OXIDATION NUMBERS
The following rules predict the oxidation number for each of the atoms/ions in a compound or ion.
General Rules (Always applicable–No exceptions)
1. For any uncombined element, the oxidation number is zero.
Examples:
Fe, Xe, O2, H2 (0 for all of these)
2. For any monatomic ion, the oxidation number is equal to the charge on the ion.
Examples:
Fe2+, F-, O2-, H+ (+2, -1, -2, and +1 respectively)
3. For any compound, the sum of the oxidation numbers must be zero.
Examples:
NaCl = Na+ + Cl- = +1 + (-1) = 0
MgF2 = Mg2+ + 2 F- = +2 + 2(-1) = 0
Fe2(Cr2O7)3 = 2 Fe3+ + 6 Cr6+ + 21 O2= 2(+3) + 6(+6) + 21(-2) = 0
4. For any ion, the sum of the oxidation numbers must equal the charge on the ion.
Examples:
NO31- = N5+ + 3 O2- = +5 + 3(-2) = -1
C2H3O21- = 2C0 + 3H+ + 2O2- = 2(0) + 3(+1) + 2(-2) = -1
Special Rules
These rules only apply to certain columns on the Periodic Table. They often combine with
General Rules 3 and 4.
Note: Rules 1-8 apply to the main group elements. Different Periodic Tables label these in
different ways, so two types of numbers are given. The currently accepted method is to use the
numbers 1-2 and 13-18 for the representative elements. Older tables used IA-VIIIA (or IBVIIIB). Each of the rules will use both of these methods with the newer method of labeling
given in parentheses.
Note: These are the most common/stable oxidation numbers. There are exceptions in addition
to the ones given below.
Note: These values refer to these elements in compounds. General Rule 1 still applies to the
elements in the elemental state.
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1. Column IA (1)
+1 (Except H with a metal, then H = -1)
2. Column IIA (2)
+2
3. Column IIIA (13) +3 (Normally, however +1 possible near the bottom of the table)
4. Column IVA (14) +4 to -4
5. Column VA (15)
+5 to -3
6. Column VIA (16) +6 to -2 (Oxygen is -2 except when combined with F, or in O22- or O2-)
7. Column VIIA (17) +7 to -1 (Fluorine is -1 in compounds)
8. Column VIIIA (18) +8 to 0 (Usually only 0)
9. Transition Metals IIIB-IIB (3-12)
If the B designation is used the values may range from +2 to the group number. If the
other designation is used, the values may range from +2 to the group number for
columns 3-8 and to the group number minus 10 for groups 11 and 12 (treat columns 9
and 10 as if they were 7 and 6 respectively).
Exceptions: Hg22+ (Hg = +1) and Au3+ (Au = +3)
When an element may have a positive or negative oxidation number, it will normally be negative if it
is to the upper right, on the Periodic Table, with respect to the other elements in the compound or ion.
When it is negative, under these circumstances, it will probably have the most negative of the possible
values noted in the rules above.
Note that in Special Rules 4-8 the range of possible values is always eight. With these elements, the
more probable oxidation states may be determined by counting from highest to lowest by twos.
In naming compounds containing a metal with a variable oxidation number (most of the transition
metals and those around lead on the Periodic Table), Stock Numbers should be used in the name.
Older nomenclature systems used different names to designate variations in the oxidation number (for
example, ferrous and ferric). We will not do so in this class.
In this class, do not use Stock Numbers except with a metal.
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Examples:
Cr
Cl
F
C
Pb
possible values:
possible values
possible values
possible values
possible values
NaH
BaO2
Cl2O7
CuS
Hg2Cl2
HgCl2
Pb3O4
H2SO4
MoO3
Na+
Ba2+
Cl7+
Cu2+
Hg+
Hg2+
Pb8/3+
H+
Mo6+
HOO2S2ClClO2S6+
O2-
0, +2, +3, +4, +5, +6
-1, 0, +1, +2, +3, +4, +5, +6, +7 (underlined preferred)
-1, 0
-4, -3, -2, -1, 0, +1, +2, +3, +4
0, +2, +4
(even though Pb and C are in the same column,
Pb only has positive values since it is a metal.)
(barium peroxide)
(actually 2 Pb2+ + 1 Pb4+)
O2-
At one time different names were used for different oxidation states for a few elements. These are no
longer proper names, however they are still seen for a few elements (iron, copper and tin), and appear
as trivia for others. These names are not acceptable on a lecture exam. These names were devised so
that the lower oxidation state of an element ended in -ous, and the higher oxidation state ended in -ic.
In general, the more likely elements to use this method were p and d block metals and metalloids with
names based upon a Latin root instead of an English root. Some examples are included in the
following table.
Name (lower state)
Charge
Name (higher state)
Charge
Antimonous
Aurous
Cerous
Chromous
Cobaltous
Cuprous
Ferrous
Manganous
Mercurous
Nickeleous
Platinous
Plumbous
Stannous
Stibonious
Thallous
Titanous
Vanadous
Sb3+
Au+
Ce3+
Cr2+
Co2+
Cu+
Fe2+
Mn2+
Hg22+
Ni2+
Pt2+
Pb2+
Sn2+
Sb3+
Tl+
Ti3+
V2+
Antimonic
Auric
Ceric
Chromic
Cobaltic
Cupric
Ferric
Manganic
Mercuric
Nickelic
Platinic
Plumbic
Stannic
Stibonic
Thallic
Titanic
Vanadic
Sb5+
Au3+
Ce4+
Cr3+
Co3+
Cu2+
Fe3+
Mn3+
Hg2+
Ni3+
Pt4+
Pb4+
Sn4+
Sb5+
Tl3+
Ti4+
V3+
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