Chapter 7: Chemical Formulas and Chemical Compounds
Section 2: Oxidation Numbers
Oxidation numbers
An oxidation number, also called oxidation state, indicates
the general distribution of electrons among the bonded
atoms in a molecular compound or polyatomic ion.
Unlike ionic charges, oxidation numbers do not have an exact
physical meaning. They are useful in naming compounds
writing formulas, and in balancing equations.
Rules for Assigning Oxidation Numbers
There are several rules for assigning the oxidation number to an
element. Learning these rules will simplify the task of
determining the oxidation state of an element.
1. The oxidation number of an atom in the elemental state is
zero.
Example: Cl2 and Al both are 0
2. The oxidation number of a monatomic ion is equal to its
charge.
Example: In the compound NaCl, the sodium has an oxidation
number of 1+ and the chlorine is 1-.
3. The algebraic sum of the oxidation numbers in the
formula of a compound is zero.
Example: the oxidation state for NaCl add up to 0
4. The oxidation number of hydrogen in a compound is 1+,
except when hydrogen forms compounds called hydrides
with active metals, and then it is 1-.
Example: H is 1+ in H2O, but 1- in NaH (sodium hydride).
5. The oxidation number of oxygen in a compound is 2-,
except in peroxides when it is 1-, or when combined with
fluorine then it is 2+.
Example: In H2O the oxygen is 2-, in H2O2 it is 1-.
6. The algebraic sum of the oxidation numbers in the
formula for a polyatomic ion is equal to the charge on
that ion.
2Example: in the sulfate ion, SO4 , the oxidation numbers of
the sulfur and the oxygen add up to 2-. Each oxygen is 2-,
4*(-2) + “S” = – 2; so the sulfur (“S”) is 6+.
Practice:
1. What is the oxidation number of the atoms in these
formulas?
Na2CrO4
Cr2O72–
Element
Oxidation #
Element
Oxidation #
Na
+1 {(2) (+1)}
Cr
+6 {(2) (+6)}
Cr
+6 {(1) (+6)}
O
–2 {(7) (– 2)}
–2 {(4) (– 2)} Ox # sum (+12) + (–14) = – 2
O
Ox # sum (+2)+(+6)+(–8) = 0
Hint: (CrO4) 2–
2. Given this unbalanced equation identify the oxidation state
of each element: Cr2O3(s) + Al(s)  Cr(s) + Al2O3(s)
Cr2O3(s): O  – 2 (“rule # 5”)
Cr  2x + (3)(– 2) = 0 => x = +3
Al(s):
Al  0 (it is in its neutral elemental form)
Cr(s):
Cr  0 (it is in its neutral elemental form)
Al2O3(s): O  – 2 (“rule # 5”)
Al  2x + (3)(– 2) = 0 => x = +3
A more extensive list of oxidation numbers is given in Appendix Table A-15.
In Section 7-1 we introduced the use of the Stock system
(roman numerals) of naming ionic compounds.
The Stock system is actually based on oxidation numbers, and
it can be used as an alternative to the prefix system for naming
binary molecular compounds.
Formula
Prefix System
Stock System
SO2
SO3
sulfur dioxide
sulfur trioxide
sulfur (IV) oxide
sulfur (VI) oxide
The international body that governs nomenclature has endorsed
the Stock system, which is more practical for complicated
compounds. Prefix-based names and Stock-system names are
still used interchangeably for many simple compounds.
In Review:
1. Oxidation numbers are assigned to the atoms composing a
compound or ion in order to indicate the general distribution
of electrons among the bonded atoms in the compound or
ion.
2. The atoms in a pure element have an oxidation number of
zero.
3. The more electronegative element in a binary molecular
compound is assigned the number that is equal to the
negative charge it would have as an anion.
4. The algebraic sum of the oxidation numbers in a compound
must equal zero.
5. Oxidation numbers are useful for naming compounds,
writing formulas, and balancing chemical equations
Homework: Page 219 #s 1 & 2; Handout; Page 235 #s 15 - 18
& 26 - 28.