Nitrogen and Oxygen react to form Nitrogen Oxides. These gasses irritate the lungs and cause acid rain.
Not sure, but I would guess it's due to environmental factors, like heat, and imperfections in the metal. Just an educated guess, though.
Benzene is more stable because it contains a ring and therefore an aromatic compound. On the other hand, 1,3,5-hexatriene, as a straight-chain alkene, is aliphatic. Aromatic compounds in general are more stable than their aliphatic counterparts because pi electrons are part of a conjugated system, meaning they are "shared" amongst all double-bonded atoms.
NH3 has 1 Nitrogen atom and 3 hydrogen atoms.
