By Thomas V. Flanagan
Gonzaga Prep School Chemistry Department
PART I – Naming the chemicals (Quick, get your “cheat sheet”!)
A. OK, so there is the formula. The first question is, “Are there any metals in the formula?”
Remember, the metals are to the left of the zig-zag line on the periodic table. If there are
no metals, your job is easy. Just use the prefix system. Don’t worry about charges or
any of that stuff. Remember that the first element gets its normal name and the second
element ends in –ide.
1. For example, take CO2. Neither C nor O is a metal, so the name is carbon
dioxide, which means one carbon and two oxygens.
2. Similarly, take P4O10. Neither P nor O is a metal, so the name is
tetraphosphorous decoxide.
B. Now, if one if the elements is a metal (and it will always be the first one in the formula),
then you CANNOT USE THE PREFIXES and you must consult the “cheat sheet” of
charges. Did I tell you that you cannot use the prefixes? I hope so.
1. You just look at the cheat sheet and find the names of the elements or groups.
Pay no attention to how many of each element or group is in the formula.
a. For example, take NaCl. Na is a metal, so we look at the cheat sheet
and see that Na is called sodium and Cl is called chloride. So the name
is sodium chloride.
b. Let’s take AlCl3. Al is a metal, so we check the sheet and so that Al is
aluminum and Cl is chloride. Its name must be aluminum chloride.
Wait! What about the fact that there are 3 chlorides? No matter. If
there is a metal involved, we don’t look at that in the name.
c. Did I say that we do not use prefixes with the metals? Hope so!
2. Now let’s suppose we have the salt PbCl2. OK, Pb is a metal. But when we
check our cheat sheet, Pb is listed in the +2 column as lead(II) and also in the +4
column as lead(IV). Yikes! What does that mean? It means that Pb can have
two different charges, so we need to figure out which one it is and write it that
How to tell? Well, you look at what the Pb is connected to. It is
connected to Cl2. OK. Since each Cl has a charge of –1 (from the cheat
sheet), then Cl2 must carry a total charge of –2 (-1 x 2). Then, since all
compounds have a net charge of zero, the Pb must have a charge of +2.
So it is the lead(II). Hence its name is lead(II) chloride.
Similarly, if you had the chemical Cu2O, it would have to be called
copper(I) oxide, since the O is a –2 and thus the Cu2 must be a +2, which
means each Cu is +1.
PART II – Writing the formulas
A. OK. You have a chemical name. What is the formula? Well, if there are any prefixes in
the name, then you ditch the cheat sheet and use the prefix conversion. For example,
triphosphorous heptachloride means 3 phosphorous and 7 chloride, so it is P3Cl7. It’s
that easy. No checking charges or any thing. Just translate those prefixes.
B. Yikes! What if there are no prefixes? Well, just use the cheat sheet and write each part
of the formula out separately. Then, if the charges match, simply slam ‘em together. If
not, use the criss-cross method.
1. For example, let’s take magnesium sulfide. OK. No prefixes, so go to the cheat
sheet. Magnesium is Mg +2 and Sulfide is S-2. They match! So the formula is
simply MgS.
2. Let’s take lead(II) oxide. Lead(II) is Pb+2 and oxide is O-2. They match! So
the formula is PbO.
3. Let’s look at tin(IV) chloride. Tin(IV) is Sn+4 and chloride is Cl-1. Oops! No
match, so we criss-cross. The formula must be SnCl4.
4. Finally, let’s look at lead(IV) oxide. OK. Lead(IV) is Pb+4 and oxide is –2. No
match! If we criss-cross, we get Pb2O4. True, but notice that the little number
could be simplified by dividing each one by 2. If we do that, we get PbO2. And
that’s the formula!!!!!
PART III – Those weird polyatomics.
A. Sometimes the formula will have more than 2 elements, such as Na2SO4 or Ba(NO2)2.
Don’t fret. It means that the ugly group (in this case the SO4 and the NO2) have their
own special names and charges. They are on the cheat sheet! Just name them, look at
the charges and do criss-cross if you have to!
B. The same goes for naming chemicals. If the name does not end in –ide, but has a name
like sodium phosphate or calcium nitrite, then you need to look at those special groups
for the formula. Don’t worry. They are listed!
PART IV – Practice, practice, practice!
Here are some samples:
Barium chloride – BaCl2
Copper(II) sulfate – CuSO4
Phosphorous trichloride – PCl3
Nitrogen dioxide – NO2
Barium nitrate – Ba(NO3)2
Sodium oxide – NaO2