Question

how we get the oxidation number

Answer 1

The oxidation number of a free element is always 0.
 The oxidation number of a monatomic ion equals the charge of the ion.
 The oxidation number of #"H"# is +1, but it is -1 in when combined with less electronegative elements.
The oxidation number of #"O"# in compounds is usually -2, but it is -1 in peroxides. The oxidation number of a Group 1 element in a compound is +1.
 The oxidation number of a Group 2 element in a compound is +2
. The oxidation number of a Group 17 element in a binary compound is -1.
The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.