Types of Primary Chemical Bonds

• Metallic– Electropositive: give up electrons

• Ionic– Electronegative/Electropositive

• Colavent– Electronegative: want electrons

– Shared electrons along

bond direction

Types of Primary Chemical BondsIsotropic, filled outer shells

+ - +

- + -

+ - +

+ + +

+ + +

+ + +

e-

e-

e-

Close-packed structures

Metals• single element, fairly electropositive• elements similar in electronegativity

cation

anion

Ionic Compounds• elements differing

in electronegativity

CERAMICS

Covalent Compounds

s2p2s2p1 s2p3

sp3

SEMI-CONDUCTORSs2

s2p4

Hybridized Bonds

one s + three p orbitals 4 (x 2) electron states(resulting orbital is a combination)

sp3 hybridization

diamond

also methane: CH4

• Elemental carbon (no other elements)

Covalent StructuresRecall: zinc blende both species tetrahedral

ZnS: +2 -2GaAs: +3 -3 or sp3 single element: C or Si or Sn

diamond

S

Zn

Another way to hybridize• Elemental carbon (no other elements)

sp2 hybridization

graphite

one s + two p orbitals 3 (x 2) electron states(resulting orbital is a combination)one unchanged p orbital

trigonal symmetry

Forms of carbon with sp2 bonds

Nobel Prize Physics, 2010

Nobel Prize Chemistry, 1996

Graphene

Fullerene

Nanotube

source: Wikipedia

Graphite*

* http://www.electronics-cooling.com/assets/images/2001_August_techbrief_f1.jpg

Structural Characteristics• Metals

– Close-packed structures (CN = 12)– Slightly less close-packed (CN = 8)

• Ionic structures– Close-packed with constraints– CN = 4 to 8, sometimes 12

• Covalent structures– Not close-packed, bonding is directional

• Any can be strongly or weakly bonded (Tm)

Diamond vs. CCP8 atoms/cell, CN = 4 4 atoms/cell, CN = 12

3 8a R

½ tetrahedral sites filled

33 3 38 98.5

3V a R R

3/ atom 12.3V R

2 4a R

33 3 34 22.6

2V a R R

3/ atom 5.7V R

Computing density• Establish unit cell contents• Compute unit cell mass• Compute unit cell volume

– Unit cell constant, a, given (or a and c, etc.)– Or estimate based on atomic/ionic radii

• Compute mass/volume, g/cc• Example: NaCl

– Contents = 4 Na + 4 Cl– Mass = 4(atom mass Na + atomic mass Cl)/No

– Vol = a3

– Units = 33

/ /#/

g mol g cmcm mol

Avogardo’s #

Cl

Na

Single Crystal vs. Polycrystalline

Rb3H(SO4)2

Ba(Zr,Y)O3-d

Periodicity extends uninterrupted throughout entirety of the sampleExternal habit often reflects internal symmetry

Regions of uninterrupted periodicity amalgamated into a larger compact

= grainsdelineated by grain boundaries

Quartz (SiO2)

Diamond

Isotropic vs. Anisotropic

* http://www.electronics-cooling.com/assets/images/2001_August_techbrief_f1.jpg

graphite* diamond

polycrystalline averaging

• Metallic– Electropositive: give up electrons

• Ionic– Electronegative/Electropositive

• Colavent– Electronegative: want electrons– Shared electrons along

bond direction

Types of Bonds Types of Materials

Isotropic, filled outer shells

+ - +

- + -

+ - +

+ + +

+ + +

+ + +

e-

e-

e-

Close-packed structures

CERAMICSMETALS

SEMI-CONDUCTORS

H

What’s Missing?

