Atoms are neither created nor destroyed in a chemical reaction. This is the law of conservation of mass: the total mass of the products equals the total mass of the reactants. So the number of atoms of each element must be the same on both sides of an equation. Take iron reacting with steam to form the iron oxide Fe three O four, and hydrogen gas. Start with the compound that has the most atoms, here Fe three O four, and count each element on both sides. There are four oxygen atoms on the right, so put a four in front of water on the left. Now there are eight hydrogen atoms on the left, so put a four in front of hydrogen gas on the right. Finally, three iron atoms on the right need a three in front of iron on the left. Never change a formula to balance an equation; change only the numbers in front, called coefficients.