Chapter 2 - 1

ISSUES TO ADDRESS...

• What promotes bonding?

• What types of bonds are there?

• What properties are inferred from bonding?

CHAPTER 2:

BONDING AND PROPERTIES

Chapter 2 - 2

Atomic Structure (Freshman Chem.)

• atom – electrons – 9.11 x 10-31 kgprotonsneutrons

• 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

Chapter 2 - 3

Atomic Structure

• Valence electrons determine all of the following properties

1) Chemical

2) Electrical

3) Thermal

4) Optical

Chapter 2 - 4

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

• Most elements: Electron configuration not stable.

SURVEY OF ELEMENTS

Electron configuration

(stable)

...

...

1s22s22p 63s23p 6 (stable)...

1s22s22p 63s23p 63d 10 4s24p 6 (stable)

Atomic #

18...

36

Element1s11Hydrogen1s22Helium1s22s13Lithium1s22s24Beryllium1s22s22p 15Boron1s22s22p 26Carbon

...

1s22s22p 6 (stable)10Neon1s22s22p 63s111Sodium

1s22s22p 63s212Magnesium

1s22s22p 63s23p 113Aluminum...

Argon...

Krypton

Adapted from Table 2.2,

Callister 8e.

Chapter 2 - 5

Electron Configurations

• 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

valence electrons

Chapter 2 - 6

The Periodic Table• Columns: Similar Valence Structure

Adapted from

Fig. 2.6,

Callister 8e.

Electropositive elements:

Readily give up electronsto become + ions.

Electronegative elements:

Readily acquire electronsto become - ions.

giv

e u

p 1

egiv

e u

p 2

egiv

e u

p 3

e

inert

gases

accept

1e

accept

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

Chapter 2 - 7

• Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

• Large values: tendency to acquire electrons.

Adapted from Fig. 2.7, Callister 8e. (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.

Electronegativity

Chapter 2 - 8

Primary Bonding• Metallic Bond -- delocalized as electron cloud • Ionic Bonding• Covalent Bonding• Ionic-Covalent Mixed Bonding

% ionic character =

where XA & XB are Pauling electronegativities

%)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

Chapter 2 - 9

• Occurs between + and - ions.

• Requires electron transfer.

• Large difference in electronegativity required.

• Example: NaCl

Ionic Bonding

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ -CoulombicAttraction

Na (cation) stable

Cl (anion) stable

Chapter 2 - 10

• Predominant bonding in Ceramics

Adapted from Fig. 2.7, Callister 8e. (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.

Examples: Ionic Bonding

Give up electrons Acquire electrons

NaCl

MgO

CaF2CsCl

Chapter 2 - 11

Net Forces dependence on interatomic separation

Fig. 2.8(b), Callister &

Rethwisch 8e.

Chapter 2 - 12

Ionic Bonding

• 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

nrB

EN = EA + ER = −−

Adapted from Fig. 2.8(b),

Callister 8e.

Chapter 2 - 13

Bonding Energies and Melting Temperature of Various Substance

Chapter 2 - 14

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 8e.

Covalent Bonding• 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

Chapter 2 - 15

• 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

column IVA

Sn 1.8

Pb 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.

EXAMPLES: COVALENT BONDING

Chapter 2 - 16

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

• Primary bond for metals and their alloys

+ + +

+ + +

+ + +

METALLIC BONDING

Chapter 2 - 17

Arises from interaction between dipoles

• Permanent dipoles-molecule induced

• Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.13, Callister 8e.

Adapted from Fig. 2.15,

Callister 8e.

SECONDARY BONDING

asymmetric electronclouds

+ - + -secondary bonding

HH HH

H2 H2

secondary bonding

ex: liquid H2

H Cl H Clsecondary

bonding

secondary bonding

+ - + -

secondary bonding

Chapter 2 - 18

Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

Comments

Nondirectional (ceramics)

Directional

(semiconductors, ceramics

polymer chains)

Nondirectional (metals)

Directional

inter-chain (polymer)

inter-molecular

Summary: Bonding

Chapter 2 - 19

• Bond length, r

• Bond energy, Eo

• Melting Temperature, Tm

Tm is larger if Eo is larger.

Properties From Bonding: Tm

r or

Energy

r

larger Tm

smaller Tm

Eo =

“bond energy”

Energy

r or

unstretched length

Chapter 2 - 20

• 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 Eo is larger.

PROPERTIES FROM BONDING: E

Chapter 2 - 21

• Coefficient of thermal expansion, α

• α ~ symmetry at ro

α is larger if Eo is smaller.

Properties From Bonding : α

= α (T2 -T1)∆L

Lo

coeff. thermal expansion

∆L

length, Lo

unheated, T1

heated, T2

r or

smaller α

larger α

Energy

unstretched length

Eo

Eo

Chapter 2 - 22

Ceramics

(Ionic & covalent bonding):

Metals

(Metallic bonding):

Polymers(Covalent & Secondary):

Large bond energylarge Tm

large Esmall α

Variable bond energymoderate Tm

moderate Emoderate α

Directional PropertiesSecondary bonding dominates

small Tm

small Elarge α

Summary: Primary Bonds