materials science atomic structure & bonding. understanding materials electronic level...
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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 ?