Oxidation and Reduction Reactions
Oxidation – reduction (redox) rxns involve a transfer of electrons (half-reaction must occur simultaneously)
- Oxidation involves the loss of electrons
o Oxidizing agent – a substance that has the potential to cause another substance to be
oxidized (lose electrons)
- Reduction involves the gain of electrons
o Reducing agent – a substance that has the potential to cause another substance to be
reduced (gain electrons)
Oxidation States:
- The oxidation number of a pure element is 0
- The oxidation number of a monatomic ion equals the charge on the ion
- The more electronegative element in a binary compound is assigned the number equal to the
charge it would have it were an ion
- The oxidation number of fluorine in a compound is always -1
- Oxygen has an oxidation number of -2 unless it is combined with Fluorine, in which it is +1 or +2,
or it is in a peroxide, in which it is -1
- Hydrogen’s oxidation state in most of its compound is +1, unless it is combined with a metal, in
which case it is -1
- In compounds, Group 1 and Group 2 elements and aluminum have oxidation numbers of +1, +2,
and +3 respectively
- The sum of the oxidation numbers of all atoms in a neutral compound in 0
- The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion
Oxidation – the process in which the atoms or ions of an element experience an increase in oxidation states
Example:
2 Na (s) + Cl2 (g) 2 NaCl(s)

Sodium undergoes an oxidation process because the sodium atoms loses an electron and
become a sodium ion – sodium is an oxidized to a sodium ion (electrons are on the right side of
the arrow)
0
+1
Na  Na+ + e-
Half-reaction
 A species whose oxidation number increases is oxidized
Reduction – the process in which the oxidation state of an element decreases
Example:
2 Na (s) + Cl2 (g) 2 NaCl(s)

Each chlorine atom accepts an electrons and becomes a chloride ion (electrons are on the left
side of the arrow)
0
-1
Cl2 + 2e-  2Cl
Half-reaction
A species that undergoes a decrease in oxidation state is reduced
Lose
Electrons
Oxidation
Gain
Electrons
Reduction