Atomic Bonding In Materials
Instructor: Ashraf Bastawros
Office 2347 Howe Hall
Tel:294-3039
Material Sciences and Engineering
MatE271
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Goals for this unit
- Understand how atoms bond together to form
molecules and solids
- Learn mechanisms of the major bond
- Associate materials types with bond types
- metals, ceramics, polymers
- Hint at property dependencies on bond types
property dependencies on bond types
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Review of Atomic Bonding
• Fundamental concepts
o Bohr atomic model
o electrons revolve around nucleus in discrete orbitals
o Wave-mechanical atomic model
o Electrons exhibit both wave-like and particle-like properties
o Electrons position based on probability distribution
Bohr atomic model
(C)
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Review of Atomic Bonding
• Electron configuration
o Ground State (all e in lowest energy states)
o Valance electrons (e in outermost shell, bonding)
o Electronegativity (ability to accept electrons)
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Electronegativity
Electronegativity - Ability of an element to readily accept electrons
generally increases as shown. The more electrons in the outer shell,
and the less shielded the outer shell to the positive nucleus, the more
electronegative the element.
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Week 1
Bonding of Atoms into Solid Structures
-
Primary Bond Types
§
Ionic
§ Covalent
(semiconductors)
§ Metallic
(metals)
§ Mixed ionic + covalent (ceramics)
- Secondary Bond
§
Polar or van der Waals
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(polymers)
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3
Bonding Types for various Materials
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Ionic Bonding by Electron Transfer
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Ionic Bonding by Electron Transfer
o
o
o
o
o
Transfer of valence electrons to achieve “stable electron configuration”
large electronegativity between elements
Attractive force is coulombic
Bond energy; strong. high melting point
Ionic bond is non-directional, magnitude of bond equal is all directions
o In ionic materials, therefore, all positive ions must have nearest
neighbor negatively charged ions in 3-D
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Ex: NaCl – Ironically Bonded Solid
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Coulombic Forces
Coulombic interaction between ions
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Attractive Bonding Forces
Fc x109(N)
a (nm)
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Interatomic repulsion Forces
- Arise from interaction of overlapping electron
clouds as the atoms approach closely
-Very short range
FR= - λ exp (-a/ρ)
- λ and ρ are experimentally calibrated constants for each ion
pair.
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Atomic Structure and Bonding
- How atoms fit together to form a solid
-Attractive force vs.
FC (attractive)
Repulsive force
- Net force Net force
FR (repulsive)
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Balance of Bonding Forces
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Bonding energy (ev)
Bonding Energies
Interatomic separation (Ao)
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Ionic Radii (r and R or rc and ra )
- Equilibrium bond length ao
ao = r + R
-r: radius of the average
electron density for outer orbit
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Ionic Bonding
- Each type of ion will gather around itself the
maximum number of the other type of ions that
will “fit” together to form a repetitive structure
- How many larger ions will “fit” around a smaller
central ion?
- Larger ions must all “touch” central ion and
not “overlap” each other
- Called the COORDINATION NUMBER
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Coordination number (CN) based on (r/R)
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(r/R)min for CN=3
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CN vs (r/R) for Ionic Bonds
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Week 1
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Covalent Bonding
nSharing
of electrons to achieve
“stable electronic configuration”
nSmall
electronegativity
nBond
energy; weak to strong
nDirectional
bond; between
specific atoms in specific
directions
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Covalent Bonding in Polymers
Ethylene (C2H4)
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Methan (CH4)
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Bond breaking and Reforming
§The elimination of a C=C double covalent bond in
favor of two C-C single bonds usually results in
the release of energy
§Because the C=C covalent bond is so stable, it
usually requires a catalyst to initiate the breaking
and reforming of bonds necessary to produce
polymerization
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Polymer Molecules
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Covalent Bonding in Diamond
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Bond angles in tetrahedral coordination
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Silica tetrahedron (SiO4)4
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Metallic Bonding
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Secondary (van der Waals) bonding
n
n
n
n
Physical bonds, not chemical
Bond energy is very weak compared to others
Exists between almost all atoms and molecules
Arise from atomic or molecular dipoles
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Secondary bonding of water molecules
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Bond Energy or “Strength”
o The higher the bond energy
o higher melting
o higher hardness
o State as function of
bonding energy
o Solid (high)
o Liquid (intermediate)
o Gaseous (low)
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Bond strength as exhibited by
melting temperature
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Bond in various material types
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Reading Assignment
Shackelford 2001(5th Ed)
– Read Chapter 2, pp 20-53
Check class web site:
www.public.iastate.edu\~bastaw\courses\Mate271.html
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