Identify the oxidation number of each element present in the compound reaction. Oxidation numbers can be positive, negative or zero and only represent one atom at a time. The sum of oxidation numbers for all atoms in a neutral compound is zero. If the element is in its natural form, the oxidation number is zero, such as carbon with its four electrons. It has an oxidation number of zero. If an element is a positive or negative ion, such as Al^3-, the oxidation number is the charge of that ion, negative three (-3) in this case. Consult a periodic table to assign other elements: Group 1A metals are +1, Group 2A metals are +2, hydrogen is +1 when bonded to a nonmetal and -1 when bonded to a metal, oxygen is -2 unless with a peroxide and fluorine is always -1.
Compare similar reactants and products on either side of the equation. Note if any oxidation states have changed. A changing oxidation state means electrons have been moved around within the reaction, but not lost or gained. The redox reaction shows newly formed products and byproducts of a reaction, such as water or hydrogen gas.
Identify which elements have been reduced and oxidized. If there has been any change in the charge of similar elements, this is a redox reaction. If an element has increased its oxidation state, it has gained electrons and is reduced. If the element has a smaller oxidation number, it has lost electrons and is now oxidized. For example, if aluminum is +1 on the reactant side, but the product side shows aluminum with a charge of +2, you know this is a redox reaction and that aluminum, specifically, was oxidized because it lost electrons, making it more positive.