Chapter 2 - 1 ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What...

Chapter 2 - 1

ISSUES TO ADDRESS...

• What promotes bonding?

• What types of bonds are there?

• What properties are inferred from bonding?

Chapter 2: Atomic Structure & Interatomic Bonding

Chapter 2 - 2

Atomic Structure (Freshman Chem.)• atom – electrons – 9.11 x 10-31 kg 

protons neutrons

• 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 6.022 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

number of neutrons = Nnumber of protons = Z

A= Z + N (2.1)

AVAGADRO’S NUMBER = 6.022 x 1023 = NA

ATOMIC OR MOLECULAR WEIGHT =

NA x WEIGHT PER ATOM.

Chapter 2 - 4

Atomic Structure

• Valence electrons determine all of the following properties

1) Chemical

2) Electrical

3) Thermal

4) Optical

Chapter 2 -

BOHR ATOM

5

Chapter 2 -

WAVE MECHANICAL MODEL OF ATOM

6

Chapter 2 - 7

Electronic Structure

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

probability.– Each orbital at discrete energy level is 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 1, 3, 5, 7 (-l to +l)

ms = spin ½, -½

Chapter 2 - 8

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...

Adapted from Fig. 2.4, Callister & Rethwisch 8e.

Chapter 2 - 9

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

• Most elements: Electron configuration not stable.

SURVEY OF ELEMENTS

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 & Rethwisch 8e.

Chapter 2 - 10

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 - 11

Electronic Configurationsex: Fe - atomic # = 26

valence electrons

Adapted from Fig. 2.4, Callister & Rethwisch 8e.

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 - 12

The Periodic Table• Columns: Similar Valence Structure

Adapted from Fig. 2.6, Callister & Rethwisch 8e.

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

giv

e u

p 1

e-

giv

e u

p 2

e-

giv

e u

p 3

e- ine

rt g

ase

s

acc

ept

1e

-

acc

ept

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 - 13

• Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

• Large values: tendency to acquire electrons.

Adapted from Fig. 2.7, Callister & Rethwisch 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 - 14

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]

Chapter 2 -

Electrons in different shells

15

Chapter 2 -

Electrons in Sodium and Chlorine

16

TABLE 2.2 / P 25 3s1

3s2 3p5

Chapter 2 - 17

• 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 - 18

Chapter 2 -

FORCES AND ENERGIES

19

Chapter 2 - 20

Chapter 2 - 21

Chapter 2 -

Bonding Forces and Energies

22

2.13 Calculate the force of attraction between a K+ and an O2- ion the centers of which are separated by a distance of r0 =1.5 nm.

The attractive force between two ions FA is just the derivative with respect to the interatomic separation of the attractive energy expression, Equation 2.8, which is just

Solution

Chapter 2 - 23

FA = dEAdr

= d

A

r

dr =

A

r2

The constant A in this expression is defined in footnote 3. Since the valences of the K+ and O2- ions

(Z1 and Z2) are +1 and -2, respectively, Z1 = 1 and Z2 = 2, then

Chapter 2 - 24

FA = (Z1e) (Z2e)

40r2

= (1)(2)(1.602 10 19 C)2

(4)() (8.85 10 12 F/m) (1.5 10 9 m)2

=2.05 10^(-10 ) N

Chapter 2 -

IONIC FORCE / P 31 FOOT-NOTE

25

F= (Z1 *Z2 * e^2)/(4*π*ε0*r^2);

e= 1.602 *10^(-19) COULOMBS ;

ε0 = 8.85 * 10^(-12 )

Z1, Z2 = VALENCIES OF IONS

Chapter 2 - 26

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

nrBEN = EA + ER =

Adapted from Fig. 2.8(b), Callister & Rethwisch 8e.

Chapter 2 - 27

• Predominant bonding in Ceramics

Adapted from Fig. 2.7, Callister & Rethwisch 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 - 28

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 & Rethwisch 8e.

Covalent Bonding• similar electronegativity share electrons• bonds determined by valence – s & p orbitals dominate bonding

• Example: CH4

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH4

Chapter 2 - 29

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 73.4% (100%) x e1 characterionic % 4)2.15.3(

2

Ex: MgO XMg = 1.2XO = 3.5

Chapter 2 -

METALLIC BONDING

30

Chapter 2 - 31

Arises from interaction between dipoles

• Permanent dipoles-molecule induced

• Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.13, Callister & Rethwisch 8e.

Adapted from Fig. 2.15, Callister & Rethwisch 8e.

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 - 32

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 - 33

• 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 - 34

• Coefficient of thermal expansion,

• ~ symmetric at ro

is larger if Eo is smaller.

Properties From Bonding :

= (T2 -T1)LLo

coeff. thermal expansion

L

length, Lo

unheated, T1

heated, T2

r or

smaller

larger

Energy

unstretched length

Eo

Eo

Chapter 2 - 35

Ceramics(Ionic & covalent bonding):

Large bond energylarge Tm

large Esmall

Metals(Metallic bonding):

Variable bond energymoderate Tm

moderate Emoderate

Summary: Primary Bonds

Polymers(Covalent & Secondary):

Directional PropertiesSecondary bonding dominates

small Tm

small E large

secondary bonding

Chapter 2 - 36

Reading:

Core Problems:

Self-help Problems:

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