The phosphite ion, PO33-, has three resonance forms. Resonance structures are different ways of depicting the same molecule where the placement of electrons differs, but the arrangement of atoms remains the same.
In the case of the phosphite ion, the phosphorus atom is bonded to three oxygen atoms. One of the oxygen atoms is bonded with a double bond to phosphorus, while the other two oxygens are bonded with single bonds. Each of these oxygens with a single bond carries a negative charge. The double bond can shift among the three oxygen atoms, leading to multiple resonance structures.
This delocalization of electrons stabilizes the molecule as a whole and is important in understanding its chemical behavior. Thus, we recognize that the phosphite ion can be represented by three distinct resonance forms.