An atom is smallest particle in an element that has all the properties of the element. It is not possible to breakdown the atom further retaining the properties of the element. Atoms are not visible to the naked eye and are the basic building blocks. For example the atoms of element gold cannot be broken down further and each atom has the properties of gold.
Molecules are formed by the combination of two or more atoms. Unlike atoms, molecules can be subdivided to individual atoms. The atoms are bonded together in a molecule. Molecules also are not visible to the naked eye, while can be seen through highly magnifying microscopes and other scientific devices. Water is comprised of numerous water molecules. Each water molecule is made up of one oxygen atom and two hydrogen atoms. So a water molecule can be further divided into oxygen and hydrogen atoms. But these atoms cannot be subdivided. In a molecule, atoms are bonded together by single, double, or triple bonds.