POLYMERS

Long chain molecules with repeated unitsMolecules formed by covalent bondsSecondary bonds link molecules into solids

C

C

H

H

H

methane

C H

CC

HH

HH

CC

HH

HH

CC

HH

HH

CC

HH

HH

many

units

http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D.png

Polymer Synthesis

• Traditional synthesis– Initiation, using a catalyst that creates a free

radical

– Propagation

– Termination

R + C=C

R…… C – C + C=C

R…… C – C + C – C……R

unpaired electron

C=CH H

H H

R – C – C

R……C – C – C – C

R –(C-C)n– R

Polydispersity• Traditional synthesis large variation in chain length

number average n i iM x M

# of

pol

ymer

cha

ins

molecular weight

# of polymer chains of Mi

total number of chains

molecular weight

weight average w i iM w M

weight of polymer chains of Mi

total weight of all chains

width is a measure of polydispersity

= weight fraction

nM

wM

• Degree of polymerization– Average # of mer units/chain

nn

Mn

m

ww

Mn

m

Average chain molecular weight

by number

by weight

mer molecular weight

New modes of synthesis• “Living polymerization”

– Initiation occurs instantaneously– Chemically eliminate possibility of random termination– Polymer chains grow until monomer is consumed– Each grows for a fixed (identical) period

Polymers• Homopolymer

– Only one type of ‘mer’• Copolymer

– Two or more types of ‘mers’• Block copolymer

– Long regions of each type of ‘mer’• Bifunctional mer

– Can make two bonds, e.g. ethylene linear polymer• Trifunctional mer

– Can make three bonds branched polymer

Polymer Configurations• Linear

• Branched

• Cross-linked

C C C C

CC C C C

C = C

H H

H H

Polymers

C C C C

CC C C C

C = C

H H

H H

109.5°

H out H in

Placement of side groups is fixed once polymer is formedExample side group: styrene

R =

R

C C C C

CC C C C

R R R R

C = C H H

Cl HIsotactic

C C C C

CC C C C

R R R R

Syndiotactic

C C C C

CC C C C

R R R R

Atactic

• Thermal Properties– Thermoplastics

• Melt (on heating) and resolidify (on cooling)• Linear polymers

– Thermosets• Soften, decompose irreversibly on heating• Crosslinked

• Crystallinity• Linear: more crystalline

than branched or crosslinked• Crystalline has higher

density than amorphous

Formal Crystallography• Crystalline

– Periodic arrangement of atoms– Pattern is repeated by translation

• Three translation vectors define:– Coordinate system– Crystal system– Unit cell shape

• Lattice points– Points of identical environment– Related by translational symmetry– Lattice = array of lattice points

a

b

c

abg

• space filling• defined by 3 vectors• parallelipiped• arbitrary coord system• lattice pts at corners +

hcp ccp (fcc)

bccHexagonal unit cellSpecify: a, cHexagonal implies:|a1| = |a2| = ag = 120°a = b = 90°

Cubic unit cellsSpecify: aCubic implies:|a1| = |a2| = |a3| = aa = b = g = 90°

But the two types of cubic unit cells are different!

a

b

c

abg

Crystal system Lattices

triclinic

simple base-centered

monoclinic a = 90°

Convention: b = 90° instead of a

simple base-centered body-centered face-centered

orthorhombic a = b = g = 90°

hexagonal g = 120° c

aa

rhombohedral (trigonal)

a = b = g a

simple body-centered

tetragonal a = b = g = 90°

a = b

simple body-centered face-centered cubic

(isometric) a = b = g = 90°

a = b = c

6 or 7 crystal systems

14 lattices

a, b, c, a, b, g – all arbitrary

a, b, c – arbitrary

a, c – arbitraryb = aa = b = 90

a – arbitrary; a = b = ca – arbitrary; a = b = g

C or A centered for b = arbitrary

a, c – arbitrary

a – arbitrary

a, b, c – arbitrarya = g = 90

Centered Lattices

b

a

b

a

b

a

conventionalchoice

unconventionalchoice

a

b

both are primitive cellsunconventional is primitiveconventional is centered

unconventionalchoice

conventionalchoice

Types of Primary Chemical Bonds

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