1

Chapter 2

Atomic Structure & Bondingin Solids

2

Issues to address

What promotes bonding?

What types of bonds are there?

What properties are inferred from bonding?

3

Fundamental ConceptAtom

Basic Unit of an ElementDiameter : 10 –10 m.

Neutrally Charged

NucleusDiameter : 10 –14 m

Positive ChargeAccounts for almost

all mass

Electron CloudMass : 9.109 x 10 –28 g

Charge : -1.602 x 10 –9 CAccounts for all volume

ProtonMass : 1.673 x 10 –24 g

Charge : 1.602 x 10 –19 C

NeutronMass : 1.675 x 10 –24 g

Neutral Charge

atomic structure

4

Fundamental Concept

atomic structure

Periodic table of the elements

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

Atomic number, ZAtomic weight, A Cu

29

63.54

5

Electron Principle

atomic structure

Principle

classical mechanic

s

quantum mechanics

Bohr

ato

mic

m

odel

Wave

mech

anic

al

ato

m m

odel

electron structure

electron energy

electron configuration

electron position

electronnucleus

Atomic structure

6

Electron Principle

atomic structure

Bohr atomic model Wave mechanical atom model

electron structure & position

▣ structure: assume electrons as particle-like & revolve around the atomic nucleus in discrete paths.

▣ position: in terms of its orbital in discrete path.

▣ limitation: inability to explain several phenomena involving electron.

▣ further refined from Bohr model.

▣ structure: assume electrons as particle-like & wave-like.

▣ Position: in terms of probability distribution (various location around the atomic nucleus).

7

Electron Principle

atomic structure

Bohr atomic model Wave mechanical atom model

electron energy

▣ Energy are quantized.

▣ Electrons are permitted to have only specific values of energy

(have energy levels @ states).▣ Each adjacent orbital/state are separated by finite energies.▣ Electron may change energy by make a quantum jump.▣ Energy is absorbed to move to higher energy level.▣ Energy is emitted during transition to lower level.

Both model assume:

8

Electron Principle

atomic structure

Bohr atomic model Wave mechanical atom model

electron energy

emit energy(photon)

absorb energy(photon)

energy levels

9

Electron Principle

atomic structure

Bohr atomic model Wave mechanical atom model

electron energy

n=1

n=2

n=3

▣ electron in its orbital (position) & separate by energy levels.

▣ Each orbital at discrete energy levels separate into electron subshells & quantum numbers dictate the number of state within each subshell.

4. Spin quantum number, ms. (spin)

3. Magnetic quantum number, ml. (state).

2. Subsidiary quantum number, l. (orbital/subshell).

1. Principle quantum number, n. (energy level/shell).

▣ Every electron characterized by 4 quantum numbers.

10

Electron Principle

atomic structure

Bohr atomic model

electron energy

Bohr atomic model

orbitaln=1

n=2

n=3

s orbital (l=0)

p orbital(l=1)

s orbital (l=0)

Energy level

n=2n=1

Energy level/shell

Orbital/subshell

Wave mechanical atom model

11

Electron Principle

atomic structure

electron energy Wave mechanical atom model

-1.5

-3.4

-13.6 K-shell n = 1

L-shell n = 2

M-shell n = 3

N-shell n = 4

Energ

y

1s

2s2p

3s3p

3d4s

4p4d

Orbital/ subshell

State Energy level/shell

eV

Electron energy level

12

Electron Principle

atomic structure

Bohr atomic model

electron configurations Wave mechanical atom model

◈ Electron Configuration: lists the arrangement of electrons in orbital.

◈ Maximum number of electrons in each atomic shell is given by 2n2.

n # of electrons 1 2 2 8 3 18 4 32◈ Apply Pauli exclusion principle: Each electron state can hold no more than 2 electrons which

must have opposite spin.◈ Electrons have discrete energy state & tend to occupy lowest

energy state.◈ Atomic size (radius) increases with addition of shells.

