What is oxidation state? How to find or calculate it for an atom?
If you want to learn What is oxidation state? and How to find the oxidation state , you should learn the following rules to assign the oxidation number:
1. The oxidation state of group 1 elements is always +1.
Li, Na, K, Rb, Cs, Fr have +1 OS in all the compounds.
For example: OS of Na in Na2O is +1.
2. The OS of group 2 elements is always +2.
Mg, Ca, Ba etc will always have +2 OS.
For Example: Oxidation number of Mg in MgO is +2.
3. The free element or a free molecule will always have OS equal to zero.
For example: Fe and O2 will have oxidation number equal to zero.
4. Fluorine is the most electronegative element so it always has -1 oxidation number.
5. Other halogens generally have -1 oxidation number.
For example: Cl has -1 oxidation number in NaCl.
In Interhalogen compounds like IF7, Iodine has +7 Oxidation number because
Fluorine always have -1 oxidation number.
6. Oxygen generally have -2 OS in all oxides.
For example: OS of O in CO2 is -2.
In peroxides ,OS of O is -1.
For example: OS of O in H2O2( Hydrogen peroxide) is -1.
In superoxides, OS of O is -1/2.
For example; OS of O in NaO2 ( Sodium superoxide) is -1/2.
Exceptionally oxygen has +2 OS in OF2( Oxygen fluoride) because F
has -1 OS always.
7. The OS of all the ions ( simple ions or polyatomic ion) is always equal
to the charge on them.
For example: Oxidation states of Aluminium and phosphate ions is equal to the
charge on them. Al3+and (PO4)3-
8. The sum of all the oxidation states of all atoms in compounds is always zero.
For example: The sum of all Oxidation states in H2O=0
H=+1 so +1×2 = +2
O= -2 so the sum is +2 -2= 0
9. The oxidation states of transition elements isto be determined by using above
rules because they have variable oxidation states.
The OS of Mn in KMnO4 is to be calculated.
K= +1 because it is in group 1.
O=-2 x4= -8
+1+Mn -8= 0
Mn = +7
Determine the following oxidation states.
1. N in NO3–
2. S in H2SO4
3. Cr in K2Cr2O7
4. Fe in Fe2O3
5. Fe in Fe3O4
6. Mn in MnO2
7. S in SO32-
8. C in CH4
9. S in HSO4–
10. N in NO2–