Zwitterion

A molecule or ion bearing positive and negative formal charges whose sum is zero. Its charged groups remain distinct despite the species’ overall electrical neutrality.

Acid–base reaction: A chemical reaction in which protons or electron pairs move between acidic and basic species. Internal proton transfer can create oppositely charged groups within one molecule.
Formal charge: An electron-bookkeeping charge assigned to an atom in a Lewis structure. A zwitterion’s positive and negative charges are formal charges on different atoms.
Glycine: The simplest amino acid, with hydrogen as its side chain. In water near neutral pH, glycine is commonly represented as a zwitterion.
Dipolar ion: An ion with spatially separated positive and negative charges. The term overlaps with zwitterion, though usage varies across chemical contexts.
Amphoteric substance: A substance that can act as either an acid or a base, depending on its reaction partner. Amino acids form zwitterions because they contain both acidic and basic groups.
Lewis structure: A diagram showing bonding and lone-pair electrons in a molecule or ion. Lewis structures make a zwitterion’s charge locations explicit.
Amino acid: An organic molecule containing amino and carboxyl functional groups; protein amino acids also have a side chain. Its amino and carboxyl groups can carry opposite charges simultaneously.
Net charge: The algebraic sum of the electric charges in a system. A zwitterion has zero net charge without having zero formal charges.
Isoelectric point: The pH at which a molecule has zero average net electrical charge. At this pH, amino acids often occur predominantly as zwitterions.
Protonation: The addition of a proton to an atom, molecule, or ion. Protonation of one site can pair with deprotonation at another.

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