Re: What are the properties of a good insulator?

Most substances fall into one of two classes: conductor or insulator.  
Conductors permit the passage of charge (or heat) through them, while 
insulators do not.  Associated with the atoms of materials, there is an 
outer band of electrons called the valence band.  When these outer valence 
electrons can easily become detached from the nucleus and can move 
freely, the material is said to be a conductor.  Within an insulator, on 
the other hand, there are no, or at most very few, free electrons.  The 
actual conductance happens when the electrons change energy levels, or 
move from one valence band to another.  If there are no nearby empty 
levels, then the electron will not be able to gain any energy at all and 
the material behaves like an insulator.  Therefore, a property of a good 
insulator is having an atomic structure such that the valence bands are 
completely filled.  Since each band can support only a specific number of 
electrons, if the bands are filled it is difficult for the electrons to 
move about.  If the electrons cannot freely move about, then no conduction 
can take place.  Therefore, you have an insulator.  

Usually, materials that are good electrical insulators are also good heat 
insulators.  Likewise, good electrical conductors are usually good heat 
conductors.  This is because heat is a form of energy.  Insulators resist 
(or block) the transfer of energy; conductors allow the transfer of energy.
[MadSci Admin note: the energy known as "heat" can be conducted by the
very same electrons that move around to conduct the electricity,
because the movement of the electrons and nucleii *IS* heat energy!]


References:  "College Physics", by Sears, Zemansky, & Young
"Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles", by 
Eisberg & Resnick