Chemistry Reference
In-Depth Information
you are asked to determine the oxidation number shown by carbon in the
formula CF
4
. Even if you knew the possible oxidation states of carbon, as
explained in the previous paragraph, can you tell if carbon is acting like it is
+4 or -4 in this particular compound? The key to solving this problem is in
knowing that fluorine always shows an oxidation number of -1. If each fluo-
rine atom has an oxidation number of -1 and there are 4 fluorine atoms, as
shown by the subscript 4, then you have a total apparent negative charge of
-4. Remember that the net charge on each neutral compound is 0, so the
apparent charge on the carbon atom must be +4, as shown in this equation:
+4 + 4(-1) = 0
Memorizing the following rules about oxidation numbers will make it
easier to write chemical formulas. Even when you have access to an oxida-
tion table, you can save a great deal of time by memorizing some of the
basic facts.
Rules for Assigning Oxidation Numbers
1.
In a free element (e.g., Ba or I
2
) the oxidation number for
each atom is 0.
2.
With the exception of hydrogen, all of the elements in group
1 show an oxidation number of +1 in compounds.
3.
Hydrogen has an oxidation number of +1 in all compounds
except for hydrides (e.g., NaH), where it shows an oxidation
number of -1.
4.
The elements in group 2, the alkaline earth metals, show an
oxidation number of +2 in all compounds.
5.
When any of the elements in column 17, the halogens, show a
negative oxidation state, it will be -1.
6.
Oxygen shows an oxidation number of -2 in all compounds,
except for the peroxides, where it is -1.
7.
In all neutral compounds, the algebraic sum of the oxidation
numbers is 0.
8.
In polyatomic ions, the algebraic sum of the oxidation
numbers is equal to the charge on the polyatomic ion.
