1. How to Figure Out Chemical Formulas

2. 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.

3. How to Criss-Cross Steps (in order) Write the chemical symbols.
Write the combining capacities over the symbols.
Criss-cross and drop.
Reduce – if necessary – the same numbers or a 2 & 4 together.
Example: Calcium nitride
Ca N
Ca2 N3
Ca3N2

4. Try These Formulas Use the criss-cross method to find the formulas for the following.
Name of Compound
Beryllium bromide
Lithium oxide
Potassium nitride
Calcium sulfide
Magnesium nitride
Aluminum phosphide
Lithium chloride
Sodium sulfide
Magnesium oxide
Calcium iodide
Chemical Formula
BeBr2
Li2O
K3N
CaS
Mg3N2
AlP
LiCl
Na2S
MgO
CaI2

5. 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.

6. Formulas With “Shifty” Metals Write the formulas for each of the following ionic compounds that feature one of the transition metals.
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
Fe2O3
SnS
PbCl2
CuBr2
Fe3N2
PbO2
Cu3P
SnF4

7. 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!

8. Polyatomic Ionic Compounds Write the formulas for each of the following polyatomic ionic compounds. Use brackets when necessary.
Name of Compound
Sodium nitrate
Calcium chlorate
Lithium phosphate
Aluminum sulfate
Tin (IV) carbonate
Iron (II) hydroxide
Magnesium carbonate
Lead (II) bicarbonate
Potassium chlorate
Ammonium phosphate
Chemical Formula
NaNO3
Ca(ClO3)2
Li3PO4
Al2(SO4)3
Sn(CO3)2
Fe(OH)2
MgCO3
Pb(HCO3)2
KClO3
(NH4)3PO4

9. 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

10. Formulas of Covalent Compounds Write the formulas for each of the following covalent compounds.
Name of Compound
Carbon dioxide
Nitrogen trioxide
Trisulfur difluoride
Phosphorous pentoxide
Dinitrogen triiodide
Diphosphorus hexabromide
Dihydrogen monoxide
Disulfur heptachloride
Nitrogen dioxide
Carbon monoxide
Chemical Formula
CO2
NO3
S3F2
PO5
N2I3
P2Br6
H2O
S2Cl7
NO2 CO

11. THE END