Key Concepts
An oxidation number (oxidation state) is the charge an atom would carry if the molecule or ion were completely ionic.
Rules for Assigning Oxidation Numbers
- Fluorine is assigned an oxidation number of -1 in compounds
          HF oxidation number of fluorine = -1
- Oxygen is assigned an oxidation number of -2 in compounds
          H2O oxidation number of oxygen is -2
Except
- Peroxides: oxidation number of oxygen is -1
          H2O2 oxidation number of oxygen is -1
- Superoxides: oxidation number of oxygen is -½
          KO2 oxidation number of oxygen is ½
- Oxygen fluorides: OF2 oxidation number of oxygen is +2
          O2F2 oxidation number of oxygen is +1
- Hydrogen is assigned an oxidation of +1 in compounds
      HCl oxidation number of hydrogen is +1
- Group I elements (Alkali Metals) are assigned an oxidation number of +1 in compounds
      NaNO3 oxidation number of sodium is +1
- Group II elements (Alkaline-earth metals) are assigned an oxidation number of +2 in compounds
      MgBr2 oxidation number of magnesium is +2
- An atom of any element in the free state has an oxidation number of 0
      S8 oxidation number of each sulfur atom is 0
- Any monatomic ion has an oxidation number equal to its charge
      H- oxidation number of hydrogen is -1
- The sum of the oxidation numbers of all the atoms in a formula equals the electrical charge shown with the formula
- The sum of the oxidation numbers of all the atoms shown in a formula of a compound is 0
      CO2: let x be the unknown oxidation number of carbon       0 = x + (2 x -2)
      So x (oxidation number of C) = 0 + 4 = +4
- The sum of the oxidation numbers of all the atoms shown in the formula for a polyatomic ion or complex ion equals the electrical charge on the ion
      Cr2O72-: let y = unknown oxidation number of chromium       -2 = 2y + (7 x -2)       2y = -2 + 14       2y = +12       y (oxidation number of Cr) = 12 ÷ 2 = +6
Oxidation
An increase in oxidation number which corresponds to a loss of electrons
(or to an addition in oxygen or loss of hydrogen)
Example
Zn(s) -----> Zn2+ + 2e
Zinc has been oxidised since there has been a loss of electrons to form Zn2+.
The oxidation number of zinc has increased from 0 to 2+
Reduction
A reduction in oxidation number which corresponds to a gain of electrons
(or to an addition of hydrogen or loss of oxygen)
Examples
Ag+ + e -----> Ag(s)
The silver ion has been reduced since it has gained an electron to form Ag(s)
The oxidation number of silver has been reduced from +1 to 0
MnO4- + 8H+ + 5e -----> Mn2+ + 4H2O
Oxidation number of manganese (x) in MnO4- is:
-1 = x + (4 x -2)
x = -1 + 8 = +7
oxidation number of manganese in MnO4- = +7
manganese has been reduced since there is a reduction in oxidation number from +7 to +2
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