13

Electron Principleatomic structure

electron configurations Wave mechanical atom model

▣ The # of available electron states in some of the electron shells & subshells

1 K

2 L

3 M

Principle quantum number, n

shell/energy level

shell designation

Subsidiary quantum #, l

Magnetic quantum #, ml

subshells/orbital

sspspdspdf

1131351357

22262610261014

8

18

324 N

Number of states

Number of electrons

per subshell per shell

14

Electron Principle

atomic structure

electron configurations Wave mechanical atom model

example: Fe (Z = 26)electron configuration is

n = 1

n = 2

n = 3

n = 4

En

erg

y

1s

2s2p

3s3p

3d

4s

4p4d

Low

est

energ

y

state

(gro

und

state

)

Hig

hest

energ

y s

tate

Principal Quantum Numbers

Orbital letters

# of electrons

1s2 2s2 2p6 3s2 3p63d 6 4s2

15

Electron Principle

atomic structure

electron configurations Wave mechanical atom model

1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f

Method of arrangement:

16

Electron Principle

atomic structure

electron configurations Wave mechanical atom model

Electron configuration

(stable)

...

... 1s2 2s2 2p6 3s2 3p6 (stable)... 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 (stable)

Atomic #

18...36

Element1s1 1Hydrogen1s22Helium1s2 2s1 3Lithium1s2 2s24Beryllium1s2 2s2 2p15Boron1s2 2s2 2p26Carbon

...1s2 2s2 2p6 (stable)10Neon1s2 2s2 2p6 3s111Sodium1s2 2s2 2p6 3s2 12Magnesium1s2 2s2 2p6 3s2 3p113Aluminum

...Argon...Krypton

Electron configuration of some elements

17

Electron Principle

atomic structure

electron configurations Wave mechanical atom model

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

▣ Most elements: electron configuration not stable.

example: Carbon C

1s2 2s2 2p2

atomic number, Z = 6

valence electrons

18

Electron Principle

atomic structure

Valence electron

▣ Electron that occupy the outermost (valence) shell.

▣ Valence electrons ▷ those in unfilled shells (most elements) not stable & filled shells (inert gases) more stable.

▣ Participate in the bonding (unfilled shell) between atoms to form atomic & molecular aggregates.▷ determine physical (optical, thermal & electrical) & chemical properties.

19

Periodic Table

atomic structure

Electropositive & electronegative elements

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 YElectropositive elements

Electronegative elements

Inert

g

ases

20

Periodic Table

atomic structure

electronegative elements

readily give up electrons to become cations (+ions).

metallic elements. smaller electro-

negativity.

readily accept electrons to become anions (-ions).

non-metallic elements.

higher electro-negativity.

unfilled valence shell. not stable electron configuration. assume stable by losing @ gaining valence electrons to form charge ions.

filled valence shell. stable electron configuration.

unreactive chemically.

electropositive elements

Inert gases

21

Periodic Table

atomic structure

electronegativity

Smaller electronegativity Larger electronegativity

▣ Ranges from 0.7 to 4.0.▣ Large values: tendency to acquire electrons.

22

atomic bonding

type

Secondary

bonding

Ionic

bondin

g

Covale

nt

bondin

g

Atomic bonding

Meta

llic

bondin

g

Primary bonding

Type of bonding

Fluct

uati

ng

dip

ole

s bond

Perm

anent

dip

ole

s bond

23

Primary bonding

atomic bonding

Ionic bonding

▣ Strong atomic bonds due to transfer of electrons. ▣ It can form between metallic & nonmetallic

elements.▣ Electrons are transferred from electropositive to

electronegative atoms.▣ Large difference in electronegativity required.▣ Occurs between + & - ions.▣ Non Directional bonding.

24

atomic bonding

Metal (electropositive

element) - unstable

Non metal (electronegative

atom) - unstable

electron transfer

Cation(+ve charge)

- stable

Anion(-ve charge)

- stable

Ionic bond

electrostaticattraction

donates electrons

accepts electrons

Primary bondingIonic bonding

25

atomic bonding

Primary bondingIonic bonding

1s2 2s2 2p6 3s2 Mg O 1s2 2s2 2p4

[Ne] 3s2 

Mg: X = 1.2, Z = 12

O: X = 3.5, Z = 8

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

[Ne]

Example: Magnesium oxide (MgO)

Metal (electropositive

element) - unstable

Non metal (electronegative

atom) - unstable

electron transfer

Cation(+ve charge)

- stable

Anion(-ve charge)

- stable

Ionic bond

MgO

electrostaticattraction

26

atomic bonding

Primary bondingIonic bonding

Bonding Force

▣ due to electrostatic attraction.

