The idea of exchanging electron, **x _{2} (g) + 3y_{2} (g) → 2xy_{3}**

This reaction is known as oxidation reduction reaction,

The actual reaction,

**N _{2} (g) + 3H_{2} (g) ↔ 2NH_{3}**

**0 0 ↔ – 3 + 1**

The oxidation number of H atom 0 → +1. So the oxidation number increases. The oxidation number of N atom 0 → – 3. So the oxidation number decreases. H_{2} releases 1 electron and N_{2} receive 3 electron. To equal the oxidation number H_{2} multiply by 3. In the reaction, oxidation -reduction occurs.