Which particle has the smallest mass in an atom?

The particle with the smallest mass in an atom is the electron. Electrons are subatomic particles that have a mass approximately 1/1836 that of a proton. This is a significant difference, as protons and neutrons (which are located in the nucleus of the atom) are both much heavier.

Protons have a mass of about 1 atomic mass unit (amu), and neutrons also have a similar mass of about 1 amu. In contrast, the extremely lightweight nature of electrons allows them to occupy regions of space around the nucleus without significantly contributing to the overall mass of the atom.

The nucleus itself, being composed of protons and neutrons, collectively has a much greater mass than the individual electrons. Therefore, the electrons, due to their much smaller mass, are the lightest subatomic particles found in an atom.