When two atoms join together what do they form

A molecule

Explanation:

When two or more atoms join together. If atoms combine that have two or more different elements, its called a compound. All compounds are molecules, but not all molecules are compounds.

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.