Properties

Structure Processing

• Electronic level (subatomic)• Atomic (molecular level, chemical composition)• Crystal (arrangement of atoms or ions wrt one another)• Microstructure (can study with microscopes)• Macrostructure (can see with naked eye)

“Materials Science”“Materials Engineering”

Valence electrons determine all of the following properties

1) Chemical2) Electrical 3) Thermal4) Optical

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• What promotes bonding?

• What types of bonds are there?

• Bonding material propertiesWhat properties are inferred from bonding?

(melting point, thermal expansion, elasticity)

• Electronic structure & bonding

Based on Chapter 2 (Callister)

atom – electrons – 9.11 x 10-31 kg  protons neutrons

Electric charge of proton = 1.6 x 10-19 C 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

C 12.011H 1.008 etc.

} 1.67 x 10-27 kg

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Nucleus: Z = # protons

= 1 for hydrogen to 94 for plutoniumN = # neutrons

Atomic mass A ≈ Z + N

Adapted from Fig. 2.1, Callister 6e.

electrons: n = principal quantum number

n=3 2 1

n labels shells; shells are composed of sub-shells: s, p, d, f, …

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Electrons have wavelike and particulate properties. This means that electrons are in orbitals defined by a

probability. Each orbital at discrete energy level determined by quantum

numbers. (like x, y, z) Quantum # Designation

n = principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.7)

l = subsidiary (form and number of orbitals) s, p, d, f (0, 1, 2, 3,…, n -1)

ml = magnetic (Orientation of orbitals) 1, 3, 5, 7 (-l to +l)

Pauli exclusion principlems = spin +½, -½

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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...

Adapted from Fig. 2.4, Callister 7e.

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

• Most elements: Electron configuration not stable.Electron configuration

(stable)

...

...

1s22s 22p 63s23p 6 (stable)... 1s22s 22p 63s23p 63d 10 4s 24p 6 (stable)

Atomic #

18...36

Element1s1 1Hydrogen1s22Helium1s22s 1 3Lithium1s22s24Beryllium1s22s 22p 15Boron1s22s 22p 26Carbon

...

1s22s 22p 6 (stable)10Neon1s22s 22p 63s111Sodium1s22s 22p 63s2 12Magnesium1s22s 22p 63s23p 113Aluminum

...

Argon...Krypton

Adapted from Table 2.2, Callister 7e.

Valence electrons – those in unfilled shells

Filled shells more stable Valence electrons are most available for

bonding and tend to control the chemical properties

example: C (atomic number = 6)

1s2 2s2 2p2

12

valence electrons

ex: Fe - atomic # =

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valence electrons

Adapted from Fig. 2.4, Callister 7e.

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 3d 6 4s2

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• Columns: Similar Valence Structure

Adapted from Fig. 2.6, Callister 7e.

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

give

up

1egi

ve u

p 2e

give

up

3e iner

t ga

ses

acce

pt 1

eac

cept

2e

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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• Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

• Large values: tendency to acquire electrons.

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.

donates accepts electrons electrons

 

Dissimilar electronegativities  

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

[Ne] 3s2 

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Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6 [Ne] [Ne]

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• Occurs between + and - ions.

• Requires electron transfer.

• Large difference in electronegativity required.

• Example: NaCl

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

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Energy – minimum energy most stable Energy balance of attractive and repulsive

terms

Attractive energy EA

Net energy EN

Repulsive energy ER

Interatomic separation r

rA

nrBEN = EA + ER =

Adapted from Fig. 2.8(b), Callister 7e.

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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.

Give up electrons Acquire electrons

NaCl

MgO

CaF2CsCl

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C: has 4 valence e-, needs 4 more

H: has 1 valence e-, needs 1 more

Electronegativities are comparable.

Adapted from Fig. 2.10, Callister 7e.

• similar electronegativity share electrons• bonds determined by valence – s & p orbitals

dominate bonding

• Example: CH4shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH4

• Molecules with nonmetals• Molecules with metals and nonmetals• Elemental solids (RHS of Periodic Table)• Compound solids (about column IVA)

He -

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

SiC

C(diamond)

H2O

C 2.5

H2

Cl2

F2

Si 1.8

Ga 1.6

GaAs

Ge 1.8

O 2.0

colu

mn IVA

Sn 1.8Pb 1.8

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 isadapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

3.5

• Prevalent in ceramics and polymers

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

• Primary bond for metals and their alloys high electrical conductivity. Why? What about ionic/covalent?

+ + +

+ + +

+ + +Adapted from Fig. 2.11, Callister 6e.

Much easier to deform materials with metallic than with ionic bonding. Why?

• Sliding atom planes over each other (deformation) very unfavorable energetically in ionic solids!

metals are ductile & ceramics (ionic) are brittle

Metallic Bond -- delocalized as electron cloud  

Ionic-Covalent Mixed Bonding

% ionic character =  

where XA & XB are Pauling electronegativities

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%)100( x

1 e

(XA XB )2

4

ionic 70.2% (100%) x e1 characterionic % 4)3.15.3(

2

Ex: MgO XMg = 1.3XO = 3.5

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Arises from interaction between dipoles

• Permanent dipoles-molecule induced

• Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.13, Callister 7e.

Adapted from Fig. 2.14, Callister 7e.

asymmetric electron clouds

+ - + -secondary

bonding

HH HH

H2 H2

secondary bonding

ex: liquid H2

H Cl H Clsecondary bonding

secondary bonding+ - + -

secondary bondingsecondary bonding

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Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

Comments

Nondirectional (ceramics)

Directional(semiconductors, ceramicspolymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)inter-molecular

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• Bond length, r

• Bond energy, Eo

• Melting Temperature, Tm

Tm is larger if Eo is larger.

r o r

Energyr

larger Tm

smaller Tm

Eo =

“bond energy”

Energy

r o r

unstretched length

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• Coefficient of thermal expansion,

• ~ symmetry at ro

is larger if Eo is smaller.

= (T2 -T1)LLo

coeff. thermal expansion

L

length, Lo

unheated, T1

heated, T2

r or

smaller

larger

Energy

unstretched length

Eo

Eo

Why does thermal expansion Why does thermal expansion occur?occur?

As atoms vibrate, easy to move apart, but difficult to move closer

As temperature increases, average bond length increases.

Bond Energy vs. Distance Curve is asymmetric

• Elastic modulus, E

• E ~ curvature at ro

cross sectional area Ao

L

length, Lo

F

undeformed

deformed

L F Ao

= E Lo

Elastic modulus

r

larger Elastic Modulus

smaller Elastic Modulus

Energy

ro unstretched length

E is larger if curvature is larger.

E similar to spring constant

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Ceramics(Ionic & covalent bonding):

Metals(Metallic bonding):

Polymers(Covalent & Secondary):

Large bond energylarge Tm

large Esmall

Variable bond energymoderate Tm

moderate Emoderate

Directional PropertiesSecondary bonding dominates

small Tm

small E large

secondary bonding