Oxidation Number
An oxidation-reduction process involves the
simultaneous loss of electrons by one
specie(s) and gain of electrons by another
specie(s). The specie that loses electrons is
said to be oxidized. The specie that gains
electrons is said to be reduced. A mnemonic
commonly used to remember these facts is
"LEO says GER." In words, "Loss of
Electrons is Oxidation," and "Gain of
Electrons is Reduction."
In a redox process, the oxidized specie is
called a reducing agent because it is the
specie that donates electrons to allow the
reduction to take place. Similarly, the
reduced specie is called a oxidizing agent.
http://www.scientia.org/cadonline/Chemistry/stoichiometry/redoxtit.ASP
Oxidation Number
When balancing redox reactions, the movement of all the electrons must be
traced. The movements are traced through the assignment of oxidation
numbers to each atom, ion, or molecule involved in the reaction. A series of
rules helps determine the oxidation number of an element.
Rule
Molecules containing only one element have an
oxidation number of 0.
Examples
Ti
S8
0
0
Simple ions (ions of one atom) have an oxidation
number equal to the charge on the ion.
Zn2+
Br¯
+2
-1
All alkali metals (Group 1A or 1) have an oxidation
number of 1; all alkaline earth metals (Group IIA or
2) have an oxidation number of 2. Aluminum is
always 3. Fluorine is always -1.
Li
Mg
Al
+1
+2
+3
O in CaO
-2
O in H2O2
-1
O in OF2
+1
Oxygen has an oxidation number of -2, except in
peroxides where it is -1 and where it is in
combination with more electronegative elements like
F, where it accommodates fluorine's demand for an
electron.
Hydrogen has an oxidation number of +1 except in
binary combination with metals.
H in HNO3 +1
H in NaH
Halogens (Group VIIA or I7) have an oxidation
number of -1 unless they are combined with an
element of higher electronegativity (generally
oxygen).
Cl in HCl
Cl in HOCl
-1
-1
+1