Atom is a basic buidling block of matter. This doesn't mean that there are no smaller particles than atom, but only atom is stable enough to exist on its own indepently or in a combination with other atoms in a molecul. This very nice video gives more detailed info on the topic of atoms. Then click the title, read more and do the quiz :-)
What is a molecul? A molecul would be a combination of different atoms in differen numbers and variations, bonded together. A good example for this is water with a chemical formula H2O, meaning the molecul of water is made of two atoms of hydrogen (H) and one atom of oxygen (O).
