Mass and atoms are the properties conserved in every ordinary chemical reaction. Because the atom is the smallest unit of matter, it is indivisible. Because atoms are indivisible, mass is neither created nor destroyed during a chemical reaction.
In a chemical reaction, atoms rearrange themselves into a new structure, with the number of atoms remaining constant. For example, the reaction between hydrogen and oxygen involves three total atoms: two atoms of hydrogen and one atom of oxygen. When hydrogen and oxygen react, the three atoms combine to form H2O, or water. H2O retains the same number of hydrogen and oxygen atoms present before the reaction, as well as the same mass.