1. Oxidation States
The oxidation state of an element indicates the number of electrons lost, gained, or shared as a result of chemical bonding.
The change in the oxidation state of a species lets you know if it has undergone oxidation or reduction.
Oxidation numbers are positive or negative numbers, but don’t confuse them with positive or negative charges on ions or valences.

2. Oxidation States Rules
Rule 1: The sum of all oxidation numbers in a neutral compound is zero.
H2O
NaCl
CO2

3. Oxidation States Rules
Rule 2: The oxidation number of an element in its free (uncombined) state is zero — for example, Al(s) or Zn(s).
This is also true for elements found in nature as diatomic (two-atom) elements.
H2, O2, N2, F2, Cl2, Br2, or I2
As well as other single elements.
For example: sulfur found as S8

4. Oxidation States Rules
Rule 3: The sum of all oxidation numbers in a polyatomic ion is equal to the charge on the ion.
(This rule often allows chemists to calculate the oxidation number of an atom that may have multiple oxidation states, if the other atoms in the ion have known oxidation numbers.)
SO42-
PO43-
NO3-

5. Oxidation States Rules
Rule 4: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example:
Na+ = +1
S-2 = -2

6. Oxidation States Rules
Rule 5: Alkali-Metals and Alkali-Earth Metals in a compound:
Group 1 elements are always +1.
Group 2 elements are always +2.
K3N
MgO

7. Oxidation States Rules
Rule 6: The oxidation state of hydrogen in a compound
is +1 (with non-metals).
But, if the hydrogen is part of a binary metal hydride (compound of hydrogen and a metal), then the oxidation state of hydrogen is -1.
NaH H = -1
CaH2 H = -1

8. Oxidation States Rules
Rule 7: The oxidation number of oxygen in a compound is usually –2.
But, if the oxygen is with a peroxide-type of compound
(for example, hydrogen peroxide), then the oxygen has an oxidation number of –1.
If the oxygen is bonded to fluorine, the number is +2.
H2O O = -2
H2O2 O = -1
OF2 O = +2

9. Oxidation States Rules
Rule 8: The oxidation number of halogens (Group 17) in a compound is usually –1.
Fluorine is ALWAYS -1
If other halogens are bonded to oxygen, the number is +1.
NaCl Cl = -1
HF F = -1
OF2 F = -1

10. Oxidation States Practice
S2O3 O2
Na2O
P2O5
NH4+
Na2Cr2O7