How to define molecules, particles and atoms?

Particles :
In microscopic view, particles (粒子) are the very tiny units which makes up all different substances. The term 「particles」was proposed many years ago by scientists to explain the nature of matter. Common particles includes atoms, molecules and ions.

Atoms :
In Chemistry, atoms are the basic unit of particles. An atom contains a nucleus (consisting protons and neutrons) and some electrons moving around the nucleus. If two atoms have the same number of protons, they are the same element. There are about 117 elements have been found or synthesized.

Molecules :
A molecule is a group of atoms, and this group of atoms can exist in nature or under certain conditions. For example, 2 hydrogen atoms and 1 oxygen atom group together to form a water molecule.
A monatomic molecule contains only 1 atom (e.g. He), a diatomic molecule contains 2 atoms (e.g. HCl), a triatomic molecule contains 3 atoms (e.g. CO2), and etc.
(Note that molecules must be neutral in charge. If a group of atoms carries positive or negative charge, this group of atoms is an ion, not a molecule.)

Actually they are more or less the same. All of them means the smallest matter in the world. But we use different terms in diiferent area.