OXIDATION NUMBER/ Definition, Rules, Concept Points, Examples/

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A compound or molecule is electrically neutral - have "zero" algebraic sum of charges - chemical species. Atoms of elements, in compound ,carry charge, that charge on atoms in chemical species is called OXIDATION NUMBER of element in compound. Oxidation number is also referred as OXIDATION STATES.

In case of polyatomic molecules, algebric sum of oxidation number of each element will be equal to charge on polyatomic molecules.

Examples of both cases , compound and polyatomic molecules, is discussed below. 

Definition


Oxidation number is the charge number either positive or negative on atoms of elements in a compound. This charge represents the ability of share, gain or loss of electron to form bonds with other elements in compound.

Rules For Assigning Oxidation Numbers


oxidation number or state of certain elements are assigned by following rules.
  1. Oxidation number or state of elemental forms like H2, O2, P4, S8 Cu, Al etc are always "zero"
  2. Elements of IA(Alkali metals) group is always +1
  3. Element of IIA (Alkaline earth metals) is always +2
  4. Elements of VIIA (halogen) is usually -1.Positive if attached to oxygen.Florine is always -1. 
  5. Oxygen in normal oxide is -2 (in peroxide is -1)
  6. Hydrogen is +1 attached with non metal and -1 when attached with metals.
  7. The sum of all the oxidation number of all atoms in a neutral compound is "Zero"
  8. The sum of all the oxidation number of all atoms in a polyatomic ion is equal to change on that ion.
Examples

Note: In  examples charge shown in red is oxidation state of single atom and charges in black is  total charge on total atoms in that compound and polyatomic ion.

Example 1: Oxidation state /number of "C" in carbon monoxide CO .
Considering the rule 5, CO is a normal oxide ,"O" in CO has -2 oxidation state so Carbon "C" would have +2 to make the sum  0 "Zero"


Example 2:Oxidation state /number of "C" in methane "CH4"
Considering the rule 6, In CH4  as H is attached to C a non metal so its oxidation number is +1. Here are 4 hydrogen in CH4 so total oxidation number  for 4  hydrogen is +4. To make the sum zero Carbon "C" will have -4 oxidation state.

Example 3:Oxidation state / number of " H" in Zinc hydride "ZnH2"
By considering rule 6, in ZnH2 Hydrogen"H" is attached with metal zinc "Zn" hence oxidation state of "H" would be -1 instead of +1.Here in ZnH2 there are 2 hydrogen atoms so total charge on H will be -2 to make the sum zero "Zn" will have +2 oxidation state.


Example4: Oxidation state / number of Chlorine "Cl" in ClF3.
By considering rule 4, usually halogens have -1 oxidation state /number  but whenever both elements in a compound are halogens then florine "F" would have normal -1 state while other attached halogen will gain positve oxidation state acordingly. in ClF3 florine will have -1 oxidation state , there are 3 F atoms so total charge on F will be -3, to make sum zero "Cl" will have + 3 oxidation state.

Example 5 : Oxidation state/ number of Hydrogen peroxide H2O2
By cosidering rule 6,  H2O2 is peroxide rather simple oxide. Here oxidation number of "H" is 
+1 as per rule due to two atoms of "H"  total charge on "H" will be +2.To make the sum "zero" so oxidation number of "O" will be -1, "O" atom number is also two hence total charge on "O"  will be -2. 
Example 6: Oxidation number of "Br" in BrO3 -1 (a polyatomic ion)
Considering the rule 4,in given polyatomic ion "Br"(a halogen) is attached with "O" hence it will have positive oxidation instead -1. we have to have sum of charges equal to -1 ."O" will have normal -2 oxidation state as in normal oxides. in BrO3 there are 3 atoms of "O" so total charge on "O: will be -6, to gain sum of charge equal to -1 "Br" will have +5  charge as shown below.






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