Section 7-2
Oxidation Numbers
• “Imagined charges”
assigned to atoms in
covalent molecules
• In fact, they can be
quite arbitrary.
They Are Useful
• In naming
compounds, in
writing formulas, in
balancing chemical
equations, and in
studying certain
types of chemical
reactions (Ch. 19).
Assigning Oxidation Numbers:
General Rule
• Shared electrons are assumed to belong to the
more electronegative atom in each bond.
Specific Rules For
Assigning Oxidation Numbers
I.
Ex:
Na
O2
S8
Atoms of a pure
element have an
oxidation number
of zero.
II. Electronegativity
• The more electronegative
element in a binary
compound is assigned the
number equal to the
negative charge it would
have as an anion, the less
electronegative one is
positive (as if it were a
cation).
III. Fluorine
• Assigned a value of
-1 in all compounds
because it is the most
electronegative
element.
IV. Oxygen
• Assigned a number of -2
in almost all compounds.
Exceptions:
There are exceptions, like
in H2O2, when you would
have to find the oxidation
number by looking at the
molecule as a whole (i.e.
charge of H)
V. Hydrogen
• +1 in all compounds
with elements that
are more
electronegative than
it is.
• It is -1 when it is
combined with
metals.
Algebraic Sums
VI. In a neutral
compound all
oxidation numbers
add up to zero.
VII. In a polyatomic
ion the sum is
equal to the charge
of the ion.
Because of Rules I-VIII
• It is often possible to
assign oxidation
numbers when they
are not known.
Using Oxidation Numbers For
Formulas and Names
• Both metals and
nonmetals can have
more than one oxidation
number.
Fe = +2 or +3
SO2; S = +4
SO3; S = +6
Both Used Interchangeably For Simple
Compounds
Phosphorous trichloride PCl3 Phosphorous (III) chloride
Dinitrogen monoxide
N2O Nitrogen (I) oxide
Assignment
• 7.2 Worksheet
• 7.2 Textbook Problems
(Warning!!! I’s and l’s
look a lot alike, think
about the formula, does
it make sense?)
Due Wednesday BOP