What Is an Oxidation Number?
An oxidation number, also known as an oxidation state, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. It is a bookkeeping tool used by chemists to track the movement of electrons in oxidation-reduction (redox) reactions.
Section 2: Rules for Assigning Oxidation Numbers
Several key rules govern the assignment of oxidation numbers. An atom in its pure elemental form (e.g., Na, O₂, P₄) has an oxidation number of 0. For a simple monatomic ion, the number equals its charge (e.g., Na⁺ is +1, Cl⁻ is -1). In compounds, fluorine is always -1, oxygen is usually -2 (except in peroxides), and hydrogen is usually +1 (except in metal hydrides). The sum of oxidation numbers in a neutral compound must be zero, while in a polyatomic ion, it must equal the ion's charge.
Section 3: A Practical Example
Let's find the oxidation number of sulfur (S) in sulfuric acid (H₂SO₄). We know hydrogen is +1 and oxygen is -2. The overall charge of the molecule is 0. We can set up an equation: 2(+1) + (S) + 4(-2) = 0. This simplifies to +2 + S - 8 = 0. Solving for S, we find that the oxidation number of sulfur in H₂SO₄ is +6.
Section 4: Importance in Chemistry
Oxidation numbers are essential for identifying and understanding redox reactions. When an atom's oxidation number increases during a reaction, it has been oxidized (lost electrons). When its oxidation number decreases, it has been reduced (gained electrons). This concept is fundamental for balancing complex chemical equations, understanding electrochemistry (like batteries), and analyzing metabolic processes.