Read the formula before changing a number
Reactants appear before the arrow and products after it. A subscript belongs to the substance’s formula: O₂ contains two oxygen atoms per molecule. A coefficient multiplies the whole formula. In 2H₂O, count four hydrogen atoms and two oxygen atoms. An unwritten coefficient or subscript means one.
Balance amounts, not identities
A chemical reaction rearranges atoms; balancing accounts for the same number of each element before and after. Change the coefficients in front of formulas. Do not change a subscript to make the counts match, because that changes the substance. Work through one element at a time, then recount all of them.
Finish with a complete atom check
Use the smallest whole-number coefficients for the final equation. Count each element independently on both sides, including every formula where it appears. Coefficients describe relative numbers of particles or amounts in moles; they are not a direct ratio of masses. Balancing also does not tell you how fast the reaction happens.
Worked example
Balance Fe + O₂ → Fe₂O₃
Begin with oxygen: O₂ contains two oxygen atoms, while Fe₂O₃ contains three. Six is a shared total, so use 3O₂ and 2Fe₂O₃. The product side now has 2 × 2 = 4 iron atoms, requiring 4Fe. The balanced equation is 4Fe + 3O₂ → 2Fe₂O₃. Check four iron atoms and six oxygen atoms on each side; the subscripts have not changed.