Explanation:
In sulfite ion that is [tex]SO_3^{2-}[/tex], bears negative charge of '2-' on overall ion. Negative charge on the one oxygen atom is de-localized to other oxygen atoms present in the molecule of sulfite ion. Due to this resonance sulfite ions get stabilized.
For resonance structures for the sulfite ion, [tex]SO_3^{2-}[/tex] refer to image attached.
