Chemistry Reference
In-Depth Information
In addition to the reference tables, the important thing to remember is
that you want to construct chemical formulas with no net apparent charge.
This means that you want to combine the elements in a ratio that would
result in a net oxidation number of zero. It is also important to note that we
will use parentheses if, and only if, we need to multiply a polyatomic ion
with a subscript. The use of parentheses seems to be the thing that students
have the most trouble with. Let's try several examples together.
Suppose your instructor asked you to write the proper chemical for-
mula for aluminum chloride. The first step is to look up the elemental sym-
bol and the oxidation number of each of the elements involved. Because
this compound contains only monatomic ions, we can find both of the re-
quired oxidation numbers in the first group of oxidation numbers.
Example 1
Write the correct chemical formula for aluminum chloride.
Name of the compound—aluminum chloride
Elemental symbols and oxidation numbers—Al
3+
and Cl
-
Next, we figure out in what ratio the ions must combine in order to
have a net charge of zero. In this example, we need 3 chloride ions to can-
cel out the aluminum ion, because +3 + 3(-1) = 0. We indicate the fact
that we need 3 chlorides by using a subscript of 3.
Name of the compound—aluminum chloride
Elemental symbols and oxidation numbers—Al
3+
and Cl
-
Proper chemical formula—AlCl
3
Notice that the oxidation numbers are not part of the proper chemical
formula; they are use to determine the ratio by which the elements combine,
but they don't get written with the formula. Now, we will try a slightly harder
example. Suppose you need to write the proper formula for aluminum sul-
fide. Once again, the first step will be to look up the proper elemental
symbol and oxidation number for each of the elements in the compound.
Example 2
Write the correct chemical formula for aluminum sulfide.
Name of the compound—aluminum sulfide
Elemental symbols and oxidation numbers—Al
3+
and S
2-
