7 structure of materials-bonding (1)

Structure of Materials: bonding and properties

ZnS

NaCl

Cs Cl

1 mm

Crystal units

Microstructure (grains, grain boundaries) of a metal alloy

Atomic Structure

atom Ø What are reactive particles?

C 12.011; H 1.008 etc

Atomic number = # of protons in nucleus of atom (Z) = # of electrons of neutral species

Atomic mass unit (amu): 1/12 of the atomic mass of the most common isotop of carbon

Atomic mass A ≈ Z + N (Z = # protons, N = # neutrons)

Electronic Structure •  Electrons have wavelike and particulate properties.

–  Orbitals –  Quantum numbers.

Nucleus Increasing energy

orbital electrons: n = principal quantum number

n=3 2 1

ω

Fe

Fc

Discrete orbitals

Quantum # Designation n = principal quantum number (energy level-shells) K, L, M, N, O (1, 2, 3, etc.) l = subshells (orbitals) s, p, d, f (0, 1, 2, 3,…, n -1) ml = magnetic 1, 3, 5, 7 (-l to +l) ms = spin ½, -½

Pauli exclusion principle Each electron state can hold no more than two electrons, which must have opposite spin.

Electron Energy States

Electrons...

“Absorb or emit energy”

Stable electron configurations...

Electron Configurations

• Why?

• Most elements: Electron configuration not stable. Electron configuration 1s1 1s2 (stable) 1s22s1 1s22s2 1s22s22p1 1s22s22p2 ... 1s22s22p6 (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 ... 1s22s22p63s23p6 (stable) ... 1s22s22p63s23p63d104s246 (stable)

•  What are valence electrons? •  Their role in bonding?

–  example: C (atomic number = 6)

1s2 2s2 2p2

valence electrons

• Columns: Similar Valence Structure

Electropositive elements: Readily give up electrons to become + ions.

Electronegative elements: Readily acquire electrons to become - ions.

give

up

1e

give

up

2e

give

up

3e

iner

t gas

es

acce

pt 1

e ac

cept

2e

O

Se Te Po At

I Br

He Ne Ar Kr Xe Rn

F Cl S

Li Be H

Na Mg

Ba Cs Ra Fr

Ca K Sc Sr Rb Y

• Ranges from 0.7 to 4.0, • tendency to acquire electrons.

Electronegativity

Bonding

Bonding force

Bonding energy

Ø Primary bonds

Ø Secondary bonds

• Example: NaCl

Ionic Bonding

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

Requirements?

Give up electrons Acquire electrons

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

CsCl

MgO

CaF2

NaCl

O 3.5

Ionic bonding What type of materials?

Bonding energy high (150-3700 kcal/mol) Nondirectional

Conductivity? Ductility? Melting temperature?

Covalent Bonding

•  Example: CH4

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH 4

Requirements?

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

co

lum

n IV

A

Sn 1.8Pb 1.8

Covalent bonding

What type of materials?

Bonding energy high (75-300 kcal/mol) Directional

Conductivity? Melting temperature?

Metallic bonding

What type of materials? Requirements?

Bonding energy high (25-200 kcal/mol) Nondirectional

Conductivity? Ductility?

Primary Bonding

•  Metallic Bond

•  Ionic-Covalent Mixed Bonding % ionic character = where XA & XB are Pauling electronegativities

%) 100 ( x

€

1−e− (XA−XB)2

4

#

$

% % %

&

'

( ( (

Ex: MgO XMg = 1.3 XO = 3.5

van der Waals

Secondary bonding

Hydrogen bonding

Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variable large-Diamond small-Bismuth

Variable large-Tungsten small-Mercury Smallest but important! Biopolymers….

In What Materials?

Bonding: a summary

How bonding affects material properties?

• Melting Temperature, Tm

Properties From Bonding: Tm

r o r

Energy

• Coefficient of thermal expansion, α

Properties From Bonding : α

= α ( T 2 - T 1 ) Δ L L o

coeff. thermal expansion

Δ L

length, L o unheated, T 1

heated, T 2

r o r

Energy unstretched length

Eo

Eo

• Elastic modulus, E

ΔL F Ao

= E Lo

Elastic modulus

Properties from bonding: elastic modulus

r

larger Elastic Modulus

smaller Elastic Modulus

Energy

ro unstretched length

Ceramics (Ionic & covalent bonding):

Metals (Metallic bonding):

Polymers (Covalent & Secondary):

Large bond energy large Tm large E small α

Variable bond energy moderate Tm moderate E moderate α

Secondary bonding dominates small Tm small E large α

Summary

secondary bonding