solids in chemistry; crystals, semiconductors, etc

[sarahsbs]

hello,

I have a few questions that hopefully someone will be able to shed some light on for me...... I read in my textbook that
if the bond length btw two atoms is close
to the sum of their atomic radii, the bond is not polar. I don;t quite
know why. I thought polarity only had to do with the electronegativity of the bonding atoms.

Similarly, I am having trouble determining what type of bond structure
is used for various compounds. I know how to tell if it is ionic or
metallic, but I don't know how to tell the difference between a
covalent bond and a molecular bond by its formula name; (ex. CO vs
SiC... CO being molecular, SiC being covalent). Any suggestions?

Lastly, does anyone understand why a plastic crystal has a lower
enthalpy than a liquid crystal? Maybe I am misunderstanding enthalpy,
but I thought it represented E required to break bonds, and it would
seem to me that is would be harder to break the bonds in a plastic
crystal (where there is still some structure) than in a liquid
crystal.

also, in a p-type semiconductor, does the conduction band have anything to do with the conduction of electricity? I understand that electricity is conducted bc the holes in the structure move (bc e- are jumping out of position to fill the +ly charged hole, thereby leaving another hole in its wake), so there is a moving charge. Is that is all that is required for electricity to be conducted?
Am I current in thinking that these e- are not in fact anywhere near the conduction band (as in an n-type semi conductor)? My book says that there is a new energy level greated in the band gap, but the diagram shows this level as being much closer to the valence band than to the conduction band.

Any thoughts?
Any help would be much appreciated

thanks!
sarah

[RobJim]

In an ionic bond, the outer shell of the cation is emptied, making the cation significantly smaller than the neutral atom. The electrons lost by the cation fill up the outer shell of the anion. When an outer shell is filled to make an anion, it stays the same size as the neutral atom. So, in an ionic bond, the distance between nuclei should be less than between two neutral atoms. A polar bond will be something like an ionic bond in this respect (internuclear distance).

This is only a guess though.

I believe a molecular bond and a covelant bond are the same thing.

I don't know about the rest of your questions offhand. Inorganic and physical chemistry aren't my thing.