Rust is a red-brown-orange substance often seen on the surface of iron objects. Rusting is the oxidation of iron that occurs in the presence of oxygen (O2) and water (H2O). Chemically, the rust represents the hydrated iron oxide commonly written as Fe(OH)3. When balancing the chemical reaction of oxidation, you need to equate the number of each atom type on the left and right sides of the equation by using appropriate coefficients.
Write down the initial reagents of of the reaction iron rusting: Fe + O2 + H2O
Write down the product of the reaction: Fe + O2 + H2O = Fe(OH)3
Multiply the number of water (H2O) molecules by 6 and iron hydroxide by 4 to balance the number of hydrogen atoms. Fe + O2 + 6H2O = 4Fe(OH)3
Multiply the number of oxygen (O2) molecules by 3 to balance the number of oxygen atoms. Fe + 3O2 + 6H2O = 4Fe(OH)3
Multiply the number of iron (Fe) atoms by 4 to balance the number of iron atoms. 4Fe + 3O2 + 6H2O = 4Fe(OH)3