Valence electrons
the electrons in the outermost shell of the uncharged atom
Polyatomic ion
A group of covalently bound atoms that act like a single ion.
CO32- 	Carbonate 
HCO3- 	Bicarbonate 
SO42- 	Sulfate 
PO43- 	Phosphate 
ionic bonding
a bond formed typically between a metal and a non-metal
Calcium
an example of a metal
oxygen
an example of a non-metal