How to Assign Oxidation States

Rules for Assigning Oxidation States to Atoms

1. The oxidation state of an atom in its elemental state is zero.
   Example: Each oxygen atom in O₂ has an oxidation state of 0.
2. The oxidation state of a monatomic ion is equal to the charge on the ion.
   Example: Cl⁻ = −1, Mg²⁺ = +2.
3. The sum of the oxidation states of all atoms in a compound is zero.
   Example: In H₂SO₄, the sum of oxidation states of H, S, and O = 0.
4. The sum of the oxidation states of all atoms in a polyatomic ion equals the charge on the ion.
   Example: In CO₃²⁻, the sum of oxidation states = −2.
5. The oxidation state of a Group 1 metal atom in a compound is +1.
   Example: Na in NaOH = +1.
6. The oxidation state of a Group 2 metal atom in a compound is +2.
   Example: Mg in MgCl₂ = +2.
7. The oxidation state of oxygen in a compound is almost always −2.
   Exceptions: In H₂O₂, O = −1; in OF₂, O = +2.
8. The oxidation state of hydrogen in a compound is almost always +1.
   Exception: In metal hydrides (e.g., MgH₂), H = −1.

Using oxidation states to spot oxidation and reduction

1. If in a reaction an element increases its oxidation state, then it has been oxidised.
2. If in a reaction an element decreases its oxidation state, then it has been reduced.

• The hydrogen and oxygen atoms in H₂ and O₂ have oxidation states of zero because they are in their elemental state.
• The hydrogen atoms in H₂O have oxidation states of +1, and the oxygen atom in H₂O has an oxidation state of −2. Note that the sum of the oxidation states of all the atoms in H₂O is zero.
• The hydrogen atoms have increased their oxidation states from 0 to +1. This is oxidation.
• The oxygen atom has decreased its oxidation state from 0 to −2. This is reduction.

Homework Question

In which two of the following species does the nitrogen have the same oxidation state?

1. NO
2. NO₂
3. N₂O
4. NH₃
5. HNO₂
6. NO₃⁻
7. Mg₃N₂

View Answer

Step-by-Step Oxidation State Calculations

NO
Oxygen is −2. Neutral molecule: N + (−2) = 0 → N = +2.

NO₂
Each oxygen is −2. Total O = −4. Neutral molecule: N + (−4) = 0 → N = +4.

N₂O
Oxygen is −2. Neutral molecule: 2N + (−2) = 0 → 2N = +2 → N = +1.

NH₃
Hydrogen is +1. Total H = +3. Neutral molecule: N + 3 = 0 → N = −3.

HNO₂
Hydrogen is +1, oxygen is −2 each (total −4). Equation: N + 1 − 4 = 0 → N = +3.

NO₃⁻
Each oxygen is −2. Total O = −6. Ion charge = −1. Equation: N − 6 = −1 → N = +5.

Mg₃N₂
Mg is +2 each. Total Mg = +6. Neutral compound: 6 + 2N = 0 → 2N = −6 → N = −3.

Comparison Table

Species	Oxidation State of N	Process
NO	+2	Oxidation
NO2	+4	Oxidation
N2O	+1	Oxidation
NH3	−3	Reduction
HNO2	+3	Oxidation
NO3−	+5	Oxidation
Mg3N2	−3	Reduction

Answer

The nitrogen has the same oxidation state in NH₃ and Mg₃N₂ (both −3).