The Concept of Atoms

Despite the conditions given above, however, and although a molecule can be broken down into fragments, matter cannot be regarded as indefinitely divisible.  The kinetic theory of gasses requires the assumption that a gas is not an expansible or continuous medium but an aggregation of particles far removed from one another. The solution of one substance in another and the passage of electrical charges through liquids are only explicable on the assumption that matter is divisible into discrete particles. From the point of view of the chemist, the atom is the smallest particle into which matter can be split. If substance (e.g. an appropriate element) is subject to the dividing process then the smallest particle will not be a molecule but an atom. A substance that is made up of atoms is called a compound and the smallest particle of a compound is a molecule. However, if the substance is an element then the smallest particle of an element (if it is monatomic) will be an atom. 

Atoms are extremely small an invisible to the eye and have extremely small masses. They consist of a central nucleus surrounded by a cloud of electrons. The electrons are bound to the nucleus by electromagnetic forces. In the same manner atoms can be bound to each other to form compounds.