4

)3.15.3(

2

=

=

Ex: MgO XMg = 1.3XO = 3.5


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Covalent Bonding

Formed when electrons are shared betweenatoms

Between non-metals and non-metals or hydrogenand non-metals

They share electrons so that both of them canhave a stable octet

Covalent bonds are HIGHLY directional bonds

There are two types of covalent bonds:

1. Non-polar: result when two exact non-metalsequally share electrons.

2. Polar: result when two different non-metals shareelectrons. 33

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Covalent Bonding

C: has 4 valence e-,needs 4 more

H: has 1 valence e-,

needs 1 more

Electronegativitiesare comparable.

similar electronegativity share electrons

bonds determined by valence s& porbitalsdominate bonding

Example: CH4shared electronsfrom carbon atom

shared electronsfrom hydrogenatoms

H

H

H

H

C

CH4


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Covalent Bonding (Examples)

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Covalent Bonding (Properties)

1. Generally have much lower melting and boilingpoints than ionic compounds

2. Soft and squishy (compared to ionic compounds)

3. Tend to be more flammable than ionic compounds

4. Do not conduct electricity in water

5. Usually not very soluble in water

Like dissolves like (compounds tend to dissolve inother compounds that have similar properties

(particularly polarity) Water is polar solvent and most covalent compounds

are fairly nonpolar

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Metallic Bonding

Occurs in metallic substances

Arises from a sea of donated valence electrons that

floats around the surface of metals

Non-directional

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Metallic Bonding (Properties)

1. Strong materials with high melting and boiling

points (Bonding is very strong)

2. Good conductors of electricity and heat

Free electrons carry the charge of an electriccurrent when a voltage is applied.

3. Silvery surface

This may be easily tarnished by corrosiveoxidation in air and water

4. Very malleable (due to mobility of electrons) Can be readily bent, pressed or hammered into

shape.

Layers of atoms can slide over each otherwithout fracturing the structure

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Secondary Bonding (Intermolecular forces)

Secondary, Van der Waals, or Physical bonds are weakin comparison to primary bonds

Exists between virtually all atoms or molecules, but itspresence may be obscured if any of the three primarybonding types is present

Forces arise from atomic or molecular dipoles

An electric dipole exists whenever there is someseparation of positive and negative portions of an atomor molecule

Secondary

FluctuatingInduced Dipole

PermanentDipole Bond

Polar MoleculeInduced Dipole

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Secondary Bonding

Arises from interaction between dipoles

Permanent dipoles-molecule induced

Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

asymmetric electronclouds

+ - + -secondarybonding

HH HH

H2 H2

secondarybonding

ex: liquid H2

H Cl H Clsecondary

bonding

secondarybonding

+ - + -

secondary bonding


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Permanent Dipole Bonds

Exist in all polar molecules Permanent dipole moments exist by virtue of an

asymmetrical arrangement of positively and

negatively charged regions

Magnitude of bond greater than for fluctuatinginduced dipoles

+ - + -

Secondary Bonding

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Fluctuating Induced Dipoles Temporary dipole induces a dipole in a neighbouring

molecule

This results in a weak and temporary force of attractionbetween two atoms.

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Fluctuation ofelectron cloud

Fluctuation ofelectroncloud


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Hydrogen Bonding

A special case of secondary bonding Exists between molecules in which one is

covalently bonded to fluorine, oxygen or nitrogen.

Example: Water molecules

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Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variable

large-Diamond

small-Bismuth

Variable

large-Tungstensmall-Mercury

Smallest

Comments

Nondirectional(ceramics)

Directional(semiconductors, ceramicspolymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)inter-molecular

Summary: Bonding


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Ceramics

(Ionic & covalent bonding):

Metals

(Metallic bonding):

Polymers(Covalent & Secondary):

Large bond energylarge Tmlarge Esmall

Variable bond energymoderate Tmmoderate Emoderate

Directional PropertiesSecondary bonding dominates

small Tmsmall Elarge

ITB (Institut Teknologi Brunei) Engineering Materials Lecture 2 - Structure & Bonding PCE1ENM 2013

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