Download - Lecture 2.1 - Atomic Bonding in Solids
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ATOMIC BONDING IN SOLIDSSCIMATP LECTURE 2-1
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OUTLINE OF PRESENTATION Atomic Models
Bohr Model Wave-Mechanical
model
Atomic Bonding
Types of Chemical Bonds Ionic Bonding Covalent Bonding
Nonpolar Polar
Metallic Bonding
Physical Properties arising from these bonds Metallic Ionic Covalent
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ATOMIC MODELS
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ATOMIC MODELS – BOHR MODEL Electrons move in discrete orbits around the nucleus Energy is quantized
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ATOMIC MODELS – BOHR MODEL Electrons move in discrete orbits around the nucleus Energy is quantized
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ATOMIC MODELS – BOHR MODEL Electrons fill lower energy states first before filling the
higher energy states
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ATOMIC MODELS – WAVE MECHANICAL MODEL Electron behaves like a wave and a particle
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PARTICLE VS. WAVE?
Exact Position
No ExactPosition
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ATOMIC MODELS – WAVE MECHANICAL MODEL Electron behaves like a wave and a particle Instead of orbits, we have electron clouds Electron’s position is described in terms of
probability
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COMPARISON OF BOHR AND WAVE-MECHANICAL MODEL
Bohr Model
Wave-Mechanical
Model
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ATOMIC BONDING
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EA + EB > EAB
WHY DO ATOMS BOND?
A B“Nature is inherently lazy”= minimization of energy
Stability
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ENERGY VS SEPARATION DISTANCE
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term used to describe the linkages between atoms joined together to form molecules or crystalsform when electrons can be simultaneously near two or more nuclei.
CHEMICAL BOND
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ILLUSTRATING CHEMICALS BONDS
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TYPES OF CHEMICAL BONDS
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IONIC BONDS Found in compounds that are composed of
both metallic and nonmetallic elements. Atoms either lose or gain electrons
Octet rule The attractive bonding forces are coulombic
The (+) and (-) ions attract each other
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COVALENT BONDS
Atoms share electrons instead of gaining or losing them.
Mostly nonmetals, as well as semiconductors, exhibit this bond
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NONPOLAR COVALENT BONDS
when electrons are shared equally Same atoms are involved
O2, H2, or Cl2
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POLAR COVALENT BONDS
when electrons are shared but shared unequally
different atoms the more electronegative atom will have a
stronger attraction, and will acquire a slightly negative charge
H2O
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POLAR COVALENT BONDS
The existence of partial charges means that a polar covalent bond behaves as if it were partially ionic
There is no sharp distinction between ionic and covalent bonds
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METALLIC BONDS found in metals and their alloys Electron “sea model”
Unlike the two previous bonding types, valence electrons are not bound to any particular atom
Electrons are free to roam to other atoms Can be thought of as “ion cores” (i.e. atomic
nuclei and nonvalence electrons) immersed in a sea of electrons
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METALLIC BONDS
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PHYSICAL PROPERTIES ARISING FROM THESE BONDS
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COVALENT BONDS Found in some polymeric materials and other
elemental solids such as carbon, silicon, and germanium, as well as other ceramics.
Usually nonconductive (electricity and heat) No free electrons unlike metallic bonds
Can be very weak (e.g. bismuth, which melts at 270oC) or very strong (e.g. diamond, which melts at temperatures greater than 3550oC.)
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METALLIC BONDS Found in metals Bonding may be weak or strong
energies range from 68 kJ/mol (0.7 eV/atom) for mercury to 850 kJ/mol (8.8 eV/atom) for tungsten.
Good conductors of heat and electricity Due to free or “delocalized” electrons
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METALLIC BONDS Ductility and malleability http://www.youtube.com/watch?v=lcXMIzprd
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IONIC BONDS Mostly found in ceramics Insulators (heat and electricity)
No delocalized electrons High melting temperatures (strong ionic
bonds) Hard but brittle
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IONIC BONDS
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SUMMARY Atomic Models
Bohr Model Wave-Mechanical model
Atomic Bonding “Nature is inherently lazy.”
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SUMMARY Types of chemical bonds
Covalent – polymers and carbon Metallic – metals Ionic – ceramics
Physical Properties arising from these bonds
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REFERENCES
Callister, William Jr. (2005) Materials Science and Engineering: An Introduction (7th Ed.), John Wiley and Sons, Inc.
Castillon, Gwen B. (2013) Classification of Materials. A powerpoint presentation
Scullion, Frank. (2012). Explaining why metals are both malleable and ductile, retrieved June 11, 2013 from http://www.youtube.com/watch?v.=lcXMIzprd14