Chemistry Reference
In-Depth Information
Example 4
Write the correct chemical formula for calcium nitrate.
Name of the compound—calcium nitrate
Elemental symbols and oxidation numbers—Ca
2+
and NO
3
-
In this example, we need 2 nitrate ions to balance out the calcium ion,
because +2 + 2(-1) = 0. Normally, we would just add a subscript of 2 to
the negative ion. However, the nitrate already has a subscript of 3. We
can't just change the existing subscript to 2, because that would simply leave
us with one nitrite (NO
2
-
) ion. Remember that I said that we cannot alter
the subscripts that come with these polyatomic ions. When we want to ex-
press multiple polyatomic ions, we use parentheses, as shown below:
Name of the compound—calcium nitrate
Elemental symbols and oxidation numbers—Ca
2+
and NO
3
-
Proper chemical formula—Ca(NO
3
)
2
Let's try one more example that illustrates the proper use of parenthe-
ses. Find the symbols and oxidation numbers to write the proper formula
for aluminum hydroxide. Remember that hydroxide isn't an element, so
you will find it with the polyatomic ions.
Example 5
Write the correct chemical formula for aluminum hydroxide.
Name of the compound—aluminum hydroxide
Elemental symbols and oxidation numbers—Al
3+
and OH
-
You can see that we will need 3 hydroxide ions to balance out the alumi-
num ion, because +3 + 3(-1) = 0. Many students are tempted to simply add
a subscript of 3 after the hydrogen to get the formula AlOH
3
, but this is not
the correct formula! The reason is because we need 3 hydroxide ions, and
adding the 3 subscript in this fashion only gives us 3 hydrogen atoms. The
subscript as shown would not act as a multiplier for the oxygen. What we
need to do, in order to use the 3 as a multiplier for the whole polyatomic ion,
is to put the hydroxide in parentheses and put the subscript outside of them.
Name of the compound—aluminum hydroxide
Elemental symbols and oxidation numbers—Al
3+
and OH
-
Proper chemical formula—Al(OH)
3
