chapter5

chapter5 - CHAPTER 5 FREE ELECTRON THEORY Free Electron...

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    CHAPTER 5 FREE ELECTRON  THEORY
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    Free Electron Theory Many solids conduct electricity. There are electrons that are not bound to atoms but are able to move through the whole crystal. Conducting solids fall into two main classes; metals and semiconductors.                                                         and  increases by the addition of small  amounts of impurity. The resistivity normally decreases monotonically with decreasing  temperature.                                                            and can be reduced by the addition of  small amounts of impurity.  Semiconductors tend to become insulators at low T. 6 8 ( ) ;10 10 metals RT m ρ - - - Ω- ( ) ( ) pure semiconductor metal RT RT - ?
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    Why mobile electrons appear in some  solids and others? When the interactions between electrons  are considered this becomes a very difficult question to answer. The common physical properties of metals;  Great physical strength  High density  Good electrical and thermal conductivity, etc.        This chapter will calculate these common properties of metals using the assumption  that  conduction  electrons  exist  and  consist  of  all  valence electrons from all the metals; thus metallic Na, Mg and Al will be assumed to have 1, 2 and 3 mobile electrons per atom respectively.         A simple theory of ‘  free electron model’  which works remarkably well will be described to explain these properties of metals.
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    Why mobile electrons appear in some  solids and not others? According  to  free  electron  model  (FEM),  the  valance electrons are responsible for the conduction of electricity, and  for  this  reason  these  electrons  are  termed conduction electrons.  Na 11    1s 2  2s 2  2p 6  3s 1 This valance electron, which occupies the third atomic shell, is the electron which is responsible chemical properties of Na. Valance electron (loosely bound) Core electrons
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    When we bring Na atoms together to form a Na metal, Na  has  a  BCC  structure  and  the  distance  between  nearest neighbours is 3.7 A˚ The radius of the third shell in Na is 1.9 A˚ Solid  state  of  Na  atoms  overlap  slightly.  From  this  observation it follows that a valance electron is no longer  attached  to  a  particular  ion,  but  belongs  to  both  neighbouring ions  at the same time.
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This document was uploaded on 02/10/2010.

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chapter5 - CHAPTER 5 FREE ELECTRON THEORY Free Electron...

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