04-12-2012, 01:56 PM
MATERIALS Classification, Properties and Applications
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Classes of MaterialsMetals
Strong, deformable and impact resistant, hard and do not break readily under high stress
Electrical conductors
Always opaque
Ceramics
Strong but not deformable, brittle, high hardness, stable at high temperatures
Insulators
Can be transparent
Plastics
Made of hydrocarbons (carbon and hydrogen atoms), softer, cannot be shaped, cannot be used at high temperatures
Insulators
Can be transparent
Structure of Atom
Atom – Small, positively charged nucleus, surrounded by electrons
Nucleus is composed of protons and neutrons
Neutral Atom – same no of protons and electrons
Ions – due to imbalance of protons and electrons
Atomic number – Number of protons in nucleus
Proton and neutron are 1840 times heavier than electrons
Atomic Mass – No of protons and neutrons in the nucleus of an atom
Isotopes – Different numbers of neutrons
Binding energies
Electrons –bound to the positively charged nucleus by Coulomb attraction
Electrons - waves or particles, placed in concentric shells by the principal quantum number n
Binding energy – work done to remove electrons from the atom
K Shell – Principal quantum number n = 1, contains only one orbital of symmetry – 1s; l = 0
L Shell – n=2, one 1s orbital and three p orbital (2p); l = 1 and m = -1, 0 and +1
M Shell – n = 3, one 3s orbital, three 3p orbital and five 3d orbital
N Shell – n = 4, one 4s, three 4p, five 4d, and seven 4f.
Valence and Core Electrons
Valence Electrons
Electrons with the lowest binding energies have largest orbital and interact with other atoms to form chemical bonds
Core Electrons
Atoms in the inner shells are tightly bound
Binding energies – Absorption or emission of X –rays or by photoelectron emission
Elements with similar occupation of their outermost shell have similar chemical properties
For eg all elements have one s electron in their higher shell, namely hydrogen, lithium, sodium, potassium, rubidium and cesium show chemical behavior.
Energy Bands
Bands closest to the atomic nucleus, called Core levels
Furthest band from the nucleus that has electrons in it, called the Valence band, all keep their electrons tightly in place.
Next band out from that is called the “Conduction band” and there, the electrons are free to roam around freely.
In Metals - Valence band and a conduction band overlap, and electricity flows freely and easily through them.
Insulators - Wide gap between the valence band and the conduction band, making it almost impossible for an electron to get excited enough to jump from one to the other, so they block the flow of electricity.