Molecule is one of the basic units of matter. It is the smallest particle into which a substance can be divided and still have the chemical identity of the original substance. If the substance were divided further, only molecular fragments or atoms of chemical elements would remain. For example, a drop of water contains billions of water molecules. If one of those water molecules were separated from the rest, it would still behave as water. But if that water molecule were divided, only atoms of the elements hydrogen and oxygen would remain.