Question

how to calculate the oxidation numbers

Answer 1

The process of finding oxidation number is simple.
For example, you have to find the oxidation number of Mn in KMnO4. Let oxidation number of Mn be x. Hence we know that metals have +1 oxidation numbers and oxygen have 3 oxidation state : -2, -1 (Peroxides)and -1/2(Superoxides). In this case, it is -2.
So now calculate it
KMnO4
=> +1+x+4(-2)=0 (Since there is no charge on it)
=> +1+x-8=0
=> x=+7
Oxidation state of Mn is +7.

Hence finding oxidation number is simple. You have to remember the oxidation state of various elements in order to calculate it.

Answer 2

Hey mate here is your answer .
1. The oxidation no. of a free electrons is always zero.
2.The oxidation no. of a monatomic ion is equal the charge of the ion.
3.The oxidation no. of H is +1 , but it is -1 in when combined with less electronegative elements .
4. The oxidation no. of oxygen is usually -2 , but it is -1 in peroxide.
5.Group 1 elements in a compound is +1 .
6.Group 2 elements in a compound is +2.
7.Group 17 elements in a compound is -1 .
8.The sum of oxidation no. of all the atoms is a neutral compound is Zero.
9.The sum of oxidation no. in a polyatomic ion is equal to the charge of the ion.
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Hope that much information is enough for you.