A zwitter ion is

The correct answer is Option (3) → An ion with a positive charge and a negative charge at different points in it

• An amino acid carries an acidic –COOH group and a basic –NH₂ group in the same molecule.

• In aqueous solution the carboxyl group loses a proton and the amino group takes it up:

      \( \displaystyle \ce{H2N-CH(R)-COOH <=> H3N^{+}-CH(R)-COO^{-}} \)

• The product is a dipolar ion, the zwitter ion: it has a positive charge on the nitrogen and a negative charge on the carboxylate oxygen, and the two charges cancel, so the ion as a whole is neutral.

Why the other options are incorrect

Option (1) A negatively charged ion without a metal atom in it — Incorrect. A zwitter ion carries both charges and has no net charge; an ion with only a negative charge is an anion.

Option (2) A positively charged ion without a metal atom in it — Incorrect. Again only one kind of charge; a zwitter ion also has a negative centre (–COO^–).

Option (4) A neutral molecule that has a free amino group and a free carboxyl group — Incorrect. This is the un-ionised form \( \ce{H2N-CH(R)-COOH} \); in the zwitter ion the proton has moved from –COOH to –NH₂, giving –NH₃⁺ and –COO^–.

NCERT (2026-27 edition):
10.2.2 Classification of Amino Acids – “… In aqueous solution, the carboxyl group can lose a proton and amino group can accept a proton, giving rise to a dipolar ion known as zwitter ion. This is neutral but contains both positive and negative charges.”