Illustrated Glossary of Organic Chemistry

Zwitterion: A molecule having a net formal charge of zero, but negative and positive formal charges on individual atoms within its structure. The charged atoms must be joined by one or more covalent bonds. The charged atoms of NaCl and NH4Cl are not covalently bonded, so these substances are not zwitterions.

Neutral form
Not a zwitterion

Cationic form
Not a zwitterion

Anionic form
Not a zwitterion

Zwitterionic form
Proton transfer allows glycine to exist in neutral, ionic, and zwitterionic forms.

Neutral resonance contributor
Not a zwitterion

Zwitterionic resonance contributor
Acetone has a neutral (significant) and zwitterionic (not significant) resonance contributors.