rules to calculate oxidation number-

1. cation is written first in the formula.

2. oxidation number of a free element is always zero.

3. the oxidation number of a monoatomic ion eqauls the charge of the ion.

na+ = +1, ca+2 = +2

4. usual oxidation number of  H= +1, O= -2,  group 1 = +1, group 2= +2, group 6= -2, group 7= -1.

5. the sum of all the oxidation numbers of all the atoms in a neutral compound is 0.

6. the sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. for eg-  the sum of the oxidation numbers for SO4 2- is -2.