Chemical
Composition
&
Reactions
Oxidation Numbers
• Keep track of electrons
during bonding
• Tell how many electrons
are involved in the bond
• Tell whether electrons are
gained, lost, or unequally
shared
Oxidation Numbers
• Help in predicting formulas
• The more electronegative
element gains electrons.
Oxidation Numbers
Rule 1
(free-element)
The oxidation number of
atoms and elements is zero.
Oxidation Numbers
Rule 1
(free-element)
Examples:
He — no bonds
O2 — equal sharing
Oxidation Numbers
Rule 2
(ions)
The oxidation number of a
monatomic ion is equal to
the charge of the ion.
Oxidation Numbers
Rule 2
(ions)
Example:
Mg loses two electrons, so
its charge is +2; therefore,
its oxidation number is
also +2.
Oxidation Numbers
Rule 2
(ions)
Example:
Cl gains one electron, so
its charge and oxidation
number are −1.
Oxidation Numbers
Rule 3
(zero-sum)
The sum of all the
oxidation numbers of all
the atoms in a compound
is zero.
Compounds are not
electrically charged.
Oxidation Numbers
Rule 3
(zero-sum)
Example:
Na loses 1 electron to Cl.
Cl gains 1 electron.
Na+ and Cl− make NaCl.
Na+ + Cl−
NaCl
Oxidation Numbers
Rule 3
(zero-sum)
Example:
Al loses 3 electrons to Br.
Br gains 1 electron.
Al+3 and Br− make AlBr3.
Al+3 + 3 Br−
AlBr3
Oxidation Numbers
Rule 4
(specific oxidation number)
A. Alkali metals
(Group 1)
+1
B. Alkaline-earth metals
(Group 2)
+2
Oxidation Numbers
Rule 4
(specific oxidation number)
C. Hydrogen is usually +1
except when bonded to
metals.
Oxidation Numbers
Rule 4
(specific oxidation number)
D. Oxygen is usually −2
except when bonded
to fluorine.
Exception:
O2−2 (peroxide) O is −1.
Oxidation Numbers
Rule 4
(specific oxidation number)
E. Halogens (Group 17)
have an oxidation
number of −1 when
bonded to metals.
Oxidation Numbers
Rule 4
(specific oxidation number)
E. Metals lose an
electron, so the
halogen gains the
electron.
Oxidation Numbers
Rule 4
(specific oxidation number)
E. When halogens bond to
other nonmetals, the
element with the highest
electronegativity is
negative (−); the others
are positive (+).
Practice Problem
Find the oxidation numbers
of all the elements in CaCl2.
Ox. #’s
+2 −1
CaCl2
+2 −2 = 0
Total of ox. #’s of all
atoms of element
Practice Problem
Find the oxidation numbers
of all the elements in Li2O.
Ox. #’s
+1 −2
Li2O
+2 −2 = 0
Total of ox. #’s of all
atoms of element
Practice Problem
Find the oxidation numbers
of all the elements in H3PO4.
Ox. #’s
+1 +5 −2
H3PO4
+3 +5 −8 = 0
Total of ox. #’s of all
atoms of element
Practice Problem
Find the oxidation numbers
of all the elements in HMnO4.
Ox. #’s
+1 +7 −2
HMnO4
+1 +7 −8 = 0
Total of ox. #’s of all
atoms of element
Question
Which are the oxidation
numbers for CO2?
1. +2, −2
2. +4, −2
3. −4, +2
4. −2, +2
5. None of these
Question
Which are the oxidation
numbers for Fe2O3?
1. +2, −2
2. +3, −3
3. +3, −2
4. +2, −3
5. None of these
Question
Which are the oxidation
numbers for NaOH?
1. +2, −2, +1
2. +2, −1, −1
3. +1, +2, +1
4. +1, −2, +1
5. None of these
Practice Problem
Write the formula for the
product of Ca and Br.
+2 −1
Ca Br2
+2 −2 = 0
Practice Problem
Choose the correct formula
for product of Li and O.
1. LiO
+1 −2
2. LiO2
Li2O
3. Li2O
+2 −2 = 0
4. Li3O2
5. Li2O3
Question
Write out the formula for
reaction of Al and O.
1. AlO
2. Al2O
3. Al2O2
4. Al2O3
5. Al3O2
+3 −2
Al2O3
+6 −6 = 0
Na2O
H2SO4
barium + iodine
calcium + oxygen
polyatomic ions
covalently bonded groups of
atoms that carry a charge
Oxidation Numbers
Rule 5
The sum of the oxidation
numbers of all the atoms in a
polyatomic ion equals the
charge of the ion.
Oxidation Numbers
Rule 5
Examples:
O−2 + H+  OH−
N−3 + 4 H+  NH4+
P? + 4 O−2  PO4−3
4 O−2 = O−8 but the sum is −3.
Therefore, P must be +5.
Pb(OH)2
+
NH4
+
−3
PO4