Final answer:
The subatomic particle with the least mass is the electron, having a mass of about 0.00055 amu, which is significantly lighter than both the proton (1.0073 amu) and the neutron (1.0087 amu).
Step-by-step explanation:
When comparing subatomic particles such as electrons, protons, and neutrons, the electron is the particle with the least mass. Despite all three being crucial components of an atom, an electron has an extremely small mass compared to protons or neutrons. Specifically, the mass of an electron is about 0.00055 amu (atomic mass units), whereas a proton has a mass of approximately 1.0073 amu, and a neutron has a mass of about 1.0087 amu.
Protons and neutrons, collectively known as nucleons, have masses that are nearly identical. However, the neutron is very slightly more massive. The vast difference in mass between electrons and nucleons (protons and neutrons) means that almost all the atomic mass is concentrated in the atom's nucleus, not in the electrons that orbit it.