Ah, the fresh scent of lemon! I love that it gives a clean, fresh scent when it's used.
The natural occurring chemical that gives off this wonderful citrus smell is limonene.
History of Lemon
According to some research it's been found that the lemon is a cultivated hybrid deriving from wild species such as the citron and mandarin. The citron seems to have been the first citrus fruit known in the Mediterranean world. {source}