27

atomic bonding

Primary bondingIonic bonding

Bonding Energy ▣ Energy – minimum energy most stable

▷ Energy balance of attractive & repulsive terms

Attractive energy EA

Net energy EN

Repulsive energy ER

Interatomic separation r

rA

nrBEN = EA + ER = __

28

atomic bonding

Primary bondingIonic bonding

Bonding Energy

Eo =

“bond energy”

Energy

r o r

29

atomic bonding

Primary bondingIonic bonding

▣ Predominant bonding in Ceramics

Give up electrons Acquire electrons

NaClMgO

CaF2CsCl

30

Primary bonding

atomic bonding

Covalent bonding

▣ share valence electrons.

▣ similar @ small differences in electronegativity.

▣ bonds determined by valence – s & p orbitals dominate bonding.

▣ Directional bonding.

overlapping electron clouds

31

Primary bonding

atomic bonding

Covalent bonding

Example: Methane (CH4)

C: X = 2.5, Z = 6

H: X = 2.1, Z = 1

shared electrons from carbon atom

shared electrons from hydrogen atoms H

H

H

H

C

C: 1s2 2s22p2

H: 1s1

4 valence electrons

1 valence electron

32

Primary bonding

atomic bonding

Ionic & Covalent bonding

Percent ionic character

▣ Ionic-Covalent Mixed Bonding

▣ % ionic character =   where

XA = electronegativity value for element A

XB = electronegativity value for element B

4

2

1)XX(

BA

e %)100( x

33

Primary bonding

atomic bonding

Ionic & Covalent bonding

Example: Magnesium oxide (MgO)

XMg = 1.3 XO = 3.5

4

2

1)XX(

BA

e %)100( x % ionic character =

= 70.2 % ionic

Percent ionic character

34

Primary bonding

atomic bonding

Metallic bonding

▣ Atoms in metals are closely packed in crystal structure.

▣ Loosely bounded valence electrons are attracted towards nucleus of other atoms.

▣ Electrons spread out among atoms forming electron clouds.▷ these free electrons are reason for a good electric conductivity & ductility.

▣ Non-directional bonding▷ outer electrons are shared by many atoms.

35

Primary bonding

atomic bonding

Metallic bonding

positive ion

valence electron charge cloud

36

Secondary bonding

atomic bonding

Fluctuating dipoles bond

▣ Arises from interaction between dipoles.▣ Very weak electric dipole bonds due to

asymmetric distribution of electron densities.

asymmetric electron clouds

+ - + -secondary

bonding

HH HH

H2H2

secondary bonding

example: liquid H2

▣ general case:

37

Secondary bonding

atomic bonding

Permanent dipoles bond

▣ Also, arises from interaction between dipoles.▣ Weak electric dipole bonds due to molecule

induced.

Example 1: liquid HCl acid

Example 2: polymer

+ - + -

secondary bonding

▣ general case:

secondary bonding

H Cl H Clsecondary bonding

38

Summary bonding

atomic bonding

Primary bonding Secondary bonding

TypeIonic

Covalent

Metallic

Secondary

Bond EnergyLarge

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

CommentsNondirectional (ceramics)

Directional(semiconductors, ceramicspolymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)inter-molecular

39

Summary bonding

atomic bonding

Primary bonding Secondary bonding

40

Properties from bonding

atomic bonding

Melting temperature

Tm is larger if bonding energy, Eo is larger.

ro r

Energy

larger Tm

smaller Tm

41

Properties from bonding

atomic bonding

Coefficient of thermal expansion

= α (T2-T1)LLo

coeff. thermal expansion

L

length, Lo

unheated, T1

heated, T2

a is smaller if Eo is larger.ro

r

smaller α

larger α

Energy

unstretched length

Eo

Eo

42

atomic bonding

Summary

Properties from bonding

Ceramics

(Ionic & covalent bonding):

Large bond energylarge Tm

large Esmall α

Metals(Metallic bonding):

Variable bond energymoderate Tm

moderate Emoderate α

Polymers(Covalent & Secondary):

Directional PropertiesSecondary bonding dominates

small Tm

small E large α

secondary bonding

43

End of Chapter

2