Which particle in an atom has the smallest mass?

The electron is the particle in an atom that has the smallest mass. While protons and neutrons, which are found in the nucleus of an atom, have significantly larger masses—roughly 1,800 times that of an electron—the electron’s mass is so small that it is often considered negligible in comparison when discussing atomic mass.

In context, the proton and neutron are both fundamental components of the nucleus, contributing significantly to the atom's overall mass due to their much greater mass relative to the electron. The nucleus itself is not a single particle but rather a collection of protons and neutrons, which also makes it substantially more massive than an electron.

Understanding the mass of these particles is fundamental in chemistry, as it contributes to concepts such as atomic mass and stability of atoms.