Oxidation Number

Oxidation States (Oxidation Numbers)

  1. Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.

Oxidation State of Some Elements

1. The oxidation state of an element is zero.
Example

Element
Oxidation State
Mg
0
H2
0
Br2
0

2. For a simple ion with single atom, the oxidation state is equal to the charge.
Example

IonOxidation State
Cu2+ +2
Br– -1
O2- -2
Al3+ +3

3. Some elements almost always have the same oxidation states in their compounds:

Example 1: The oxidation state of oxygen is always -2 except peroxide, which is -1.

CompoundOxidation state of oxygen
H2O-2
H2SO4-2
ZnO-2
KClO3-2
H2O2-1


Example 2: The oxidation state of hydrogen is always +1 except hydride, which is -1.

CompoundOxidation state of hydrogen
NH3+1
HCl +1
NaOH +1
MgH2 -1
NaH -1

4. The sum of the oxidation states of all the atoms or molecule in a neutral compound is zero.

Example:

IonSum of Oxidation State
H2O 0
CO2 0
NH3 0

5. The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.

Example:

IonSum of Oxidation State
NO3– -1
CO32- -2
PO43- -3
NH4+ +1

    Working Out the Unknown Oxidation State of an Element in a Compound

    1. The sum of the oxidation state of each element in a compound are equal to the charge of the compound.
    2. This rule can be used to find the unknown oxidation number of an element is a compound.

    Example 1

    Find the oxidation state of all the elements in a Chlorate(V), ClO3– ion.

    Answer:
    Oxidation number of O = -2

    Oxidation number of Cl = x

    x + 3(-2) = -1
    x = -1 + 6 = +5

    Example 2
    Find the oxidation state of all the elements in a Potassium manganate(VII), KMnO4 ion.

    Answer:
    Oxidation number of K = +1`

    Oxidation number of O = -2

    Oxidation number of Mn = x

    (+1) + x + 4(-2) = 0
    x = -1 + 8
    x = +7

    Example 3
    Find the oxidation state of all the elements in an Ammonium ion, NH4+ ion.

    Answer:
    Oxidation number of H = +1

    Oxidation number of N = x

    x + 4(+1) = +1
    x = +1 – 4
    x = -3

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