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The key to identifying oxidation-reduction reactions is recognizing
when a chemical reaction leads to a change in the oxidation number of one
or more atoms. It is therefore a good idea to take another look at the
rules for assigning oxidation numbers. By definition, the oxidation number
of an atom is equal to the charge that would be present on the atom if the
compound was composed of ions. If we assume that CH4 contains C4-
and H+ ions, for example, the oxidation numbers of the carbon
and hydrogen atoms would be -4 and +1.
Note that it doesn't matter whether the compound actually contains
ions. The oxidation number is the charge an atom would have if the
compound was ionic. The concept of oxidation number is nothing more than a
bookkeeping system used to keep track of electrons in chemical reactions.
This system is based on a series of rules, summarized in the table below.
| Rules for Assigning Oxidation Numbers |
- The oxidation number of an atom is zero in a neutral
substance that contains atoms of only one element. Thus, the
atoms in O2, O3, P4, S8,
and aluminum metal all have an oxidation number of 0.
- The oxidation number of monatomic ions is equal to the
charge on the ion. The oxidation number of sodium in the Na+
ion is +1, for example, and the oxidation number of chlorine
in the Cl- ion is -1.
- The oxidation number of hydrogen is +1 when it is combined
with a nonmetal. Hydrogen is therefore in the +1
oxidation state in CH4, NH3, H2O,
and HCl.
- The oxidation number of hydrogen is -1 when it is combined
with a metal. Hydrogen is therefore in the -1
oxidation state in LiH, NaH, CaH2, and LiAlH4.
- The metals in Group IA form compounds (such as Li3N
and Na2S) in which the metal atom is in the +1
oxidation state.
- The elements in Group IIA form compounds (such as Mg3N2
and CaCO3) in which the metal atom is in the +2
oxidation state.
- Oxygen usually has an oxidation number of -2. Exceptions
include molecules and polyatomic ions that contain O-O
bonds, such as O2, O3, H2O2,
and the O22- ion.
- The nonmetals in Group VIIA often form compounds (such as
AlF3, HCl, and ZnBr2) in which the
nonmetal is in the -1 oxidation state.
- The sum of the oxidation numbers of the atoms in a
molecule is equal to the charge on the molecule.
- The most electronegative element in a compound has a
negative oxidation number.
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Any set of rules, no matter how good, will only get you so far. You
then have to rely on a combination of common sense and prior knowledge.
Questions to keep in mind while assigning oxidation numbers include the
following: Are there any recognizable ions hidden in
the molecule? Does the oxidation number make sense in terms of the known
electron configuration of the atom? |