Question

how do we find oxidation numbers

Answer 1

We cannot find the oxidation number of an equation as a chemical equation has no oxidation number.

We can only find the oxidation number of an element in a compound.

Oxidation number of an element is defined as the charge which its atom has or appears to have when all the other atoms from the molecule are assumed to be removed as ions by counting the shared electrons with more electronegative atom.

For example, in hydrogen chloride molecule, chlorine is more electronegative than hydrogen. So, the shared pair is counted towards chlorine atom. As a result, chlorine gets 8 electrons in its outermost shell with one extra electron and acquires a unit negative charge. Hence, oxidation state of chlorine is -1. On the other hand, hydrogen atom without electron has a unit positive charge. Hence, oxidation number of hydrogen is +1.



General rules for assigning oxidation number to an atom:The rules have been formulated on the basis of the assumption that electrons in a covalent bond belong entirely to the more electronegative atom.

Oxidation number (O.N) of:

Free elements  = 0Fluorine   = -1Simple ions   = Charge on themOxygen   = -2; in peroxides(-1); F2O(+2); F2O2(+1)Hydrogen     = +1; in metal hydrides(-1)Sum of O.N of atoms in molecules   = 0Sum of O.N of atoms in polyatomic ions = Charge on them

Let us apply the above rules to calculate the oxidation number of some elements.

(a)O.N of C in CO2: Let the O.N of C be a

O.N of each O atom = -2

Sum of O.N of all atoms = a + 2(-2)=a-4

As it is a neutral molecule, the sum must be equal to zero.

  a-4 = 0  or  a  = +4

(b) O.N of N in NH4+:Let O.N of N be a

 O.N of each H atom= +1

Sum of O.N of all atoms = a + 4(+1)=a+4

Since it is an ion, Sum must be equal to charge on the ion.

   a+4 = +1  or  a = -3