How to Figure Out Chemical Formulas

How to Figure Out
Chemical Formulas
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
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 With “Shifty” Metals
 The criss-cross method is also used when dealing
with metals that have more than one combining
capacity value. (Copper, Iron , Lead or Tin)
 The combining capacity value for these metals can
be found in the name of the compound that
contains them.
 Examples:
 Iron (II) oxide – the Iron (Fe) used here has a combining
capacity of 2.
 Lead (IV) sulfide – the Lead (Pb) used here has a
combining capacity of 4.
Formulas With “Shifty” Metals
Write the formulas for each of the following ionic compounds that
feature one of the transition metals.
1.
2.
3.
4.
5.
6.
7.
8.
Name of Compound
Iron (III) oxide
Tin (II) sulfide
Lead (II) chloride
Copper (II) bromide
Iron (II) nitride
Lead (IV) oxide
Copper (I) phosphide
Tin (IV) fluoride
Chemical Formula
1. Fe2O3
2. SnS
3. PbCl2
4. CuBr2
5. Fe3N2
6. PbO2
7. Cu3P
8. SnF4
Polyatomic Ionic Compounds
 Polyatomic ions are clusters of atoms that stay together as one
unit and carry an overall charge. Most polyatomic ions are
negatively charged.
 The charge of the polyatomic ion is the same as its combining
capacity so that is the number you will use in the criss-cross
method.
Brackets
 You may need more than one polyatomic ion in your formula…This
means that you will have to use brackets. Use brackets whenever
you need more than one polyatomic ion in the formula.
 Example: Iron (III) nitrate…
 Start with symbols and combining capacities…
Fe3 NO31
 After the criss-cross, you may get one of two possibilities…
FeNO33
OR
Fe(NO3)3
 The second option is right because it says you need two nitrate clusters to go
with every iron atom. The first option, which did not use brackets, reads as
one iron atom with one nitrogen atom and thirty-three oxygen atoms.
 Fe(NO3)3 is the correct formula! Use those brackets!
Polyatomic Ionic Compounds
Write the formulas for each of the following polyatomic ionic
compounds. Use brackets when necessary.
Name of Compound
Chemical Formula


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



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1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Sodium nitrate
Calcium chlorate
Lithium phosphate
Aluminum sulfate
Tin (IV) carbonate
Iron (II) hydroxide
Magnesium carbonate
Lead (II) bicarbonate
Potassium chlorate
Ammonium phosphate
NaNO3
Ca(ClO3)2
Li3PO4
Al2(SO4)3
Sn(CO3)2
Fe(OH)2
MgCO3
Pb(HCO3)2
KClO3
(NH4)3PO4
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
THE END