gains two electrons to become S2−. In order to balance the charge, two sodium
ions must be present to neutralize the charge on one sulfur ion, creating a
compound with a formula of Na 2 S.
When an oxygen anion (O2−) bonds with a magnesium cation (Mg2+), the
charges balance each other already, and only one of each ion is needed—thus the
formula is MgO. When nitrogen ions (N3−) bond with barium ions (Ba2+), in
order to zero out the total charge, two nitrogen (−3 × 2 = −6) and three barium
(+2 × 3 = +6) ions are necessary, creating a compound with the formula Ba 3 N 2.
Notice the cation always comes first in the formula of an ionic compound.
Polyatomic Ions
There are some ions that are created of multiple elements that stay bonded
together and act as a single unit when forming ionic compounds. These ions are
called polyatomic ions. There are dozens of different polyatomic ions, but your
best bet is to memorize the names, formulas, and charges of the following six,
which are by far the most common.
NO 3 −: Nitrate SO 4 2−: Sulfate
OH−: Hydroxide PO 4 3−: Phosphate
CO 3 2−: CarbonateNH 4 +: AmmoniumTo determine the formula of a compound containing a polyatomic ion, the same
rules are applied. When a potassium ion (K+) bonds with a sulfate ion, two
potassium ions must be present to balance out the charge, creating a compound
with the formula K 2 SO 4.
The only catch here is that if there are multiple polyatomic ions present in a
compound, parentheses must be used to show that. When an aluminum ion
(Al3+) bonds with a nitrate ion, to represent the three nitrate ions that are
necessary to balance the charge, the formula would be Al(NO 3 ) 3 , NOT AlNO 33
(which would imply 33 oxygen atoms!).