Oxidation Numbers - Chemistry

1. Fundamental Concepts

Definition: Oxidation numbers (or oxidation states) are assigned to atoms in a chemical compound to help keep track of the distribution of electrons among the atoms.
Rules for Assigning Oxidation Numbers:
- The oxidation number of an atom in its elemental form is always 0.
- The oxidation number of a monatomic ion is equal to its charge.
- The sum of the oxidation numbers in a neutral compound is 0.
- The sum of the oxidation numbers in a polyatomic ion equals the charge on the ion.
- In compounds, oxygen typically has an oxidation number of -2, and hydrogen typically has +1.

Assigning Oxidation Numbers: $${\text{Oxidation number of O}} = -2$$ $${\text{Oxidation number of H}} = +1$$

Charge Balance in Ions: $${\text{Sum of oxidation numbers in } \text{SO}_4^{2-} = -2}$$

Application: $${\text{Used to balance redox reactions and understand electron transfer}}$$

Problem: Determine the oxidation number of sulfur in $$ \text{H}_2\text{SO}_4 $$.

Validation: The sum of the oxidation numbers should be 0. $$ 2(+1) + (+6) + 4(-2) = 2 + 6 - 8 = 0 $$ ✓

Problem: Determine the oxidation number of chromium in $$ \text{Cr}_2\text{O}_7^{2-} $$.

Validation: The sum of the oxidation numbers should be -2. $$ 2(+6) + 7(-2) = 12 - 14 = -2 $$ ✓

Systematic Approach: Always start by assigning known oxidation numbers (e.g., H, O, F).
Charge Balance: Use the overall charge of the compound or ion to set up the equation.
Step-by-Step Calculation: Solve for the unknown oxidation number systematically.
Validation: Always check that the sum of the oxidation numbers equals the overall charge of the compound or ion.