Oxidation Numbers

Understanding Oxidation Numbers

An oxidation number (or state) is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. This concept helps chemists track how electrons are transferred in reactions, particularly redox (reduction-oxidation) reactions.

Rules for Assigning Oxidation Numbers

1. Free Elements: The oxidation number of an atom in a pure element (e.g.,$O₂$, $N₂$, $Na$) is 0.
2. Simple Ions: The oxidation number of an atom in a monoatomic ion is the same as its charge (e.g., $Na⁺ = +1$, $Cl⁻ = -1$).
3. Sum of Oxidation Numbers:

• For a neutral compound, the sum of the oxidation numbers of all atoms is 0.
• For a polyatomic ion, the sum of the oxidation numbers of all atoms equals the charge of the ion (e.g., $SO₄²⁻$: sum = -2).

4. Group I Elements: Always have an oxidation number of +1 in compounds (e.g., $NaCl: Na = +1$).
5. Group II Elements: Always have an oxidation number of +2 in compounds (e.g., $CaCl₂: Ca = +2$).
6. Hydrogen: Has an oxidation number of +1 in most compounds, but -1 in metal hydrides (e.g., $NaH: H = -1$).
7. Oxygen: Usually has an oxidation number of -2 in compounds, except:

• In peroxides (e.g., $H₂O₂$), where it is -1.
• When bonded to fluorine, it takes on a positive value (e.g., $OF₂: O = +2$).

8. Halogens (Group 17): Typically have an oxidation number of -1, unless they are bonded to oxygen or other halogens with higher electronegativity.

Oxidation and Reduction in Terms of Oxidation Numbers

• Oxidation refers to an increase in oxidation number (loss of electrons).
• Reduction refers to a decrease in oxidation number (gain of electrons).

Calculating Oxidation Numbers for Transition Metals

Transition metals often have variable oxidation states, which can be determined based on the compounds they form. For example:

• In $Fe₂O₃$, the oxidation number of $Fe$ can be calculated by knowing that $O$ is -2.
• With three $O$ atoms, the total negative charge is -6.
• To balance this, the two $Fe$ atoms must have a total oxidation number of +6, so each $Fe$ is +3. Tip: In the names of compounds containing transition metals, Roman numerals indicate the oxidation state (e.g., Iron (III) oxide means $Fe$ is in the +3 state).

Applications: Bleaches as Oxidising Agents

Bleaches often work through redox reactions. For example:

• Sodium hypochlorite ($NaOCl$) acts as an oxidising agent in cleaning and disinfecting. In this case, chlorine in $NaOCl$ has an oxidation number of +1.
• Sulfur dioxide ($SO₂$), a reducing agent, is commonly used to prevent oxidation in food preservation.