Material Science and Engineering Lecture 2

7/27/2019 Material Science and Engineering Lecture 2

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Why study bonding?

Because the properties of materials (strength,

hardness, conductivity, etc..) are determined by the

manner in which atoms are connected.

Also by how the atoms are arranged in space

What determines the nature of the chemical

bond between atoms?

Electronic structure (distribution of electrons inatomic orbitals)

Number of electrons and ectronegativity(tendency

for an atom to attract an electron)


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Classes of Materials


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Review of Bonding:

Bonding joins of two atoms in a stable arrangement using

only valence electrons. Valence electrons are outer shellelectrons.

Through bonding, atoms attain a complete outer shell

Structure and Bonding

con gura on w c s e a no e gas. Covalent bonds result from the sharing of electrons between

two elements.

Ionic bonds result from the transfer of electrons from oneelement to another.

Polarity of a bond depends upon the electronegativity and

size of the atoms forming the bond.


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Structure and Bonding

Elements in the same row are similar in size

but size decreases going to the right.

Elements in the same column have similar

electronic and chemical properties butbecome more metallic going down the family.


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Ranges from 0.7 to 4.0,

He-

Ne-

Ar

-

Kr

-

F4.0

Cl

3.0

Br

2.8

Li1.0

Na

0.9

K

0.8

H2.1

Be1.5

Mg

1.2

Ca

1.0Ti

1.5

Cr

1.6

Fe

1.8

Ni

1.8

Zn

1.8

As

2.0

Large values: tendency to acquire electrons.

Electronegativity

Smaller electronegativity Larger electronegativity

Xe

-

Rn-

I

2.5

At2.2

Rb

0.8

Cs0.7

Fr0.7

Sr

1.0

Ba0.9

Ra0.9

Adapted from Callister 6e. , The figure is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd

edition,


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Types of atomic and molecular bondsPrimary atomic bonds

Ionic (large interatomic forces, nondirectional,

electron transfer, coulombic forces)

Covalent (large interatomic forces, localized

Metallic (large interatomic forces) nondirectional

Secondary atomic and molecular bonds

Permanent dipole bondsFluctuating dipole bonds


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An ionic bond generally occurs when an element on the far

left side of the periodic table combines with an element on

the far right side, ignoring noble gases.

A positively charged cation formed from the element on the

left side attracts a negatively charged anion formed from the

element on the right side. An example is sodium chloride,

Ionic Bonding Electron Transfer

NaCl.


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Na metal Cl nonmetal

Occurs between + and - ions.

Requires electron transfer.

Large difference in electronegativity required.

Example: NaCl

Ionic Bonding

unstable unstableelectron

+ -CoulombicAttraction

Na (cation)stable Cl (anion)stable


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He-

Ne-

Ar

-

Kr

-

F4.0

Cl

3.0

Br

2.8

Li1.0

Na

0.9

K

0.8

H2.1

Be1.5

Mg

1.2

Ca

1.0

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

CsCl

MgO

CaF2

NaCl

O3.5

Examples: Ionic Bonding

Give up electrons Acquire electrons

Xe-

Rn-

I2.5

At2.2

Rb0.8

Cs0.7

Fr0.7

Sr1.0

Ba0.9

Ra0.9

Adapted from Callister 6e.


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Requires shared electrons

Example: CH4

C: has 4 valence e,

needs 4 more

shared electronsfrom carbon atom

HCH4

Covalent Bonding

H: has 1 valence e,

needs 1 more

Electronegativities

are comparable.

shared electronsfrom hydrogen

atoms

HH

H

C

Adapted from Callister 6e.


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Hydrogen forms one covalent bond.

When two hydrogen atoms are joined in a NONPOLAR covalent

bond, each has a filled valence shell of two electrons. It is

non olar due to e ual sharin of es.

Bonding in Molecular Hydrogen (H2):

Covalent Bonding Electron Sharing


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He

-

Ne-

Ar-

Kr

F4.0

Cl3.0

Br

Li1.0

Na0.9

K

H

2.1Be1.5

Mg1.2

Ca Ti Cr Fe Ni Zn As

SiC

C(diamond)

H2O

C2.5

H2

Cl2

F2

Si1.8

Ga Ge

O2.0

columnIVA

Examples: Covalent Bonding

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Molecules with nonmetals

Molecules with metals and nonmetals

Elemental solids (RHS of Periodic Table)

Compound solids (about column IVA)

-

Xe-

Rn

-

.

I2.5

At

2.2

.

Rb0.8

Cs0.7

Fr0.7

.

Sr1.0

Ba

0.9

Ra

0.9

. . . . . ..

GaAs

.

Sn1.8

Pb

1.8

Adapted from 6e. (Fig. 2.7 is

adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition

Prevalent in ceramics and polymers


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Arises from a sea ofdonated valence electrons(1, 2, or 3 from each atom).

+ + +

Metallic Bonding

Primary bond for metals and their alloys

high electrical conductivity.

Why? What about ionic/covalent?

+ + +

+ + + Adapted from Callister 6e.


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Metallic versus Ionic Bonding

Much easier to deform materials with metallicthan with ionic bonding. Why?

Sliding atom planes over each other (deformation) veryunfavorable energetically in ionic solids!

metals are ductile & ceramics (ionic) are brittle


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Arises from interaction between dipoles

Fluctuating dipoles

HH HH

H2 H2

secondarybondin

ex: liquid H2asymmetric electron

clouds

+ - + -secondary

bonding

Secondary Bonding

Permanentdipoles-molecule induced

+ - secondarybonding

+ -

H Cl H Clsecondarybonding

secondarybonding

-general case:

-ex: liquid HCl

-ex: polymer

.

Adapted from Fig. 2.14,

Callister 6e.

Adapted from Fig. 2.14,Callister 6e.


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Ionic

Covalent

Large!

Variable

large-Diamond

-

Type Bond Energy Comments

Nondirectional (ceramics)

Directional

(semiconductors, ceramics

Summary: Bonding

Metallic

Secondary

Variable

large-Tungsten

small-Mercury

smallest

Nondirectional (metals)

Directionalinter-chain (polymer)

inter-molecular


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Ceramics

(Ionic & covalent bonding):

Large bond energylarge Tm

large E

small a

Metals

(Metallic bonding):

Variable bond energymoderate Tm

moderate E

Summary: Bonding In Materials

moderate a

Polymers

(Covalent & Secondary):

secondarybonding

Secondary bonding dominatessmall Tm

small Elarge a

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Material Science and Engineering Lecture 2