several questions about solids; crystals, bonding etc

I have a few questions... I read in my textbook that
"non-stoichiometric compounds are readily understood by lattice
vacancies." Does anybody know why that is the case?

My book further suggests 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.

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.

Any help would be much appreciated

thanks!
sarah

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