Materials Science

Atomic structure & bonding

Understanding materials

Electronic level (subatomic) Atomic (molecular level, chemical composition) Crystal (arrangement of atoms or ions wrt one another) Microstructure (can study with microscopes) Macrostructure (can see with naked eye)

Properties

Structure Processing

“Materials Science” “Materials Engineering”

Structure of the Atom

Niels Bohr model of the atom

Electrons –ve orbit nucleus in discrete orbital shells

Electrons have quantized positions, and specific energies

Stable configurations have full outer shell

Valence electrons(participate in bonding)

d p s

Nucleus contains Protons (1 Hydrogen – 94 Plutonium) and Neutrons.Atomic mass ~ Protons + Neutrons

Electron energy states Lowest energy state is

filled first Electrons are

characterized by quantum numbers: Principal (1,2,3…) relates to

distance from nucleus Second (s,p,d,f) relates to

shape of subshell

Max. number of electrons per subshell: s=2, p=6, d=10, f=14

Stable elements

Stable electron configurations: Have complete s and p sub-shells (octet) Tend to be very unreactive.

Z Element Configuration

2 He 1s2

10 Ne 1s22s22p6

18 Ar 1s22s22p63s23p6

36 Kr 1s22s22p63s23p63d104s24p6

Survey of elements

Most elements are not stable… Why? Valence electrons determine physical and chemical properties (bonding).

Element Hydrogen Helium Lithium Beryllium Boron Carbon ... Neon Sodium Magnesium Aluminum ... Argon ... Krypton

Atomic # 1 2 3 4 5 6

10 11 12 13

18 ... 36

Electron configuration 1s1 1s2 (stable) 1s22s1 1s22s2 1s22s22p1 1s22s22p2 ... 1s22s22p6 (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 ... 1s22s22p63s23p6 (stable) ... 1s22s22p63s23p63d104s246 (stable)

Adapted from Table 2.2, Callister 6e.

The periodic table – History

Credited to Dmitri Mendeleev (1834 – 1907).

Russian chemist and inventor Recognized periodicity

amongst the elements We now know that the atomic

structure of elements determines the properties observed

The periodic table Columns have similar valence structure

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

Adapted from Fig. 2.6, Callister 6e.

He

Ne

Ar

Kr

Xe

Rn

inert

gase

s acc

ept

1e

acc

ept

2e

giv

e u

p 1e

giv

e u

p 2e

giv

e u

p 3e

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

The periodic table – properties

Elements are grouped into columns, that have similar numbers of valence electrons… and hence properties.

Electron donors (left) are metals and electron acceptors (right) non-metals.

Through bonding, atoms can achieve a full outer electron shell, with lower energy and more stability.

Bonding in solids

Primary bonding Ionic (ceramics – some covalent) Covalent (polymer C=C bonds) Metallic (metals)

Secondary bonding (weaker) Van der Waals (polymers) Hydrogen (similar to VdW)

We will consider the mechanisms and characteristics.

Ionic bonding – origin Compounds of metallic and non-metallic elements (e.g. NaCl, Al2O3,

MgO, many ceramics) Requires electron transfer (+ve and –ve ions) and large difference

in electronegativity. Atomic number of Na = 11 (1 valence electron) Atomic number of Cl = 17 (7 valence electrons)

Na (metal) Unstable

Cl (nonmetal) unstable

electron

+ - Coulombic Attraction

Na (cation) stable

Cl (anion) stable

Ionic bonding – examples

Give up electrons Acquire electrons

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

CsCl

MgO

CaF2

NaCl

O 3.5

Ionic bonding – characteristics

Bonding is: Non-directional Relatively strong

Material properties: Often ceramic (e.g. Alumina, Al2O3)

High melting points (e.g. 2,200°C) High elastic modulus (e.g. E=400 GPa) Brittle (difficult for atoms to slide/ rearrange) Electrical and thermal insulators (no free electrons)

Covalent bonding – origin Stable electron configurations by sharing electrons

between atoms Shared electrons belong to both atoms Typically non-metal compounds (polymers C-C & C-H

bonds)

