Particle, corpuscle, atom, molecule in technical physical or chemical use can mean a submicroscopic division of matter.
Particle, the oldest and most general of these terms, is applied especially to any of certain minute entities which have more specific designations such as ion, molecule, atom, electron, proton, and alpha particle . Particle is often used to emphasize the idea of indivisibility, commonly suggesting the entities (as protons, neutrons, and electrons) of which all matter is believed to be composed.
Corpusele may be interchangeable with particle, but more often it is specifically equivalent to elementary particle and may be applied to energy quanta (as photons or phonons) when these are considered as particulate entities.
According to the common modern concept, an atom is the smallest particle of an element that can exist either alone or in combination with smaller particles of the same or of a different element.
Molecule denotes the smallest particle of an element or of a chemical combination (as a compound) that retains chemical identity with the substance in mass. Molecules are usually composed of two or more atoms, either of the same or of different elements.