lecture 2.1 - atomic bonding in solids
TRANSCRIPT
ATOMIC BONDING IN SOLIDSSCIMATP LECTURE 2-1
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
ATOMIC MODELS
ATOMIC MODELS – BOHR MODEL Electrons move in discrete orbits around the nucleus Energy is quantized
ATOMIC MODELS – BOHR MODEL Electrons move in discrete orbits around the nucleus Energy is quantized
ATOMIC MODELS – BOHR MODEL Electrons fill lower energy states first before filling the
higher energy states
ATOMIC MODELS – WAVE MECHANICAL MODEL Electron behaves like a wave and a particle
PARTICLE VS. WAVE?
Exact Position
No ExactPosition
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
COMPARISON OF BOHR AND WAVE-MECHANICAL MODEL
Bohr Model
Wave-Mechanical
Model
ATOMIC BONDING
EA + EB > EAB
WHY DO ATOMS BOND?
A B“Nature is inherently lazy”= minimization of energy
Stability
ENERGY VS SEPARATION DISTANCE
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
ILLUSTRATING CHEMICALS BONDS
TYPES OF CHEMICAL BONDS
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
COVALENT BONDS
Atoms share electrons instead of gaining or losing them.
Mostly nonmetals, as well as semiconductors, exhibit this bond
NONPOLAR COVALENT BONDS
when electrons are shared equally Same atoms are involved
O2, H2, or Cl2
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
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
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
METALLIC BONDS
+
+
+
+
+
+
+
+
+
PHYSICAL PROPERTIES ARISING FROM THESE BONDS
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.)
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
+
+
+
+
+
+
+
+
+
METALLIC BONDS Ductility and malleability http://www.youtube.com/watch?v=lcXMIzprd
14
IONIC BONDS Mostly found in ceramics Insulators (heat and electricity)
No delocalized electrons High melting temperatures (strong ionic
bonds) Hard but brittle
IONIC BONDS
SUMMARY Atomic Models
Bohr Model Wave-Mechanical model
Atomic Bonding “Nature is inherently lazy.”
SUMMARY Types of chemical bonds
Covalent – polymers and carbon Metallic – metals Ionic – ceramics
Physical Properties arising from these bonds
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