By John Brennan
Updated Mar 24, 2022

In redox chemistry, electron transfer is tracked through oxidation numbers. When an element’s oxidation number rises—or becomes less negative—it has been oxidized; a decrease, or a move toward more negative, indicates reduction. Remember the mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) to keep the direction straight. An oxidizing agent accepts electrons and is itself reduced, while a reducing agent donates electrons and becomes oxidized.

Step 1

Write the balanced chemical equation for the reaction. For example, the combustion of propane is represented as:

C₃H₈(g) + 5 O₂ → 3 CO₂(g) + 4 H₂O(l)

Step 2

Assign oxidation numbers to every element using these core rules:

• Elements in their elemental form have an oxidation number of 0 (e.g., O₂, N₂).
• Fluorine is always –1 in compounds. Oxygen is –2 except in peroxides or when bonded to fluorine.
• Hydrogen is +1 when bonded to nonmetals and –1 when bonded to metals.
• Halogens (group 17) are –1 unless paired with oxygen or a more electronegative halogen, in which case they are +1.
• Group 1 metals are +1; group 2 metals are +2.
• The sum of oxidation numbers in a neutral molecule equals 0; in an ion it equals the ion’s charge.

For instance, the sulfate ion, SO₄^2–, has a net charge of –2, so the oxidation numbers of S and O must add to –2.

Step 3

Compare the oxidation numbers on the reactant side to those on the product side. A species whose number drops (or becomes more negative) has gained electrons—reduced. A species whose number rises (or becomes less negative) has lost electrons—oxidized.

In the propane combustion example, oxygen starts at 0 and ends at –2 in both CO₂ and H₂O, so oxygen is reduced. Propane’s carbons go from –3 (in C₃H₈) to +4 (in CO₂), indicating oxidation.

Step 4

Identify the oxidizing and reducing agents:

• The substance that oxidizes another is the oxidizing agent.
• The substance that reduces another is the reducing agent.

Thus, in the propane‑oxygen reaction, O₂ is the oxidizing agent and C₃H₈ is the reducing agent.

Step 5

Keep in mind that a compound can play either role depending on the partner species. Some substances readily lose electrons and are typically viewed as reducing agents; others excel at accepting electrons and act as oxidizing agents. The actual role is dictated by the specific reaction context.

Things Needed

• Periodic table
• Pencil
• Paper

TL;DR

Mastering oxidation numbers takes practice. Try assigning numbers to various compounds until the patterns become intuitive.

References

• "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.