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH4

E.g. Methane (CH4) C – has 4 valence

e, needs 4 more H – has 1 valence

e, needs 1 more. Electronegatvities

are comparable

Covalent bonding – examples

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

colu

mn IVA

Sn 1.8Pb 1.8

Covalent bonding – examples

Compounds containing elements on right side of table (GaAs, Si3N4)

Non-metallic molecules (H2, Cl2) Some elemental solids (Si, C)

C – Diamond Prevalent in polymers Occurs in ceramics

Covalent bonding – characteristics Bonding is:

Directional (exists in specific orientation) Very strong

Material properties: Often polymers, glasses and ceramics (e.g. Diamond) Less dense than ionic/metallic bonded materials (directional

bonding makes is harder to ‘pack’ atoms) High elastic modulus (e.g. E~1000 GPa) High melting point (e.g. 3,550°C) Brittle (strong, directional atomic bonds) Electrical and thermal insulators …But polymers have low melting point and stiffness?...

Metallic bonding – origins Metals and alloys

Low number of valence electrons (1, 2, 3 from each atom) Valence electrons become delocalized

Electrons are not bound to any particular atom

Electrons are free to drift throughout the metal ‘Sea of electrons’ around

positive ion cores High electrical conductivity

Metallic bonding- characteristics Bonding is:

Non-directional Intermediate strength

Material properties: Metals (e.g. Aluminium – Al, Tungsten – W) Intermediate melting point (Al~660°C, W~3,410°C) Intermediate elastic modulus (Al~70, W~400 GPa) Close packing of atoms (high density) High electrical and thermal conductivity (free

electrons) Ductile (planes of atoms can slide over each other).

Van der Waals – origins

Secondary bond is weak in comparison to Primary bonds

It arises from atomic or molecular dipoles, e.g. asymmetric molecules

Secondary bonding in inert gases or between covalently bonded molecules

Van der Waals – examples

• Permanent dipoles-molecule induced

• Fluctuating dipoles

+ - secondary bonding + -

H Cl H Clsecondary bonding

secondary bonding

HH HH

H2 H2

secondary bonding

ex: liquid H2asymmetric electron clouds

+ - + -secondary bonding

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.14, Callister 6e.

Adapted from Fig. 2.14, Callister 6e.

Van der Waals – characteristics

Bonding is: Weak Directional

Material properties: Polymers (between covalent chains) Low stiffness (E<5 GPa) Low melting point (<400°C) Very ductile

Hydrogen bonding

Weak, secondary bond (H-H)

Occurs from interaction and delocalisation of hydrogen electrons

Not significant for this course

Atomic bonding - Summary

TypeIonic

Covalent

Metallic

Secondary

Bond EnergyLarge!

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

CommentsNondirectional (ceramics)

Directional(semiconductors, ceramics

polymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)

inter-molecular

Bonding energies

The forces result from the energy potential between the atoms

Force is the space differential of energy

This energy equilibrium reveals fundamental properties of materials

Bonding in materialsCeramics(Ionic & covalent bonding):

Metals(Metallic bonding):

Polymers(Covalent & Secondary):

secondary bonding

Large bond energylarge Tm

large E

Variable bond energymoderate Tm

moderate E

Secondary bonding dominatessmall Tm

small E

Typical bond properties

Bond strength determines fundamental properties (MPt & stiffness) But: strength of materials is dependent on defects within materials

(e.g. chalk). We will consider defects later…

Bond ExampleeV / atom

M’Pt (°C) E (GPa)

Ionic MgO 5 2,800 250

Covalent C (diamond) 7 3,550+ 1,000

MetallicAl 3 660 70

W 8 3,410 400

VdW PVC 0.5 210 3

Exercise: Bonding in materials…

What bonding would you expect in… CaF2 ?

Bronze (Cu-Sn alloy) ? Polyethylene ? SiC ? Solid Xe ?

Which has… The highest melting point ? Most ductile ? The greatest electrical conductivity ?