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

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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

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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

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