Name________________________________________________Date___________________Period____
Criss-Cross Method of Ionic Formulas (Polyatomic Compounds)
In a compound of aluminum ion (Al3+) and nitrate ion (NO3-) there must be three nitrate ions. So, the formula for
this compound is Al(NO3)3. Recall that nitrate is a polyatomic ion, so this combination of atoms remains intact
when writing a chemical formula. Also notice that the proper way to designate more than one polyatomic ion
involves the use of parenthesis.
An easy technique for creating a neutral combination of two charged ions is called the criss-cross technique. When
writing a formula for an ionic compound the charges from each ion are simply switched to become the subscript
values written to designate the number of atoms present in a compound. See examples below.
Common Polyatomic Ions
NH4+
Ammonium
OH-
Hydroxide
CN-
Cyanide
NO3-
Nitrate
ClO3-
Chlorate
C2H3O2-
Acetate
SO42-
Sulfate
CO32-
Carbonate
PO43-
Phosphate
HCO3-
Bicarbonate
HSO4-
Bisulfate
Fill out the chart using the criss-cross method. The bolded part of the name indicates a polyatomic ion.
Name
Cation
Anion
potassium oxide
K+
O2-
sodium sulfide
Na+
S2Cl-
ammonium chloride
calcium nitrate
Ca2+
calcium bromide
Ca2+
strontium hydroxide
Sr2+
aluminum sulfite
Al3+
Br-
Formula
gallium permanganate
Ga3+
ammonium carbonate
sodium oxide
Na+
barium chlorate
Ba2+
magnesium bicarbonate
Mg2+
calcium hydroxide
Ca2+
potassium phosphate
K+
aluminum acetate
Al3+
barium sulfate
Ba2+
strontium sulfide
Sr2+
rubidium phosphate
Rb+
O2-
S2-
ammonium nitrite
Classify the following as either atoms (A), molecules (M), monatomic ions (MI), or polyatomic ions (PI).
Cu
Co
H2O
CO
Au3+
NH4+
PO43-
NH3
S2-
Au
SO42-
Cu2+
SO3
OH-
Naming Compounds with Polyatomic Ions Practice
When naming a compound containing a polyatomic ion, simply state the name of the polyatomic ion in the formula.
Highlight (or shade) the polyatomic ions in each formula below.
Name the ionic compounds below.
Formula
Name
Formula
Ca(OH)2
Al(NO3)3
Ba(NO3)2
NaHCO3
Mg(C2H3O2)2
CaHSO4
SrCO3
NaOH
CaSO4
KClO3
Na3PO4
NH4Cl
Sr(CN)2
Ca3(PO4)2
Ba(MnO4)2
(H3O)2S
NH4NO3
(NH4)2SO4
Name
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