Define Oxidation number

Oxidation number is the charge acquired by an atom when it accepts or loses electrons. For example, Na⁺ ion is formed from Na by losing an electron. Thus its oxidation number is +1. 

 Na → Na⁺  + e^-

Similarly, Zn⁺² is formed from Zn by losing two electrons. Thus its oxidation number is +2.  

 Zn → Zn⁺²  + 2e^-

Similarly, Chlorine (Cl) forms Cl^- ion by accepting an electron. Hence its oxidation number s -1. 

 1/2 Cl₂ + e^- → Cl^-  + e^-