In Other Words…:
In Other Words… A polyatomic ion is a covalently bonded molecule that only behaves like an ion when it forms bonds. Due to an excess or deficiency of electrons, the molecule has either a positive or negative charge.
Take Phosphate, for example. Its chemical formula is PO4, with a charge of -3. It isn’t an ion itself, but it can act as an anion in a chemical bond with a charge of -3.