General Rules:
* Free element: The oxidation number of an element in its elemental form is always 0. So, N₂ (nitrogen gas) has an oxidation number of 0.
* Monatomic ions: The oxidation number of a monatomic ion is equal to its charge. For example, N³⁻ (nitride ion) has an oxidation number of -3.
* Oxygen: Oxygen usually has an oxidation number of -2, except in peroxides (like H₂O₂) where it's -1.
* Hydrogen: Hydrogen usually has an oxidation number of +1, except in metal hydrides (like NaH) where it's -1.
* Sum of oxidation numbers: The sum of the oxidation numbers in a neutral compound must equal zero, and in a polyatomic ion, it must equal the charge of the ion.
Examples:
* NH₃ (ammonia): H is +1, so 3 H atoms contribute +3. To balance, N must be -3.
* HNO₃ (nitric acid): O is -2, so 3 O atoms contribute -6. H is +1. To balance, N must be +5.
* N₂O (nitrous oxide): O is -2. To balance, each N must be +1.
* NO₂ (nitrogen dioxide): O is -2, so 2 O atoms contribute -4. To balance, N must be +4.
Important Notes:
* Oxidation numbers are assigned, not measured: They're a way to keep track of electron transfer in reactions.
* The oxidation number can be fractional: In some cases, like in N₂O, the oxidation number for nitrogen is not a whole number.
Let me know if you have a specific nitrogen-containing molecule in mind, and I can help you determine its oxidation number.
