Step 1 - Determining the oxidation number
The oxidation number of severals elements may vary depending on to which atoms they are bonded. Nonetheless, there are still some useful tips we can use to calculate the oxidation number:
I) The oxidation number of simple substances is always 0. Therefore, O in O2 or H in H2 will always have oxidation number 0.
II) The oxidation number of ions is equal their charge. Therefore, the oxidation number of Cl(-), for example, is -1.
III) The oxidation number of H is usually +1 and of O usually -2 in compound substances.
Step 2 - Calculating the oxidation number
a) N2 is a simple substance. Therefore, the oxidation number of N in this substance must be 0.
b) ZnCl2 is an ionic substance formed by the ions Zn(2+) and Cl(-). Using tip II, we know that their oxidation number must be equal their charge. Therefore, the oxidation number of Zn is +2.
c) The same reasoning as b) applies here. The oxidation number of Cl in ZnCl2 is thus -1.
Answer: a) 0; b) +2; c) -1