For a very long time, atoms are said to be indivisible not until the start of the technological revolution when the three subatomic particles stood out: electrons, protons and neutrons. But as the advancement in particle science, (a lot) smaller particles have been discovered and now being disputed over which particles is the smallest in terms of mass. These smallest particles mainly belong to fermions, hadrons and bosons and some of which are classified to be "massless" and are yet bound to certain observations.
If we not include those of the "massless" molecules, the smallest in terms of mass is the neutrino (muon and tau neutrinos), under leptons which is a type of Fermions. Before, neutrinos were also considered "massless" until they found out that it has a mass of less than 1.7 keV,or around 3.311 x 10^-31 kilograms.