syn-propanethial-S-oxide

The chemical that causes the tears is syn-propanethial-S-oxide, a corneal irritant.

Because the gas evolved from the cutted onions, named syn-propanethial-S-oxide, is irritating for the eyes.

Onions

Peeling, cutting or crushing an onion's tissue releases enzymes called allinases, which convert these molecules to sulfenic acids. The sulfenic acids, in turn, spontaneously rearrange to form syn-propanethial-S-oxide, the volatile irritating chemical that triggers the tears. (Taken from Scientific American)