how can we find the oxidation state of given element?
Oxidation state of an atom is defined as the charge which it possesses in ionic form or appears to acquire 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 state of hydrogen is +1.
General rules for assigning oxidation state 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 state of :
- atoms in free elemental state (like H2, Na, O2, Ag etc) = 0
- Oxidation state of simple monoatomic ions = Charge on them ( For example : Halogens (like Fluorine, chlorine) = -1, Na+ = +1, Ca+2 = +2 etc)
- Oxygen = -2; in peroxides(-1); F2O(+2); F2O2(+1)
- Hydrogen = +1; however in metal hydrides it is (-1)
- Sum of oxidation states of all the atoms in molecules = 0
- Sum of oxidation states of atoms in polyatomic ions = overall charge on them
Let us apply the above rules to calculate the oxidation state of some elements.
(a) Oxidation state of C in CO2 : Let the O.N of C be a
Oxidation state of each O atom = -2
Sum of oxidation state 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) Oxidation state of N in NH4 + Let O.N of N be a
Oxidation state of each H atom= +1
Sum of oxidation state 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