Transcript 4-ch50182-atomc

Materials Science
Atomic structure & bonding
Understanding materials
Properties
“Materials Science”
Structure
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“Materials Engineering”
Processing
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)
Structure of the Atom
Valence electrons
(participate in bonding)
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d
p s
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Nucleus contains Protons
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(1 Hydrogen – 94 Plutonium)
and Neutrons.
Atomic mass ~ Protons + Neutrons
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
Electron energy states
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Lowest energy state is
filled first
Electrons are
characterized by
quantum numbers:
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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
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Stable electron configurations:
 Have
complete s and p sub-shells (octet)
 Tend to be very unreactive.
Survey of elements
Electron configuration
1s1
1s2
(stable)
1s22s1
1s22s2
1s22s22p1
Adapted from Table
2
2
2
2.2, Callister 6e.
1s 2s 2p
...
1s22s22p6
(stable)
1s22s22p63s1
1s22s22p63s2
1s22s22p63s23p1
...
1s22s22p63s23p6
(stable)
...
1s22s22p63s23p63d10 4s246
(stable)
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Most elements are not stable… Why?
Valence electrons determine physical and chemical properties (bonding).
The periodic table – History
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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
Adapted
from Fig. 2.6,
Callister 6e.
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
The periodic table – properties
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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
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Primary bonding
(ceramics – some covalent)
 Covalent (polymer C=C bonds)
 Metallic (metals)
 Ionic
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Secondary bonding (weaker)
 Van
der Waals (polymers)
 Hydrogen (similar to VdW)
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We will consider the mechanisms and
characteristics.
Ionic bonding – origin
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Compounds of metallic and non-metallic elements (e.g. NaCl, Al2O3,
MgO, many ceramics)
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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
Na (cation)
stable
+
Coulombic
Attraction
-
Cl (anion)
stable
Ionic bonding – examples
NaCl
MgO
CaF2
CsCl
H
2.1
Li
1.0
Be
1.5
Na
0.9
Mg
1.2
K
0.8
Rb
0.8
Ca
1.0
Sr
1.0
Cs
0.7
Ba
0.9
Fr
0.7
Ra
0.9
Ti
1.5
Cr
1.6
Give up electrons
Fe
1.8
Ni
1.8
He
-
Zn
1.8
As
2.0
O
F
3.5 4.0
Cl
3.0
Ne
-
Br
2.8
I
2.5
Kr
Xe
Rn
-
At
2.2
Acquire electrons
Ar
-
Ionic bonding – characteristics
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Bonding is:
 Non-directional
 Relatively
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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
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Stable electron configurations by sharing electrons
between atoms
Shared electrons belong to both atoms
Typically non-metal compounds (polymers C-C & C-H
bonds)
E.g. Methane (CH4)
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C – has 4 valence e,
needs 4 more
H – has 1 valence e,
needs 1 more.
Electronegatvities
are comparable
Covalent bonding – examples
H2
H
2.1
Li
1.0
Na
0.9
K
0.8
Be
1.5
Mg
1.2
Ca
1.0
Rb
0.8
Cs
0.7
Sr
1.0
Fr
0.7
Ra
0.9
Ba
0.9
column IVA
H2O
C(diamond)
SiC
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
Ga
1.6
C
2.5
Si
1.8
Ge
1.8
F2
He
O
2.0
As
2.0
Sn
1.8
Pb
1.8
GaAs
F
4.0
Ne
-
Cl
3.0
Ar
Kr
-
Br
2.8
I
2.5
At
2.2
Xe
-
Rn
-
Cl2
Covalent bonding – examples
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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
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Bonding is:
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Directional (exists in specific orientation)
Very strong
Material properties:
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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
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Metals and alloys
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Low number of valence electrons (1, 2, 3 from each atom)
Valence electrons become delocalized
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Electrons are not bound to any particular atom
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Electrons are free to drift
throughout the metal
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‘Sea of electrons’ around
positive ion cores
High electrical conductivity
Metallic bonding- characteristics
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Bonding is:
 Non-directional
 Intermediate
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strength
Material properties:
(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).
 Metals
Van der Waals – origins
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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
• Fluctuating dipoles
• Permanent dipoles-molecule induced
Adapted from Fig. 2.14,
-general case:
-ex: liquid HCl
-ex: polymer
Callister 6e.
Adapted from Fig. 2.14,
Callister 6e.
Van der Waals – characteristics
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Bonding is:
 Weak
 Directional
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Material properties:
 Polymers
(between covalent chains)
 Low stiffness (E<5 GPa)
 Low melting point (<400°C)
 Very ductile
Hydrogen bonding
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Weak, secondary
bond (H-H)
Occurs from
interaction and
delocalisation of
hydrogen electrons
Not significant for this
course
Atomic bonding - Summary
Type
Bond Energy
Comments
Ionic
Large!
Nondirectional (ceramics)
Variable
Directional
Covalent large-Diamond (semiconductors, ceramics
small-Bismuth
polymer chains)
Metallic
Secondary
Variable
large-Tungsten
small-Mercury
Nondirectional (metals)
smallest
Directional
inter-chain (polymer)
inter-molecular
Bonding energies
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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 materials
Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers
(Covalent & Secondary):
Large bond energy
large Tm
large E
Variable bond energy
moderate Tm
moderate E
Secondary bonding dominates
small Tm
small E
Typical bond properties
Bond
Example
eV /
atom
Ionic
MgO
5
2,800
250
7
3,550+
1,000
Al
3
660
70
W
8
3,410
400
PVC
0.5
210
3
Covalent C (diamond)
Metallic
VdW
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M’Pt (°C) E (GPa)
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…
Exercise: Bonding in materials…
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What bonding would you expect in…
 CaF2
?
 Bronze (Cu-Sn alloy) ?
 Polyethylene ?
 SiC ?
 Solid Xe ?
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Which has…
 The
highest melting point ?
 Most ductile ?
 The greatest electrical conductivity ?