What is the Definition of Atom and Molecule Atom: All the matter is made up of atoms. An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms of most of the elements are very reactive and do not exist in the free state. They exist in combination with the atoms of the same element of another element. Atoms are very, very small in size. … [Read more...] about What is the Definition of Atom and Molecule