Polyatomic ions are groups of atoms covalently bonded together, but the group of atoms as a whole has a charge. They are different from monatomic ions, which contain only one atom. Polyatomic ions can be positively or negatively charged, and they are named based on the number and type of atoms they contain. Examples of polyatomic ions include nitrate ion (NO−3), hydroxide ion (OH-), acetate (CH3COO–), phosphate (PO43-), permanganate (MnO4–), oxalate (C2O42-), and many others. Polyatomic ions are formed when a neutral molecule gains or loses electrons, and the group of covalently bonded atoms carries a net charge because the total number of electrons in the molecule is not equal to the total number of protons in the molecule.