Atomic particles. The reason is that when an atom is broken, that substance has no other properties , that is, the substance of which the atom is broken , then that substance no longer exists. This is why atoms are referred to as the smallest particles of an elemental substance. As in the case of the smallest particle of a compound, it refers to a molecule. Just as an atom of a few elements can be found by breaking a molecule , but no properties of that compound are present , so no idea of that element can be found by breaking an atom. So the atom is better known as the smallest particle of any element. Electrons , protons , neutrons are considered to be the three main elemental particles.