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Conductors

Al-Azhar University of GazaFaculty of Science

Chemistry Department

Preparation by:

Mohammed H. Rida

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Solid materials are classified into:

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Definition of conductors

a conductor is an object or type of material that

permits the flow of electric charges in one or more directions ,commonly

metal.

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Metallic crystals• Except for the actinides, most metals crystallize in• A- simple cubic packing • B- hexagonal close-packed (hcp)• C- and cubic close packed (ccp)

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Properties of metals

Metals have high conductivity, or low resistance, to the passage of electricity and heat.

nonmetals have low conductivity or high resistance.

most can be shaped by hammering or bending. This is possible because the bonding is non directional.

each atom is rather weakly bonded to all neighboring atoms, rather than to individual atoms, as is the case in discrete molecules.

When force is applied, the atoms can slide over each other and realign into new structures with nearly the same overall energy.

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Metallic Solids• Metals are excellent conductors of both heat and electricity and

this is not a coincidence.

* A metal is a good conductor of electricity because there are a large number of free electrons (the valence electrons).

* A small electric field across the metal causes the negative electrons to migrate to the positive side and this produces an electrical current.

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MOLECULAR ORBITALS AND BAND theory

When molecular orbitals are formed from two atoms,

each type of atomic orbital gives rise to two molecular

orbitals.

When n atoms are used, the same approach results in n

molecular orbitals.

 Because the number of atoms is large, the number of

orbitals and energy levels with closely spaced energies is

also large.

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The result is a band of orbitals of similar energy, rather

than the discrete energy levels of small molecule.

These bands then contain the electrons from the atoms. The highest energy band containing electrons is called the

valence band;

the next higher empty band is called the conduction band

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In conductors, the valence band is either not fully occupied with electrons, or the filled valence band overlaps with the empty conduction band.

In general, both states occurs at the same time, there is free movement of electron ,the electrons can therefore move inside the partially filled valence band or inside the two overlapping bands .

In conductors there is no band gap between the valence band and conduction band.

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The conduction in metals arises from: (a) the delocalization of the electron energy levels over the entire solid.

(b) the availability of empty orbitals in a given band permitting movement of the electrons. 

* Almost no energy is required to promote the electrons to the open level.

* In fact the energy separation of the constituent s and p orbitals are

close enough that for the solid the s and p bands overlap. 

The highest occupied level in the band is known as the Fermi Level.

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In fact the energy separation of the constituent s and p orbitals are close enough that for the solid the s and p bands overlap. 

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Common conductors:

• Most familiar conductors are metals

Cu is the most common material used for electrical wiring

Ag is the best conductor but its expensive ,

Au is used for high quality surface to surface contact because it doesn’t Corrode .

There is Some non-metallic conductors like graphite ,solution of salt

And some polymer.

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The conductance of metals decreases with increasing temperature,

because the increasing vibrational motion of the atoms interferes with the motion of the electrons and increases the resistance to electron flow.

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Superconductors.

exhibits no resistance to electrical conductivity, & will oppose an external

magnetic field, a phenomenon referred to as the Meissner effect

Photograph of the Meissner effect for a rare-earth magnet above a sample of YBCO (YBa2Cu3O7)immersed in liquid nitrogen. The onset of strong diamagnetism (“superdiamagnetism,” as observed by the repulsion of an external magnetic field) is the most reliable method to determine superconductive behavior.

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SUPER CONDUCTlVlTY

The conductivity of some metals changes abruptly near liquid helium temperatures (frequently below 10 K), and they become superconductors,

In this state, the metals offer no resistance (zero resistance) to the flow of electrons, and currents started in a loop will continue to flow indefinitely (several decades at least) without significant change.

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*For chemists, one of the most uses of this effect is in

superconducting magnets used in nuclear magnetic resonance

instrument in which they allow generation of much larger magnetic

fields than can be obtained with ordinary electromagnets.

*A major goal of superconductor research is a material that is

superconducting at higher temperatures, to remove the need for

liquid helium and liquid nitrogen for cooling.

*a year later, first material with a critical point above boiling point

of nitrogen (77 K) was discovered, known as:

- YBa2Cu3O7, with a critical point of 92K

- Hg0.8Tl0.2Ba2Ca2Cu3O8.33, Tc = 138 K

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Resistivity ρ (Ω·m) Material

0 Superconductors

10−8 Metals

variable Semiconductors

variable Electrolytes

1016 Insulators

relative to the conductivity of pure water at 25 °C. An EC meter is normally used to measure conductivity in a solution. A rough summary is as follows: