Valence electrons in insulators

[Superposed_Cat]

I was wondering why the valence electrons aren't free to move in insulators? Thanks for any help in advance.

 

[UltrafastPED]

Takes too much energy to reach the conduction band.

For a diagram etc. see http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html#c2

 

[DrDu]

See also this discussion further down on this page:

https://www.physicsforums.com/showthread.php?t=715010

 

[hokhani]

I was wondering why the valence electrons aren't free to move in insulators? Thanks for any help in advance.

In fact they move with their group velocities but since the band is full, movement of the electron at k would neutralize the movement of another one at -k.