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الموضوع: Assigning Oxidation Numbers

  1. #1
    عضو ذهبي الصورة الرمزية ماجده
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    Aug 2011
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    افتراضي Assigning Oxidation Numbers

    Assigning Oxidation Numbers

    Oxidation numbers (ON) are not real charges. They are the results of an accounting method whereby we can keep track of electrons during a chemical reaction. In a few cases, the ON is actually a real charge but only in rare monatomic ions like Cl- and Na+.
    Oxidation numbers are defined using two premises.
    1. The bonds in compounds for which you wish to assign oxidation numbers are assumed to be 100% ionic in nature.
    2. The electrons in the bonds are thus divided such that the more electronegative element gets both electrons in a bond and the less electronegative element ends up with a net loss of electrons.

    Using this method, we need to be up on the relative electronegativities of the elements and it takes time to figure out which way the electrons go and the resulting charges. For example, if we take the bonds in water H2O. There are two O-H bonds in each molecule.
    Taking them one at a time, in an O-H bond, there two electrons and we assign them both to the oxygen since it is more electronegative than hydrogen. Doing this to the second bond as well leaves us with an oxygen ion with 8 electrons for a net charge of -2 and two hydrogen ions with no electrons for a net charge of +1. This takes considerable thought processes and can be simplified using a set of rules based on these processes but which gives results directly.
    RULES: (taken in order of importance.)
    1. The sum of the oxidation numbers (ON) of the atoms in a species (ion, molecule, atom) is equal to the charge on that species.
      For example,
      He(atom) ---> ON = 0
      H2(molecule) ---> both atoms must have ON=0 so sum is 0
      Cl- (ion) ---> ON = -1
      Na+ (ion) ---> ON = +1
    2. Alkali metals in a compound with non-metals have ON=+1
      example: NaCl ---> ON(Na) = +1\ON(Cl) = -1 [rule 1]
    3. Alkali Earth metals in a compound with non-metals have ON=+2
      example: MgO ---> ON(Mg) = +2 \ON(O) = -2 [rule 1]
    4. Hydrogen in cpds. has ON = +1
    5. Halogens have ON=-1 as in HF or HCl. (This is always true for fluorine but for the others, except if bonded to other halogens [interhalogen] or to oxygen)
      example ClO- ---> ON(Cl) is not -1 since O is more electronegative than Cl.
    6. Oxygen in compounds is always ON(O)=-2. Except with Fluroine (rule 5) or in the case of peroxides O22- [ON(O)=-1] or super oxides O2- [ON(O) = -1/2].
    7. For all elements where these rules don't work, use the overriding premise and work out the oxidation numbers, i.e., the more electronegative element gets the electrons (or the negative oxidation number).

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  2. #2
    عضو ذهبي الصورة الرمزية م.عبد الرحمن
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    Aug 2011
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    افتراضي رد: Assigning Oxidation Numbers

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  3. #3
    عضو مميز الصورة الرمزية قمر بلحاج
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    افتراضي رد: Assigning Oxidation Numbers

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  4. #4
    مراقب عام الصورة الرمزية Eiman
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    Jul 2011
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