Polyatomic ions - Oxoanions

CO₃^2–
(carbonate)

NO₃^–
(nitrate)

PO₄^3–
(phosphate)

SO₄^2– (sulfate)

Polyatomic ions are groups of atoms that have an overall charge.

The four polyatomic anions shown are referred to as oxoanions with oxygens bonded to a central atom.

Oxoanion names end in -ate. Contrast this with monatomic anion names that end in -ide.

To help remember these formulae consider:
C and N are adjacent in row 2 of the periodic table.

Carbonate and nitrate each have three oxygens.

P and S are adjacent in row 3 of the periodic table.

Row 3 atoms are larger, and phosphate and sulfate each have four oxygens.

The anion in each pair having a central element closer to the center of the periodic table has the oxoanion with the higher charge.

Group Numbers
1	2					H	He
Li	Be	B	CO₃^2– carbonate	NO₃^– nitrate	O	F	Ne
Na	Mg	Al	Si	PO₄^3– phosphate	SO₄²^- sulfate	Cl	Ar
K	Ca	Ga	Ge	As	Se	Br	Kr