Why is a benzyl radical particularly stable?
The radical can be stabilized by resonance delocalization over the entire benzene ring, which relieves the energy burden of having a radical on an isolated carbon.
Which type of material can be permanently deformed by a combination of heat and pressure?
These types of materials are called thermoset, as opposed to thermoplastics.
Why is fluorine the most electronegative element?
Fluorine is one electron short of reaching a filled outer shell (the "octet" rule), which would bring stability to the atom. Its relatively small size in the halogen series, combined with the above, makes it the most electronegative element.