How to Figure Out
Chemical Formulas
Chapter 2
Science 9
The Criss-Cross Method
 The criss-cross method is an easy way to
determine the formula of any ionic compound.
 The ionic compound can be the simple (binary)
ionic compound with just two elements, or one
that has a polyatomic ion as part of it. (
 All you need is the “combining capacity” of the
elements/ions with which you will be working.
 You can get these combining capacity values
directly off of the periodic table for most elements
or they are part of the list for either the “shifty”
metals and polyatomic ions.
How to Criss-Cross
Steps (in order)
1. Write the chemical
symbols.
2. Write the combining
capacities over the
symbols.
3. Criss-cross and drop.
4. Reduce – if necessary
– the same numbers
or a 2 & 4 together.
Example: Calcium nitride
1.
Ca N
2.
Ca2 N3
3.
Ca3N2
How to Write Formulas
Try These Formulas
Use the criss-cross method to find the formulas for the following.
Name of Compound
Chemical Formula
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Beryllium bromide
Lithium oxide
Potassium nitride
Calcium sulfide
Magnesium nitride
Aluminum phosphide
Lithium chloride
Sodium sulfide
Magnesium oxide
Calcium iodide
BeBr2
Li2O
K3N
CaS
Mg3N2
AlP
LiCl
Na2S
MgO
CaI2
Formulas of Covalent Compounds
 Covalent compounds are between two nonmetals and it
involves the sharing of electrons – no bullying involved.
 The names of covalent compounds have prefixes
(starters) that tell you how many of each element is
required in the formula.
 You must remember the prefixes and you must remember
that covalent compounds do not reduce to lowest terms
like those ionic ones do:
Mono – 1
Tetra – 4
Hepta - 7
Di – 2
Penta – 5
Octa - 8
Tri – 3
Hexa – 6
Nona - 9
 Examples:
 Carbon tetrachloride = CCl4
 Dinitrogen dioxide = N2O2
 Disulfur trihydride = S2H3
Formulas of Covalent Compounds
Write the formulas for each of the following covalent compounds.
Name of Compound
Chemical Formula
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Carbon dioxide
Nitrogen trioxide
Trisulfur difluoride
Phosphorous pentoxide
Dinitrogen triiodide
Diphosphorus hexabromide
Dihydrogen monoxide
Disulfur heptachloride
Nitrogen dioxide
Carbon monoxide
CO2
NO3
S3F2
PO5
N2I3
P2Br6
H2O
S2Cl7
NO2
CO
Write name and formula
for Covalent Compounds