Chapter 2 - 1

Properties depend on - arrangement of atoms- interactions between atoms

Study Bonding:

• 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 kg 

protons neutrons

• nucleus of atom: “size of a point on a page” -! فراغ

• atomic number = No. of protons in nucleus of atom = No. 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 Unit 1 amu/atom = 1g/mol

C 12.011H 1.008 etc.

} 1.67 x 10-27 kg

Chapter 2 - 3

Atomic Structure

• Valence electrons: in the outer-shell

• determine all of the following properties1) Chemical: reactivity

2) Electrical: conductivity

3) Thermal: conductivity

4) Optical: transmittance

Chapter 2 - 4

Electronic Structure - Review

• 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. Quantum # Designation

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

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

ml = magnetic (direction in space) 1, 3, 5, 7 (-l to +l)

ms = spin ½, -½

Chapter 2 - 5

Electron Energy States

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...

Chapter 2 - 6

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

• Most elements: Electron configuration not stable.

SURVEY OF ELEMENTS - Review

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

Chapter 2 - 7

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

• Primary Bonding: Ionic, Covalent and Metallic

• example: C (atomic number = 6)

1s2 2s2 2p2

valence electrons

Chapter 2 - 8

Electronic Configurationsex: Fe - atomic # = 26

valence electrons

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

Chapter 2 - 9

The Periodic Table• Columns: Similar Valence Structure

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

Chapter 2 - 10

• Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

• Large values: tendency to acquire electrons.

Electronegativity

Chapter 2 - 11

Ionic bond – metal + nonmetal

donates accepts electrons electrons

Dissimilar electronegativities  

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

[Ne] 3s2 

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

1s22s 22p 6 (stable)10Neon

Chapter 2 - 12

• Occurs between + and - ions.

• Requires electron transfer.

• Large difference in electronegativity required.

• Example: NaCl

Ionic Bonding

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

Chapter 2 - 13

• Predominant bonding in CeramicsExamples: Ionic Bonding

Give up electrons Acquire electrons

NaCl

MgO

CaF2CsCl

Chapter 2 - 14

C: has 4 valence e -, needs 4 more

H: has 1 valence e -, needs 1 more

Electronegativities are comparable.

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

Primary Bonding• Metallic Bond -- delocalized as electron cloud  

• 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 - 16

Arises from interaction between dipoles

• Permanent dipoles-molecule induced

• Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.14, Callister 7e.

SECONDARY BONDING

asymmetric electron clouds

+ - + -secondary

bonding

HH HH

H2 H2

secondary bonding

ex: liquid H2

H Cl H Clsecondary bonding

secondary bonding+ - + -

secondary bondingsecondary bonding

Chapter 2 - 17

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

Summary: Bonding

Chapter 2 - 18

Interactions Between Atoms

+-

+-

r

+-

+-

Spacing (r)

Atomic Radius (R)

Far Atoms: Attraction Close Atoms: Repulsion

Equilibrium spacing = ro

ro = 2 R

+-

+-

Dynamic Equilibrium

Chapter 2 - 19

Chapter 2 - 20

Bonding Curve

Chapter 2 - 21

Bonding Curve

• 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 = ro

Chapter 2 - 22

Bonding - Estimations

• Estimation of Bonding Energy (Eo) and Equilibrium Spacing (ro)• Given:

• Bonding occurs at energy minimum

• At minimum: Derivative dE/dr = g(r) = 0• Solve g(r) = 0 for r ro

• Substitute r=ro in the Energy Eqn:• Get Eo = E (r=ro)• Atomic radius R = ro/2

Eo =

“bond energy”

Energy

r o r

rA

nrBE = certain f(r) e.g. E =

For many atoms, ro is approximately 0.3 nm.

Chapter 2 - 23

• Bond length, r

• Bond energy, Eo

• Melting Temperature, Tm

Tm is larger if Eo is larger.

Properties From Bonding: Tm

r o r

Energyr

larger Tm

smaller Tm

Eo =

“bond energy”

Energy

r o r

unstretched length

Chapter 2 - 24

• Coefficient of thermal expansion,

• ~ symmetry at ro

is larger if Eo is smaller.

Properties From Bonding :

= (T2 -T1)LLo

Coefficient of thermal expansion

L

length, Lo

unheated, T1

heated, T2

r or

Smaller

Larger

Energy

unstretched length

Eo

Eo

Chapter 2 - 25

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

Summary: Primary Bonds

secondary bonding

Bonding in Solids > Liquids > Gases

Chapter 2 - 26

Reading: Focus on Atomic Bonding in Solids

Core Problems: 2.7, 2.8, 2.12, 2.13,2.17, 2.19, 2.21 (Callister 3rd Addition)

Quiz: Next Class

ANNOUNCEMENTS

Chapter 2 - 27

Quiz

Energy

Atomic Spacing r

A B

c

Rank materials A, B and C according to 1. Increasing melting temperature > > 2. Decreasing bonding energy >

>3. Increasing Atomic Radius >

>4. Increasing coefficient of thermal expansion >

>

Chapter 2 - 28

Quiz

Energy

Atomic Spacing r

A

B

c

Rank materials A, B and C according to 1. Increasing melting temperature > > 2. Decreasing bonding energy >

>3. Increasing Atomic Radius >

>4. Increasing coefficient of thermal expansion >

>